Plants from Mediterranean climates are well suited to hot, dry summers, which will become the norm in north temperate zones in the coming decades. Consequently, there is worldwide interest in their characteristics for effective use in green spaces.
British landscape designers innovatively illustrated their potential at the RHS Chelsea Flower Show this year and research examining common landscaping problems and solutions was presented at the International Horticulture Congress in Lisbon during the summer.
Humankind is now predominantly an urban dweller and the green spaces in cities and alongside roadways are precious assets utilised for rest, relaxation and sport. But land managers increasingly face inadequate budgets and escalating demands. While no landscape survives without some maintenance, competent site preparation and plant selection can reduce costs. Low-input landscapes that use plants with Mediterranean traits reduce irrigation and weeding problems by using compost mulches.
Vertical gardens maximise the benefits of landscaping in crowded urban areas. According to Dr D Brohm of the University of Applied Sciences in Dresden, Germany, these gardens offer aesthetically pleasing backgrounds and reduce temperatures inside homes and offices. Plans for vertical green space must be included in the earliest stages of building design, but "greening" buildings uses minimal expensive ground space. Hydroponically irrigated green facades controlled by computerised fertigation is the most efficient system. The solar heat is absorbed by the plants, which decreases the energy flowing into building surfaces. On hot summer days, more than 50 per cent of the sun's heat may be deflected by this technique.
The areas used for individual private urban green spaces are getting smaller as house prices escalate, but the number of plots is increasing. The requirements of private gardeners were examined by Dr Galopin from Agrocampus-Ouest in Angers, France. Small living areas, such as verandas, patios, decking and balconies, must offer permanent aesthetic appeal using round or upright easily managed plants with long and abundant flowering periods. These gardeners want vigour, shapeliness, ample flowering and minimal effort.
In China, the property boom means that urban private gardeners are encountering soils that have been badly degraded during house building, says XH Yang from the College of Horticulture & Landscape Architecture in Chongqing, China. Poor plant growth and death because of soil compaction are familiar problems for European landscapers. The Chinese advocate using compost-based fertilizers which enhance soil organic matter, improve plant growth and encourage earthworm activity.
Mediterranean-style climates are found in the north-west USA, South Africa, Australia, and in southern Europe. Western Oregon has a temperate climate with warm and dry summers and cool, wet winters. Cold winter weather and summer drought conditions limit landscape content. But hardy ground-cover plants that thrive in low maintenance green space, as described by Dr NC Bell of Oregon State University, could include Ceanothus gloriosus 'Emily Brown', C. cuneatus var. rigidus 'Snowball', rock roses such as Cistus x ardianus, C. x obtusifolius and Halimium lasianthum 'Sandling'.
Dr CI Cervelli from Sanremo in Italy is pioneering the development of new ornamental forms of native Mediterranean species. Clones of the common myrtle (Myrtus communis) have been selected for suitability as container plants displaying an attractive range of foliage characters, fruit colours and compactness. The strawberry tree (Arbutus unedo) 'Compacta' is fitted for containerisation with useful flower and fruit characters and A. x andrachnoides is similarly valuable.
Species of the composite herbaceous plant Helichrysum (H. stoechas, H. hyblaeum, H. scandens, H. italicum subsp. microphyllum, H. errerae) and a clone of Smilax aspera with variegated foliage are also seen as suitable for containerisation. Sea lavender (Limonium serotinum) is a good bedding plant. These grow well in Mediterranean climates with limited maintenance.
Wild flower meadows that use indigenous plants pioneered by Professor James Hitchmough (Sheffield University) are particularly useful for Mediterranean conditions, according to CI Ponte-e-Sousa from Universidade de evora, Portugal. They are selecting ornamental plants with low water needs from the natural grasslands around evora and Portalegre in the Alentejo region. Local forms of St John's wort (Hypericum perforatum), scabious (Scabiosa atropurpurea) and Tolpis barbata are especially effective. G Yike from the College of Ornamental Horticulture & Landscape Architecture in Beijing has found that lack of water is also a major problem in China, resulting in increasing demands for drought-resistant wild flower meadows.
