2013 Investigation of weed phenology in an establishing semi extensive green roofs

Ecological Engineering 58 (2013) 156–164

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Research paper

Investigation of weed phenology in an establishing semi-extensive green roof Ayako Nagase a,∗ , Nigel Dunnett b,1 , Min-Sung Choi b,1 a b

Chiba University, Graduate school of Engineering, Division of Design Science, 1-33 Yayoicho, Inage-ku, Chiba-shi, Chiba 263-8522, Japan University of Sheffield, Department of Landscape, Arts Tower, Western Bank, Sheffield S10 2TN, UK

a r t i c l e

i n f o

Article history: Received 24 November 2012 Received in revised form 7 April 2013 Accepted 8 June 2013 Keywords: Urban landscape Gravel mulch Substrate depth Plant species diversity Planting density Self-sowing

a b s t r a c t Although weeding is one of the most time consuming tasks in green roof maintenance, there have been few studies of weed phenology and it is not clear how planting design affects weed colonisation on green roofs.

This study investigated weeds including self-sowing planted species during the establishment period in a semi-extensive green roof in Rotherham, UK. This green roof was installed in the summer of 2005, and 54 plant species were planted in 10 cm (areas with gravel mulch) and 20 cm (areas without mulch) of the substrate. The planting density was 18–22 plants/m2 . Thirty-two quadrats (50 × 50 cm) were set up through the combinations of plant species diversity (high and low), planting density (high and low), four aspect and covering 2.5 cm of gravel mulch (with and without). Drip irrigation was installed for supplementary watering in dry seasons. All weeds and self-sowing in each quadrat were not removed.

The remainder of the roof was weeded six times in this period. Nine weed species were found on the green roof. They were all native species and could have value of biodiversity. High planting density reduced weeds effectively whereas plant diversity did not affect weed colonisation significantly. Moreover, the use of gravel mulch significantly reduced the number of weeds. Knowing phenology of expected weeds allows targeting maintenance to remove them before they set seeds. 29 species planted on this green roof were self-sowing, Allium schoenoprasum, Campanula rotundifolia, Festuca spp. and Petrorhagia saxifraga showed a very high number of seedlings. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Recently, the use of green roofs has received considerable attention and the number of papers related to green roofs is increasing rapidly. At present, the environmental benefits of green roofs seem to be most frequently studied (Berndtsson, 2010; Sailor and Hagos, 2011; Tonietto et al., 2011). Other examples of research topics are the adaptation of plants to the green roof environment (Butler and Orians, 2011; MacIvor and Lundholm, 2011; Rowe et al., 2012), substrate composition for green roofs (Emilsson, 2008; Nagase and Dunnett, 2011; Molineux et al., 2009.) and social aspects of green roofs (Francis and Lorimer, 2011; Kosareo and Ries, 2007). However, there has been little research on green roof maintenance.

Green roof maintenance tends to rely on people’s experience, and information on green roof maintenance is mainly obtained from books (Dunnett and Kingsbury, 2008; Weiler and Scholz-Barth,

∗ Corresponding author. Tel.: +81 043 290 3113; fax: +81 043 290 3121.

E-mail addresses: a-nagase@faculty.chiba-u.jp (A. Nagase), n.dunnett@shef.ac.uk (N. Dunnett), mschoi2012@gmail.com (M.-S. Choi).

1 Tel: +44 0114 222 0611; fax: +44 0114 275 4176.

0925-8574/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ecoleng.2013.06.007

2009; Snodgrass and McIntyre, 2010). Lack of scientific evidence on green roof maintenance may result in inappropriate plant selection and planting design and lead to a high maintenance cost. It has been reported that the increased maintenance cost could be a barrier to the implementation of green roofs (Zhang et al., 2012) and reduction of the maintenance cost may be an important initiative for installing green roofs. In some countries, such as Germany, Switzerland, the United States of America and Japan, governments encourage the installation of green roofs through policy, direct and indirect regulation, and financial incentives, and funding of demonstration or research projects (Carter and Fowler, 2008). However, they tend to be limited to the encouragement of new green roof construction and not green roof maintenance. Moreover, maintenance may be a continuing concern for green roof owners because the maintenance cost may not be guaranteed for a long period in many cases.

