No fines please

New research looks at urban planting directly into deep mineral mulches

By James Hitchmough and Michael Livingstone

Planting into “stony substrates” became fashionable in Germany as part of the ‘Silbersommer’ pre-designed planting mixes developed by the Arbeitskreis Pflanzenverwendung (study group for plant use and planting design) from the early 2000s.

These mixes arose from a long tradition of nature-like designed “steppe” planting into dry substrates. The mixes are sold by nurseries who participated in their development, in part a response to the diminution of planting design skills in German landscape architecture. All of these mixes have been evaluated in public spaces throughout Germany for at least 5 years before their release. We were interested in knowing more about how these approaches might work in the UK climate, where the intervals between rainfall events are much shorter. We set a PhD research team (James Hitchmough as project lead, with Nigel Dunnett and Ross Cameron) and Michael Livingstone as the PhD researcher. Financial support came from Boningale Nurseries and logistical support from the Green Estate Landscape in Sheffield. In collaboration with Nigel and Michael, we designed four mixes (20 species per mix type), which we would use in the research and that Bonningale would sell as a commercial product (see https://www.boningale.co.uk/ floratopia/) as the research proceeded.

There was an evergreen, drier mix and an evergreen less dry mix, and the same split for winter deciduous species. Tolerance of wet soils was relative, bearing in mind that all species had to be able to grow in a 150-200mm depth of mineral aggregate on top of a soil base.

We wanted to simplify the system as much as possible and avoid unnecessarily complex substrate mixes (with high mixing and embodied energy costs and the risk of weed seed contamination from green waste components from the outset). Since roots would be able to access moisture in the soil beneath, we focused on two essentially “fines free” minerals; 4-10 mm diameter pea gravel and a similar sized crushed limestone that was not absolutely fines free. Water-holding capacity in the mulch itself is extremely low.

We had a number of core research questions, including; –what effect did mineral mulch type have on establishment and growth of the different species used? –what effect did mineral mulch type have on germination and establishment of wind-blown weed seeds?

One hundred and ninety two linear metres of 2m wide planting beds were set up at Manor Top in Sheffield, with a 150-200mm deep layer of mineral aggregate overlying site topsoil 1 . Plants were established primarily by planting green roof plugs specifically grown by Boningale, plus some P9s [9cm square pots]. Most plants were established in autumn 2014 with some top up planting in summer 2015. The experiment ran from 2015 to 2019. No hand weeding was undertaken during this period.

The research is still being written up and results are presented here in very general terms to avoid future conflict with publication in academic journals. As the experiment progressed, it became obvious that weed establishment was very slow and slowest on the pea gravel. Fines between large mineral particles provide a continuity of water films that facilitate weed seed germination (Hitchmough, et al., 2001) and reduce seedlings mortality until their roots reach the soil below. As the research progressed, when the opportunity presented itself, we set up case studies to test the system under varying climatic conditions across the world.

Case study 1

Temperate maritime climate

Converted tennis court near Sheffield

Michael Livingstone

Two years into my PhD research, I gave a workshop on the system for the Landscape Institute. One of the attendees, planting designer Chrissie Dale, asked me to collaborate on a project converting a disused tennis court into a gravel garden. The client was particularly keen for it to require minimal weeding, as she already had a large garden to look after. I saw this as a good opportunity to apply my research to a larger area outside of experimental plots. I decided the easiest and cheapest option for dealing with the tarmac surface of the tennis court was to break it up but leave it in situ. This was a good “urban” test, laying gravel mulch at depths of up to 400 mm (minimum 200mm) across the tarmac rubble. A highly drought-tolerant mix with many steppe species, such as Scutellaria baicalensis and Oenothera macrocarpa ssp. glauca were planted. Two years after planting, the system has proved to be almost completely weed-free with virtually no weeding. It has a very long flowering season, starting with Pulsatilla and Primula veris in spring and flowering continuously until autumn with lateflowering prairie species.