Beware invasive aliens
There is currently worldwide alarm about the threats posed by invasive alien plants. These grow uncontrolled because natural predators are absent. Ways of characterising the invasive potential of ornamental plants have been discussed by SB Wilson from the University of Florida. There are substantial economic and ecological costs associated with managing and removing invasive plants.
Florida ranks second amongst US states in the level of devastation caused by exotic aliens to its natural ecosystems. New threats to Florida's wild flora are posed by Ruellia tweediana, 'privet' (Ligustrum sp.), Pennisetum setaceum, Stachytarpheta spp., Nandina domestica and Lantana camara, which are highly invasive elsewhere.
The 'burning bush' (Euonymus alatus) is a scourge in many parts of America, says Dr C Finneseth from the University of Kentucky. This is a popular ornamental shrub that is easy to grow and makes a profitable nursery crop, but increased use by gardeners has encouraged its spread across the country. The cultivar E. alatus 'Rudy Haag' is nearly seedless and that might now be a significant marketing advantage for nurserymen.
Selecting vigorous American native woodland plants is one means of countering invasive aliens, as suggested by Dr D Zhang, University of Maine. Successful selections have come from chokeberry (Aronia spp.), lowbush blueberry (Vaccinium angustifolium 'Burgundy'), bunchberry (Cornus canadensis), crowberry (Empetrum nigrum), bearberry (Arctostaphylos uva-ursi), Comptonia peregrina, black alder (Ilex verticillata) and sweet gale (Myrica gale).
One of Britain's most pernicious invasive aliens is the rhizomatous perennial Japanese knotweed (Fallopia japonica), says Dr A K Scott of Writtle College. The plant causes substantial damage to roads and buildings. Control is difficult and very expensive as it is resistant to many herbicides, but autumn is considered to be the optimum time for attacking F. japonica in situ because this coincides with maximum translocation of phloem fluids. Herbicide efficacy is temperature-dependent but under warm conditions the systemic herbicide picloram produced the most reliable control, according to Mark Chapman of Thurock Countryside Management. His company has cleared Japanese knotweed from the Olympic 2012 site.
The 'tree of heaven' (Ailanthus altissima) is an invasive alien in south-east France. Originating from eastern Asia, the tree of heaven is now widespread from temperate to tropical climates across Europe and America. It rapidly colonises rural, urban and forested land and threatens its environment and biodiversity, according to the European & Mediterranean Plant Protection Organization. In France, it is mainly found in the Mediterranean basin and in Corsica. It establishes readily on disturbed sites, such as railway embankments and along roads, and colonises natural open spaces like sandy seasides and riverbanks.
The highly viable winged seeds are efficiently dispersed by wind, water or birds. Its fast growth quickly beats native species for sunlight and space and the roots produce chemicals that inhibit surrounding plants' growth. Herbicides are effective and killing the aerial parts is relatively easy but subsequent sprouting from the stump and root suckering requires follow-up treatments.
The invasive weed guayule (Parthenium hysterophorus) is causing major problems in Ethiopia. It is widespread across the country and has adapted itself to survive on roadsides in towns, villages, gardens, waterways, grasslands and fields during and after cropping. This alien has reached Australia and America, indicating the ease with which it adapts to new conditions. Its presence reduces the diversity of local natural floras because few species can compete with it so they die out.
Southern Britain is expected to gain a climate resembling that currently found in Portugal and parts of Spain over the next 20 years. Plants from Mediterranean zones will thrive and become garden favourites in these conditions. They will displace many of our traditional lush and succulent herbaceous types, which are simply too resource-expensive and too time-consuming because of their water needs.
Gardens with access to plentiful water in parts of Wales, the Lake District and Scotland may become the custodians of traditional English herbaceous gardening. But where new plants or clones of existing exotics are introduced, caution is needed. Potentially useful Mediterranean plants could by virtue of rapid sexual and asexual propagation and an absence of natural predators endanger our native natural plants.