Different types of green roofs may have different maintenance requirement. Usually, a set of simple annual tasks such as plant protection, checking drainage and weeding is carried out for extensive and semi-extensive green roofs which are light-weight green roof systems in which mainly herbaceous plants are used (Dunnett and Kingsbury, 2008). Intensive green roofs, which have a thicker

A. Nagase et al. / Ecological Engineering 58 (2013) 156–164

substrate and various types of plant species, require maintenance operations such as the weeding of undesirable plants, fertilisation, infilling bare spots (with cuttings, plugs or seeds), replacing eroded substrate, pruning vegetation back from building structures and cleaning plant debris from roof drains (Getter and Rowe, 2006).

However, it is important to note that the requirement of maintenance may depend on the planting design intent. For example, lawn cover extensive roofs may require high maintenance to keep tidy and aesthetic. On the contrary, naturalistic intensive green roofs may allow weed colonisation to get more diversity benefits such as like a green roof in Ministry of land, infrastructure transportation and tourism in Japan.

Weeding is one of the most important and time-consuming maintenance tasks for any type of green roof. In this paper, weeds are defined as spontaneously colonising plants without the grower’s intention. Weeds may grow aggressively, compete for nutrients and water and often shade out desirable plants (Weiler and Scholz-Barth, 2009). Weeds can be brought in with the growing medium, with the wind, by birds or through the shoes, clothing and tools of people installing or maintaining the roof (Snodgrass and McIntyre, 2010). Weeds are also brought onto roofs with the plants, and as contaminants in any seed-mixes used. However, height above the ground in green roofs may exclude heavier weed seeds and some herbivores and dispersers. Weeding is important for garden type of green roofs for healthy growth of planted plants and for aesthetic. However, extensive green roofs, particularly those of the meadow type, may allow plant colonisation to have a fully covered vegetation layer. It is required to remove vigorous weeds such as willow, birch and buddleia regularly (Grant, 2006). These vigorous weeds tend to compete with desired plants for nutrients, water, sunlight, and other resources (Allaby, 2006). Their roots can also damage roof components such as the waterproofing membrane (Luckett, 2009).

Weed control can be more efficient if urban ecosystems are understood. Although some studies have reported plant colonisation in green roofs (Dunnett et al., 2007; Köhler, 2006; Köhler and Poll, 2010), there has been little research on the efficacy of using planting design to reduce weeds on green roofs. Seed establishment may be influenced by space, light, nutrient and moisture availability. Three methods are typically used to reduce weeding in urban landscapes with the methods being (1) exclude light at ground surface using sufficient height and density of plants, self-mulching plants or mulch (Hitchmough, 1994), (2) Use high plant diversity (Cook-Patton and Bauerle, 2012; Lundholm et al., 2010; Nagase and Dunnett, 2010, 2012), (3) remove parent plants before seed is physiologically capable of germination. Many previous ecological studies have suggested that resident biodiversity is an important determinant of invasion success, arguing that high diversity increases the competitive environment of communities and makes invasion more difficult (Funk et al., 2008; Levine, 2000; Naeem et al., 2000; Zimdahl, 2004).

Some plant species, commonly planted for green roofs, could be self-sowing; they disperse seeds freely and the seeds germinate quickly. In this paper, self-sowing is defined as seed dispersion from planted species. For example Allium spp. which was planted initially self-sow well and this is one of the most dominant species on the extensive green roof installed in 1985 in Berlin (Köhler and Poll, 2010). Self-sowing planted species can be valuable or problematic, depending on the situation (Hitchmough, 2004). In meadow planting, self-sowing may be recommended to be planted to fill gap, however, if the other plants are displaced, it could present a problem. Plant establishment from vigorous self-sowing seeds can be minimised by using species that do not produce viable seed in the region and cutting before the seed is physiologically capable of germination (Hitchmough, 1995). Several studies have been

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Fig. 1. Overview of green roof.

conducted to identify the plant species that are self-sown in Britain on the ground (Clement and Foster, 1994; Hitchmough and Woudstra, 1999). However, it is necessary to study self-sowing plants on green roofs because the microclimate on a green roof tends different from that on the ground (e.g. drought, extreme temperature, high light intensity and high wind speed).

This study investigated weeds, including self-sowing planted species, in a semi-extensive green roof, case study in Rotherham, UK. Our first goal was to understand the dynamic changes in the weeds and self-sowing plants on the green roof over a year. We aimed to suggest appropriate maintenance season and frequency, to reduce maintenance costs and to estimate maintenance costs more accurately and to enable more careful selection of self-sowing plants. Our second goal was to identify which factors (plant species diversity, plant density, mulch, substrate depth) in the planting design may affect weed colonisation. This provides useful information because the planting design tends to influence weeds colonisation significantly after the installation of green roofs. If planting design influences weed composition, then appropriate planting design could reduce maintenance costs. Plant species diversity, plant density and mulching were studied to test this hypothesis.