Case study 2

Temperate maritime climate

Horniman Museum, London

James Hitchmough

Wes Shaw, Head Gardener at the Horniman, contacted me in 2016 about a planting to interpret the relationship of indigenous cultures with grassland vegetation. The planting site was small (400m 2 ) but knowing how challenging it is for maintenance staff to distinguish between weeds and planted species in complex herbaceous planting, it was an ideal site to test deep gravel mulches. I designed three plant communities: a dry prairie (USA) and two south African communities. The top 150mm of the soil surface was stripped off and replaced by the same depth of pea gravel. We started planting the project on 3 November 2017 with randomised P9s at approximately 300mm centres. In 2019 the planting project won Horticulture Week’s “Best Planting Design” award and is much featured and discussed on social media. Seedling weed establishment has been minimal and maintenance likewise. Despite unusually hot summers, irrigation was not required in either 2018 or 2019. The vegetation is attractive for a very long season from March to November.

Case study 3

Continental temperate

“The New Silk Road Garden”, Beijing 2019 International Horticultural Expo

James Hitchmough

In 2017, I was asked to design one of this site’s four permanent “master” gardens (the others by SLA Copenhagen, West 8, and Hargreaves). Tom Stuart Smith and I collaborated, he focused on structures and hard surfaces, I on planting design. The scheme (1500m 2 ) is essentially a planted woodland with a dry steppe-like vegetation surrounded by drainage swales, woodland edges and understories. Mulches are a new idea in China, as low-cost unskilled labour is available for hand weeding, but this only works with planted monocultures where it is easy to distinguish between the planted and unplanted. The planting was complex involving repeating individuals of species in three layers with a woodland layer on top. I had just undertaken 2 years of funded research project for a major nursery and landscape company in Beijing, looking at how deep gravel mulches affected plant establishment and irrigation needs in this hot, but relatively high summer rain climate. With deep gravel mulching we found no irrigation whatsoever was required, whereas the same types of planting in public space in Beijing were irrigated at least once a week and often once a day.

After many delays, planting was completed in April 2019 just a few days before the opening, using P9’s at approximately 12-15 plants/ m 2 . Pre-planting, a 120mm layer of crushed rock (approximately 10-20mm diameter) mulch was laid over the underlying soil and planted into 5 . The planting was irrigated by hand during the establishment period and results were excellent, despite 25°C plus temperatures in April.

Case study 4

Near Mediterranean climate

Melbourne, Australia, The “Woody Meadow Project”

James Hitchmough

In 2013 I was asked by Melbourne City Council to develop meadow-like vegetation, but given the climatic predictions for this city, I suggested we should look instead at using post fire, resprouting native shrubs to create a multi-layered triennially coppiced woody equivalent; the “Woody Meadow”. In 2014, in collaboration with the University of Melbourne, we secured funding from the City to set up two demonstration-research sites to trial the concept. In addition to weed control benefits, many of the native shrub species are least affected by fungal root pathogens when growing in highly aerated, free draining substrates. We used volcanic scoria as a relatively fines free, locally available, low cost mulch substrate. The sites were planted using a 200mm layer of this in 2015, resulting in excellent plant establishment and performance without irrigation and minimal seedling weed invasion. A new research collaboration between Sheffield and Melbourne University and a much wider range of Australian Local Government agencies has been developed and we are hoping to evaluate deep mineral mulching throughout Australia.

Conclusion

Our experiences suggest that as long as you select appropriate stress tolerant species, planting into deep mineral mulches can work anywhere in the world and it reduces maintenance by keeping invading weed seedlings to the absolute minimum possible. This level of weed control only works when fines are absent or close to absent. There is now good research based evidence in the German literature (Cassian Schmidt personal communication) that the presence or fines or colloidal organic matter at the surface of plantings is directly correlated with increased weeding costs into perpetuity. This raises issues of sustainability in terms of the availability of suitable mineral substrates and the processes to remove fines from them, but this must be balanced against long term “invisible” energy and other resource inputs necessary to remove weeds from “fine dense” surfaces, such as green roof type substrates. Rounded particles have the advantage of being mobile and any winter weed seedling establishment can be managed by raking the surface after the cutting down of the dead herbaceous stems, on a sunny day in March. As the diameter of mineral particles increases (within reason) weed seedling establishment decreases. Providing the mineral mulches are fines free and 120mm or deeper, worm casting onto the surface does not appear to occur, hence the mulches are permanent and continue to function in the longer term.

James Hitchmough is Professor of Horticultural Ecology in the Dept. of Landscape Architecture, University of Sheffield

Michael Livingstone is a PhD student in the Dept. of Landscape Architecture, University of Sheffield, interested in the ecosystems service benefits of designed plant communities.