Introduction Climate and the Environment A special, arid climate prevails in Arriyadh and its surrounding region. In the summer months from June to September, the thermometer rises to an average of 43 °C. In the winter, from December to February, the temperature can fall up to 10 °C below zero. In addition, precipitation is minimal: the average annual rainfall is 99mm and there about 47 cloudy days on average, with only occasional rain. During the summer there may be rare thunderstorms with heavy downfalls. Such extreme climatic conditions place equally extreme demands on those plants, which manage to grow in Arriyadh. _e Reference Manual is aimed at the use of plants, which survive these harsh, climatic conditions as well as possible.
Climatic improvement An improvement of the micro-climate in arid regions should be a major goal in all landscape projects. _e sun and heat in summer can be merciless. Dryness, too, leads to all sorts of health problems. Experience has shown that what arid regions need more than anything else to improve the climate is trees. And, of course, the aesthetic e_ect of tree planting cannot be emphasized enough. Trees are green and soothe the eye! Trees provide enclosure, give directional guidance, reduce glare, _lter dust and stabilize the soil. Trees not only provide shade upon hard surfaces, reducing re_ection and making walking and sitting more comfortable, evapo-transpiration via their leaves also increases air humidity, so that when light breezes pass under the canopies of trees, there is a cooling e_ect, too.
Urban climate of Riyadh the urban climate of Arriyadh varies according to location and the prevailing micro-climate. Generally the city has a desert climate, but within the built-up area, there are many di_erent situations: • Wadi Hanifah – this normally dry river course, to the west of the city, temporarily contains large amounts of water a_er rainfall, but nowadays there is almost a constant _ow of water, making the air more humid than that of the surrounding urban area. Because of evaporation the valley is therefore a source of cooler air, which is bene_cial to nearby neighbourhoods. • Inner city of Arriyadh – extreme heat, dust, solarisation, desiccation, re_ection of sun and heat from hard surfaces are characteristic of the public open spaces of the downtown area. In addition frequently visited or heavily tra_cked spaces such as Kindi Plaza in the Diplomatic Quarter or roadside planting on King Fahd Road are under high pressure of use. • Public Parks with their own micro-climates – like Salam Park and the parks of the King Abdulaziz Historical Centre in Murabba – are systems in themselves, with large areas of grass lawn, water features and therefore su_cient evaporation for the plants to grow well in this more humid environment. Dense tree planting and irrigation produce a micro-climate, which provides protection and optimal conditions for plant growth. • Courtyards between buildings are protected locations, which offer a variety of situations for planting. Generally a wider range of habitats is possible for plants and there is a good supply of irrigation and higher air humidity. _ere are also sunny and shady locations within a small space. • Private farms in the suburbs or on the edge of the built-up area are more exposed to the harsh desert conditions. _ere are extreme
temperatures in summer and winter. But on the other hand good conditions for plant growth can be made possible with well-controlled irrigation, lawn areas and water features. _ese items provide another special environment for plants. • On the periphery of the city, open country and desert areas such as the _umamah Nature Park or other protected areas are o_en fenced and protected against grazing, overuse by people, uncontrolled tra_c or land use. _ese conditions o_en initialize a process of rehabilitation of natural plant cover. _is process of rehabilitation can o_en be enhanced or improved with the sowing of seed mixtures or plantings.