2. Methods This experiment was conducted between February and November 2006 on the fourth storey of a commercial building; Moorgate Crofts Business Centre in Rotherham, North England (Latitude: 53.433◦ , Longitude: −1.356◦ ). An area of 415 m2 of accessible green roof was constructed on the roof (770 m2 ) in the summer of 2005. There was additional building on the third floor, and the building was surrounded by the green roof. A picture and floor plan of the green roof are shown in Figs. 1 and 2, respectively.

The green roof consisted of a vapour control barrier (HiTen Universal vapour barrier), 9 cm of insulation (Alumasc BGT polyurethane insulation), a waterproofing membrane (Derbigum), a root barrier (Preventol B2), a geotextile made of polypropylene with fleece backing for green roof drainage (SSM45), a drainage layer (Floradrain FD 40), a filter sheet (SF) and a substrate (Zinco heather and lavender substrate: ≤15% of granules that were <0.063 mm in diameter, salt content ≤2.5%, total porosity 64%, pH 7.8, dry weight 940 kg/m3 , saturated weight 1360 kg/m3 , maximum water capacity 42%, air content at maximum water capacity 22%, water permeability ≥0.064 cm/s). The substrate analysis was

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Fig. 3. Mean monthly temperature and rainfall change in 2006 in Sheffield (Met Office, 2007).

Fig. 2. Floor plan and cross section of green roof.

carried out according to FLL guideline (Alumasc Exterior Building Products, 2006; FLL, 2008). The green roof was divided into a semiextensive area, an alpine planting area and a low-edge area (Fig. 2).

In the semi-extensive area, 20 cm of the substrate was installed.

In the low edge area and the alpine planting area, 2.5 cm of gravel mulch over 10 cm of the substrate was installed. In these areas, underneath the substrate, 7.5 cm of crushed brick (Zincolit) was used to make up the levels. These green roofs were different substrate depth but keep the same surface height. The cross section of green roof is shown in Fig. 2. All the materials were obtained from Alumasc (Northamptonshire, UK). Approximately half of the plant used was native UK species and half were non-native species. The plant list was made for three areas: semi-extensive, alpine planting and low-edge. The used plants were follows. Forbs and grasses were planted in summer 2005 and bulbs were planted in autumn 2005. 43 species of forbs, 5 species of grasses and 6 species of bulbs, 54 species in total were planted. The used plants were follows.

Forbs: Allium schoenoprasum, Armeria juniperifolia, Armeria maritima ‘Splendens’, Aster amellus, Calamintha nepeta, Campanula rotundifolia, Centaurea scabiosa, Dianthus deltoides, Erodium ciliatum, Erodium manescavii, Euphorbia cyparissias ‘Fens Ruby’, Galium verum, Geranium cinereum ‘Ballerina’, Geranium endressii ‘Wargrave Pink’, Geranium lucidum, Gypsophila repens ‘Dorothy Teacher’, Helianthemum nummularium ‘Wisley Primrose’, Kniphofia ‘Border Ballet’, Lavandula angustifolia ‘Hidcote’, Leucanthemum × superbum, Limonium latifolium, Nepeta × faassenii, Origanum laevigatum ‘Herrenhausen’, Petrorhagia saxifraga, Phlox douglasii, Primula veris, Pulsatilla vulgaris, Salvia × sylvestris ‘Blauhügel’, Sedum acre ‘Golden Queen’, Sedum album ‘Coral Carpet’, Sedum ‘Herbstfreude’, Sedum hispanicum ‘Silver Carpet’, Sedum kamtschaticum var. floriferum ‘Weihenstephaner Gold’, Sedum reflexum, Sedum sexangulare, Sedum spathulifolium var. purpureum, Sedum spurium ‘Green Mantle’, Sedum telephium ‘Matrona’, Sempervivum arachnoideum, Silene uniflora, Sisyrinchium striatum, Stachys byzantina ‘Silver Carpet’ and Verbascum phoeniceum.

Grasses: Festuca amethystina, Festuca glauca, Helicotrichon sempervirens, Melica ciliate and Stipa tenuissima.

Bulbs: Allium caeruleum, Allium karataviense, Crocus tommasinianus ‘Whitewell Purple’ Muscari armeniacum, Tulipa tarda and Tulipa praestans ‘Fusilier’.

Two plant species, H. sempervirens and K. ‘Border Ballet’ were used as accents in the planting design. Two > 100 m flower height tall species were planted in 12 spots (Kniphofia) and 4 spots (Helictotrichon), respectively (Fig. 2).

The planting density was approximately 18 plants/m2 in the semi-extensive area, 22 plants/m2 in the alpine plant area and 18 plants/m2 in the low edge area. The plants were obtained from Chapel Cottage Plants (Cambridgeshire, UK), Van Dogeweerd (Lincolnshire, UK), Barbara Austin Perennials (Wiltshire, UK), Gedney Bulbs (Lincolnshire, UK) and Mike Handyside Wildflowers (Cheshire, UK). The size of pot was 9 cm. A drip irrigation system was installed on the establishment of the green roof in 2005 and this was used once a week in June and July 2006 because of low rainfall and high temperatures. Except for the above mentioned period, no irrigation was applied during this study. Maximum and minimum temperatures and precipitation at Sheffield (latitude: 53.383◦ ; longitude: −1.483◦ ) for each month in 2006 are shown in Fig. 3. Weather information for Sheffield is shown because weather information for Rotherham was not available. Sheffield is located to 15 km away from Rotherham (South West) and same altitude to Rotherham. The climate was mild; the mean maximum temperature was 25.6 ◦ C in July and the mean minimum temperature was 1.4 ◦ C in February.

The quadrats (50 × 50 cm) were set up on the green roof in January 2006. There were four combinations of plant species diversity and planting density (1) low species diversity and low planting density; (2) low species diversity and high planting density; (3) high species diversity and low planting density and (4) high species diversity and high planting density. These four combinations were chosen in areas with mulch (substrate depth of 10 cm) and without mulch (substrate depth of 20 cm). There were four replications of each direction (NE, NW, SE and SW) so that 32 quadrats were placed. When the positions of the quadrats were being determined, several positions were tried and the positions that fulfilled the above criteria were chosen. The positions of the quadrats are shown in Fig. 2.

2.1. Plant species diversity The number of plant species was between five and six in high diversity quadrats and between two and four in low density quadrats when the quadrats were set up. Overall, the quadrats

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2.3. Measurement The number of weeds and self-sowing plants in each quadrat were counted monthly between Feb and Nov 2006. If the plant species were too small to identify, they were left until they could be identified. All weeds and self-sowing plants in each quadrat were not removed during the study. The remainder of the roof was weeded six times in this period (late February and on 19 May, 11 July, 5 August, 11 September, and 12 October in 2006).

2.4. Statistical analysis

Fig. 4. Change in the mean number of plant species per quadrat over time. Error bars represent standard errors.

were set up to capture the most of variety of species which were planted on the roof at the time of installation. However, in spring bulbs and some species had not appeared at the time of setting quadrats, emerged in some low density quadrats, increasing the number of species in these quadrats. Changes in the number of plant species (planted species) per quadrat over time are shown in Fig. 4. In high diversity quadrats, the number of plant species was stable; however, in low density quadrats, the number of species increased after March because some species such as bulb species appeared in spring.

2.2. Planting density The percentage of coverage and height (from the bottom to the highest leaf apex) of each planted plant in each quadrat was measured in each month. The percentage of coverage was measured using the 50 × 50 cm quadrat, which was divided into a 5 × 5 cm grid to give a total of 100 squares. High coverage was defined as more than 50% plant cover, and low coverage defined as less than 30% plant cover in January 2006.Changes in the mean coverage of planted species per quadrat over time are shown in Fig. 5. Sometimes, plant coverage was more than 100% because the figure was the sum of plant coverage of each species and plants overlapped each other as they grew.

Fig. 5. Change in the mean plant coverage (planted species) over time. Error bars represent standard errors.

One-way analysis of variance (ANOVA) was used to detect the effects of different plant species diversity, planting density and mulch (with and without) on the number of weeds. Linear regression was carried out to identify the relationship between planting density and number of weeds. All statistical analysis was carried out using Minitab Release 14 software.