Hardiness In recent years there have been heavy frosts in Arriyadh during the winter, occasionally leading to the death of certain plants or at least to frost damage of leaves and _owers. Until now any discussion in Arriyadh about temperatures has usually referred to extremes in the plus range. However, new planting carried out on the edge of the city, where the prevailing winter climate sees temperatures in the minus range, must also take into consideration the fact that until a suitable micro-climate has been established on peripheral open land, some plants may be unsuitable for planting due to the cold and the damage it may cause to them. Tolerance to minus temperatures and frost is known as ‘hardiness’ and while this Manual has used degrees centigrade as a measure of hardiness, for all practical reasons, hardiness in Arriyadh may be classi_ed in terms of tolerance, as follows: Low tolerance: hardy from 0°C to –2°C Moderate tolerance: hardy from –2°C to –5°C High tolerance: hardy below –5°C
Special conditions such as desiccation, drought and salt tolerance Desiccation Desiccation is a word used to describe the drying out of a living organism. In the case of plants it refers especially to their leaves, which are exposed to heat, extreme sunlight and drought in the summer months. Under these circumstances leaves can dry out as a result of the intense sun. _ey become brown. In particular, public open spaces with a lot of hard paving cause desiccation due to heat re_ection.
Drought Drought means the lack of water, either by the absence of rainfall or irrigation. _e root system dries out and the whole plant does not receive enough water. Drought resistance is the ability of a plant to survive situations of temporary droughts.
Salt tolerance Measured in parts per million of Total Dissolved Solids: Low: less than 1000 ppm Moderate: less than 2000 ppm High: less than 3000 ppm Very high: more than 3000 ppm
The use of native plants Adaptation of plants to aridity Plants have devised various methods to withstand desert conditions: for example, by evading heat and disappearing until it becomes cooler or by enduring heat by using water more e_ciently.
_ey have also developed special water-gathering functions in roots or thorns: some plants are riparian and dependent on the collection of sporadic rainfall in wadis or rowdahs. _e use of such drought tolerant species and particularly native plants is a very viable alternative to planting non-indigenous, introduced species, which can be very costly in terms of both irrigation (even recycled water) and maintenance. Native species are not only fully adapted to the climate, they are relatively disease and pest resistant and not di_cult to maintain. Native species bring back nature into the city and make people more aware of the passing seasons. _ey also increase biodiversity. In the Arriyadh Region ephemeral, natural vegetation appears seasonally, a_er rainfall has fallen and in_ltrated into the ground. _is usually happens in wadis or depressed areas, called “Rowdah”, where relatively good soil and groundwater conditions prevail. _is aspect of the ‘desert blooming’ in spring should be exploited to bring back nature into the city and is another reason why the use of native plants for landscaping in the short and long terms is an important consideration.
The use and cultivation of native species _e natural beauty of desert vegetation is inspiring in the creation of man-made landscapes. When such landscapes are planted with native species, they will provide a setting of great beauty. Just one native tree can become the accent for a whole garden and a desert– look on the edge of a garden is the ideal transition to the open desert beyond. In many projects it has also proved feasible and useful to channel storm water run-o_ from hard surfaces and desert surfaces to suitably large retention areas, wadis, swales or even planted areas, where the water will bene_t the native species. Native species look their best when planted with gravel mulch beneath them and with rock placement. Gravel mulches make an attractive groundcover in desert landscapes: in Arriyadh and its region they should be of the locally occurring limestone. Native plants may not always be available in nurseries. But nurserymen in many arid regions have now learned to cater for their growing demand and some nurseries are o_en specialised in native plants and their cultivation. Most native plants will also survive with little attention, once established, but they require proper planting and early care to succeed. In their natural surroundings, native plants grow on a wide range of sites, from sand to relatively fertile rowdahs, to rocky slopes. Good soil drainage is the one common characteristic of these locations, and it is an important requirement for native plants in man-made landscapes. Once established, native trees and shrubs can o_en survive and grow without additional irrigation, but two or more deep irrigations during the heat of the summer will greatly improve their growth rate, foliage colour and _owering e_ects.
Water efficient landscapes To continue to enjoy the new, urban landscapes of the Arabian Peninsula requires a considerable e_ort in terms of costs, water and maintenance. _ey must become ‘water-e_cient’, sustainable landscapes. Water conservation will be the basic concept behind all new developments. the inclusion of native species and a description of their use in the Reference Manual are intended to emphasize that native plants have great potential in landscape design in terms of water e_cient landscapes, especially for street planting, green belts and large parks in Arriyadh.