3. Results During the 9 months of this study, only nine plant weed species were found to have colonised in a total of 32 quadrats on the green roof. These species were Cardamine hirsuta, Cerastium fontanum, Epilobium montanum, Geranium molle, Picris echioides, Poa annua, Senecio jacobaea, Sonchus oleraceus and Taraxacum officinale. These species are commonly observed in gardens and the urban landscape in the UK and no invasive weeds were observed in this establishment period. Their life form, habitat, phenology and dispersal are summarised in Table 1. All these species are native to the UK. Half of the species are annuals, and the remaining species are perennials. Although their habitats are a wide range of places, the similarity is their habitats tend to be dry-open lands. Many plant species exist as rosettes until growth conditions improve, and they tend to overwinter. Most species are dispersed by wind.

Tracking changes in numbers of individual weeds show overall numbers dropped after August but individual species had their own peaks (Table 2). C. hirsuta, E. montanum and S. jacobaea were the most commonly found species on this green roof. The highest number of C. hirsuta were found in winter and spring, and the number dramatically declined in summer. E. montanum and S. jacobaea showed higher numbers in early summer. In contrast, the numbers of G. molle and P. echioides increased in autumn.

The effect of plant species diversity on the mean frequency of weeds per quadrat over time is shown in Fig. 6. Overall, larger numbers of weeds were observed in the high diversity quadrats than in the low-diversity quadrats. However, the difference was statistically significant only in February. The difference between highand low-diversity quadrats in the number of weeds became small after August when the number of weeds decreased.

The effect of plant density on the mean frequency of weeds per quadrat over time is shown in Fig. 7. Similar changes in weed colonisation were observed in high- and low-density quadrats; however, the number of colonising weeds was higher in the low-density quadrats than the high-density quadrats. The effect of density was statistically significant in April, May, August, October and November. In addition, there was a significant negative relationship between the number of colonising weeds (mean number of weeds in low-density quadrats and high density quadrats in each month) and planting density (y = 113–2.90 x, R2 = 51.2%, df = 1,18, F = 18.87, P < 0.01) (Fig. 8). This result suggests that less weeds colonised in high-density quadrats. Even in low density, it

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Table 1 Characteristics of weeds which were found on the green roof.

Life form

Habitat

Phenology

Dispersal

Cardamine hirsuta

Annual, sometimes biennials

Well-drained sandy or calcareous soils

Ballistic

Cerastium fontanum

Perennial

Epilobium montanum

Perennial

All types of unshaded dry land habitats Widely distributed on rocky, disturbed shaded ground

Geranium molle

Winter or more rarely, summer-annual

Pants exist as rosettes of pinnate-compound leaves until flowering stems develop at maturity; Flowers from January to June Winter green. Flowers from April to September.

Seed shed from June onwards Overwintering as short above- or below-ground stolons. Flowers from June to August, Set seeds from June to September Variable, depending on the season of germination.

Autumn-germinating plants overwinter as rosettes. Flowers within the period from April to September. Sets seeds from June to October.

Picris echioides

Annual or biennial

Poa annua

Short-lived perennial

Senecio jacobaea

Perennial

Sonchus oleraceus

Winter or summer annual

Taraxacum officinale

Perennial

Mainly recorded from limestone outcrops and distributed in wide habitats with bare ground such as wasteland and pasture.

Waysides, hedgebanks, field margins, rough places and cost Occurs in a great variety of disturbed situation, but most common on arable land and disturbed fertile soil Widely dispersed, particularly in rocky habitat, but restricted to habitats with at least a little bare ground.

Frequently recorded from demolition sites and widespread on disturbed places.

A common constituent of all but except aquatic habitat.

It is an autumn- or spring-germinating plant.

Spring/summer flowering but can flower at any time of the year.

Leaves, flowers and fruits may be found during all seasons. Most typically summer annual but can behave as winter annual in droughted habitats.

Seeds germinate mainly in autumn and seedlings overwinter in a leafy condition. Some seeds germinate in spring. Ripe seeds area dispersed from August until winter.

Autumn-germinating plants overwinter as rosettes, spring-germinating plants overwinter as achenes. Flowers and fruits from May onwards in autumn-germinating plants and June onwards in spring-germinating plants A small rosette of leaves overwinters. Flowers from May to October but mainly from April to June.

Most seeds set from May to June.

Animal, wind Wind

Ballistic

Wind

Animal, wind

Wind

Wind

Wind

Adapted from: Klingman et al. (1982) and Grime et al. (1988).

was possible to reduce the number of weeds significantly after the plant cover reached over 50%.

The effect of the combination of mulch (with and without) and substrate depth (10 and 20 cm) on the mean frequency of weeds per quadrat over time is shown in Fig. 9. The combination of gravel mulch and shallow substrate was effective in reducing weed invasion. Differences were greatest in months when high weed number was observed (February, March and June).

During the experiment, 29 species which showed self-seeding were found in 32 quadrats. The total number of self-seeded plants found in 32 quadrats over time is shown in Table 3. This result indicates that the seeds of many plant species used on this green roof are not dormant and they germinate easily. In particular, very high numbers of seedlings of A. schoenoprasum, C. rotundifolia, Festuca spp. and P. saxifraga occurred after flowering in autumn.

4. Discussion A limited number of weeds (nine species), was found on the extensive green roof over a period of nine months. Except for the quadrat areas, weeding was performed six times a year and some weed species were removed before they set seeds. This result suggests that six times weeding per year was efficient to keep low number of weed colonisation during the establishment for semiextensive green roofs in North England. Six times weeding per year may be relatively high maintenance for semi-extensive green roofs.

The maintenance requirement depends on how much tidiness is important for the green roof.

The low water-holding capacity of green roof means the period of drought stress for weeds is more severe and more frequent than at ground level (i.e. the window of opportunity for seed germination is shorter, and many plants will die before setting seeds). Most weeds in the quadrats did not flower; therefore, they germinated

Table 2 Total number of individual weeds over time (total of 32 quadrats).

February

March

April

May

June

July

Cardamine hirsuta Cerastium fontanum Epilobium montanum Geranium molle Picris echioides Poa annua Senecio jacobaea Sonchus oleraceus Taraxacum officinale

478 0 40 0 1 1 118 2 1

442 18 42 0 3 5 120 9 0

448 6 59 0 1 2 137 8 0

306 10 68 0 2 4 144 4 0

224 9 148 0 41 3 123 3 0

159 3 132 0 39 3 67 3 0

August 35 1 20 0 33 5 35 1 0

September 23 0 30 12 46 1 101 0 1

October 16 11 34 11 66 3 46 10 0

November 5 0 44 10 54 1 69 1 2

Total

641

639

661

538

551

406

130

214

197

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Table 3 Total number of self-sowing over time (total number of 32 quadrats).

Allium schoenoprasum Aster amellus Calamintha nepeta Campanula rotundifolia Dianthus deltoids Erodium manescavii Euphorbia cyparissias ‘Fens Ruby’ Festuca spp.

Galium verum Geranium endressii ‘Wargrave Pink’ Gypsophila repens ‘Dorothy Teacher’ Kniphofia ‘Border Ballet’ Leucanthemum × superbum Melica ciliate Nepeta × faassenii Origanum laevigatum ‘Herrenhausen’ Petrorhagia saxifrage Primula veris Pulsatilla vulgaris Sedum acre ‘Golden Queen’ Sedum album ‘Coral Carpet’ Sedum hispanicum ‘Silver Carpet’ Sedum kamtschaticum var. floriferum ‘Weihenstehaner Gold’ Sedum sexangulare Sempervivum arachnoideum Silene uniflora Stachys byzantina ‘Silver Carpet’ Stipa tenuissima Verbascum phoeniceum Total number of plants

February

March

April

May

June

July

August

September

October

November

0 3 0 1 9 25 0 397 0 0 0 0 3 0 0 0 0 0 0 10 6 3 0

0 0 0 0 24 29 0 840 0 0 0 0 0 0 0 1 12 2 0 4 11 4 0

21 0 3 0 35 55 15 622 0 0 0 0 0 0 0 0 46 1 0 21 14 11 0

36 0 0 10 80 80 55 1210 7 0 0 0 0 0 0 0 47 0 0 28 18 27 0

18 22 0 9 107 76 50 445 2 0 11 0 17 0 0 0 20 0 1 22 11 25 1

22 25 4 9 109 106 50 460 2 0 3 0 24 0 0 1 30 0 1 21 11 16 1

569 24 113 19 93 78 51 398 6 0 0 1 18 0 0 10 35 0 0 39 11 15 0

10,300 15 87 34 55 119 60 628 8 1 0 4 41 0 0 11 15,809 0 0 51 15 14 0

10,400 28 65 240 49 149 39 3019 0 0 0 101 53 1 1 87 11,082 0 0 25 11 13 0

3052 32 103 2492 56 173 53 3365 1 1 0 167 50 540 0 202 6407 0 0 24 7 14 0

0 0 0 0 117 0

4 0 1 0 356 0

12 2 0 2 145 0

23 2 1 2 216 0

3 3 10 2 57 1

3 3 5 5 72 1

4 1 1 5 80 1

3 1 0 32 70 2

7 0 0 41 80 0

7 0 5 46 527 1

10

12

15

16

22

24

22

22

20

22

gradually or their seeds may have continued to enter the roof. There was one exception; many flowers and seed heads of C. hirsuta were observed. C. hirsuta may spread its seeds by itself on the roof. Some species would have been transported initially by wind, and once in situ can produce large seed crops which may remain viable for several season (Archibold and Wagner, 2007).

It is important to note that this study is particularly relevant to green roofs at establishing stage. Weed species on this green roof is likely to change over time; some species may disappear over a year.

For example, C. hirsuta is a typical pioneer plant (Roxburgh and Wilson, 2003); they tend to colonise and spread in the beginning, however, they usually disappear in later. Therefore, this species

may not exist for a long time. Dunnett et al. (2007) studied plant colonisation on extensive green roofs in Sheffield after six years of instalment and showed 35 colonising species were identified in total. S. jacobaea, E. montanum, S. oleraceus and T. officinale were common plant species which could be found in their study and this study. These species may stay be present over a number of years.

The number of weeds on this green roof may increase over time, however, the number of plant species varied from year to year with no apparent significant tendency according to roof are over a 20year period on two extensive green roofs in Berlin (Köhler, 2006).

This study was conducted in a newly established green roof over a period of 9 months, further research is necessary to understand

Fig. 6. Effects of diversity on the number of weeds per quadrat over time. Values are compared within the same month. *P < 0.05. Error bars represent standard errors.

Fig. 7. Effects of density on the number of weeds per quadrat over time. Values are compared within the same month. *P < 0.05. Error bars represent standard errors.

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Fig. 8. The relationship between number of colonising weeds and planting density (y = 113–2.90x, R2 = 51.2%, df = 1, 18, F = 18.87, P < 0.01). Number of weeds and plant coverage are in low-density quadrats and high-density quadrats in each month.

the dynamic changes that occur in weed populations in the longer term.

This green roof studied in the present study was installed for aesthetic reasons and, except for the quadrat areas, weeds were removed regularly. However, notably, some weeds have a high conservation value, and selective weeding is recommended if biodiversity is important; the capacity of green roofs to act as habitats for colonising species confers substantial benefits for urban biodiversity (Francis and Lorimer, 2011). Weeds may contribute to create diverse plant structure and microhabitats. All the weeds found in this study were UK native plant species. The use of native plants on green roofs has attracted considerable attention in recent years (Butler et al., 2012). Native plants are frequently used in biodiverse green roofs to create habitats and provide resources for invertebrates (Brenneisen, 2006; Kadas, 2006). Some of the weeds, found in this study, are host plants for butterflies. For example, C. hirsuta is a host for Pieris napi, G. molle for Aricia agestis and P. annua for Thymelicus acteon. In addition, P. echioides and T. officinale can be an important nectar source (Dennis, 2010).

A large number of weeds was identified until July; however after August, the number decreased significantly. This result is related to

Fig. 9. Effects of substrate depth and mulch on the number of weeds per quadrat over time. Values are compared within the same month. *P < 0.05. Error bars represent standard errors.

the plant phenology of the three dominant weed species; C. hirsuta, E. montanum and S. jacobaea. Their germination rate decreased, and they began to die after August. This result suggests that weeding from spring to early summer is particularly important for this green roof because weeds must always be removed before they flower and set seeds (Weiler and Scholz-Barth, 2009). After August, it may be possible to reduce the frequency of maintenance. Notably, the number of S. jacobaea plants increased again in September because this species germinates in autumn as well as spring. If an autumngerminating plant species such as S. jacobaea is the most dominant species on a roof, weed management in autumn is essential.

Therefore, weed management should be planned after studying the phenology of the dominant weeds e.g. seed germination and reproductive biology.

In previous ecological studies, plant species diversity was found to confer resistance to invasion because more diverse assemblages utilise the available resources more completely, leaving little resource space for individuals of new species (Levine and D’Antonio, 1999). In this study, however, an opposite tendency was observed in that the larger number of weeds colonised areas of high species diversity, although there was a significant effect of species diversity only in February. Indeed, recent theoretical studies have consistently supported the predicted negative relationship between plant species diversity and invasiveness, although the results of empirical studies have been decidedly mixed (Levine and D’Antonio, 1999). Understanding of how invasiveness varies with plant species diversity is complicated by the fact that variation in plant species diversity is controlled by, and thus covaries with disturbance, resource availability, physical stress, competitors, consumers, etc., the factors that are also known to influence invasiveness (Rejmánek, 1989, Huston, 1994, Robinson et al., 1995, Wiser et al., 1998). Researchers have recently started to apply ecological methods, largely developed in non-urban locations, to metropolitan regions (Steiner, 2011). Further research is necessary to determine how increasing plant diversity could improve the short- and long-term functioning of green roofs (Cook-Patton and Bauerle, 2012).

A higher planting density (coverage of more than 50%) could reduce weed invasion significantly. This is supported by Rao (1999) who suggested that increasing planting density by using a higher seedling rate and narrower planting spacing is an important weed management technique as it enhances the competitiveness of planted species by suppressing or smothering weeds. Larger plant populations create shading, which prevents weed seed germination, emergence and establishment. However, a higher plant population is dependent on growth habit, leaf orientation, duration and other characteristics. Indeed, the results of this study suggested that weed invasion is affected not only by a high plant cover percentage, but also by factors such as plant structure. Further research is necessary to study how plant characteristics, including morphology, affect weed invasion.

As hypothesised, it was shown that gravel mulch reduced weed invasion. The area with gravel mulch was shallower than that without gravel mulch; this may also have helped to reduce weed invasion because a shallow substrate is generally drier than a deep substrate. In this green roof, white gravel mulch was particularly useful for interest in winter, when many plants disappeared. However, the effectiveness of mulches in suppressing weeds may depend on the characteristics of the mulch, including depth, texture and colour.

According to Hitchmough (2004), the most effective mulches for restricting colonisation through weed seed rain are synthetic materials such as polythene woven weedmats, followed by very coarse grades of bark, wood chips, and coarse mineral aggregates. It is necessary to consider the effectiveness and cost of mulch as well as the design of green roofs. However, the abovementioned research

A. Nagase et al. / Ecological Engineering 58 (2013) 156–164

was carried out on the ground; textile-type mulches may be unacceptable on roofs where access is difficult or winds could blow the material away. Further research is necessary to determine how different mulches affect weed invasion on green roofs.

Twenty-nine plant species out of the 54 planted species planted on the green roof in the present study were self-sowing. The seeds of many plant species used on this green roof are not dormant and they germinate easily.Other planted species may be self-sown but did not germinate because they required winter chilling/dormancy.

Not only the number of self-sowing plants but also their survivability, establishment and growth rate may affect the performance of green roofs. It was observed that self-sown E. manescavii, E. cyparissias ‘Fens Ruby’ and Festuca spp. established well and grew rapidly and these species could be invasive. Since a high planting density could reduce weed invasion significantly, desirable plants that generate seedling is another strategy to reduce bare surface area, as is selecting plant species that grow quickly. In contrast, seedlings of some species such as O. laevigatum ‘Herrenhausen’ and Sedum spp. grew slowly, and these species tend to remain small in size; therefore, they are less likely to disturb other planting.

5. Conclusion In this study, it was shown that six times weeding per year was efficient to keep low number of weed colonisation during the establishment in semi-extensive green roofs. Knowing phenology of expected weeds (especially dominant weeds) allows targeting maintenance to remove them before they set seeds. Some perennials that are commonly planted for green roofs are selfsowing. Vigorous self-sowing seeds can be minimised, however, self-sowing plants may be useful to fill gap and reduce number of weeds. It was possible to reduce weed colonisation using an appropriate planting design such as a higher planting density and shallow substrate with 2.5 cm depth of mulch. If the results described here are applied to the initial planting design and weed phenology is studied after the installation of green roofs, it should be possible to develop low maintenance green roofs, thus reduce maintenance costs. In this study, a practical green roof was used for improved understanding of the phenology of weeds and self-sowing plants.

Although the study benefitted from high replication and a commercial maintenance regime, the commercial design of the roof limited the power of the experimental design; the differences in plant diversity and density were small and the gravel was only applied to the thin substrate. Further research is necessary to study weeds under in more controlled planted green roofs to the factors that affect that weed colonisation on green roofs. This study was carried out during the establishment period, therefore, further research on long term green roof maintenance is required to understand dynamic change of green roofs over the time.

Acknowledgements We express our appreciation to Mortgage Crofts Centre in Rotherham for providing research field, to career-support programme for woman scientists in Chiba University for founding to proof our English, to Dr. Algirdas Paskevicius in Chiba University for helping drawing figures.

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