Gardening in the Global Greenhouse
The Impacts of Climate Change on Gardens in the UK
Case Studies
The following case studies come from the four RHS gardens.
Wising up to water shortages at Hyde Hall
Testing tender plants to the limit in Rosemoor’s frost pocket
Wisley Walled Garden sets the tone for modern gardening with a tropical twist
Making a splash with wetland plants at Harlow Carr
Wising up to water shortages at Hyde Hall
Matthew Wilson, curator, RHS Garden Hyde Hall, spares the hosepipe to produce a sea of drought-tolerant plants.
RHS Garden Hyde Hall is situated in one of the driest areas of the British Isles, with an average annual rainfall of just 600mm (24in). In 1996 only 451mm (17.7in) fell. This is a lower annual rainfall than Beirut and Jerusalem. If the rain didn’t fall so evenly throughout the year, Essex would look like a desert.
Add to this heavy, unstable Essex clay soil and the picture of a ‘challenging site’ is complete. All the work that has been done at Hyde Hall, and will be done in the future, is carried out with climate very much in mind.
The Dry Garden
Even during the devastating floods that submerged many parts of the UK in 2000, East Anglian gardeners knew that come summer, only a few days of hot, drying winds beneath clear blue skies would cause any amount of rain to evaporate, leaving plants wilting in the cracked and blanched soil.
The only way to garden successfully in these conditions is to understand the soil, the climate and the types of plant that will succeed. The Dry Garden has been designed and planted to demonstrate how plants of great diversity and interest can be grown without the need for artificial irrigation, thus preserving our precious water resources. The garden is supported by Essex & Suffolk Water, which is committed to encouraging the wise use of water through reduction of waste.
Drought-tolerant plants prefer a free-draining soil and the heavy clay at Hyde Hall needed improving with sandy grit. This helps with drainage and improves the soil structure enabling plants to absorb nutrients more readily. Drainage was further improved by creating raised mounds made from rubble-filled subsoil and covered with gritty top-soil. These could be replaced in the domestic garden by mounding up soil in the borders or creating raised beds.
The plants in the Dry Garden all originate from arid climates such as southern and central Europe, southern Africa, California, South America, Australasia and central Asia. There is a huge variety of plants that will grow in such conditions and for gardeners that have an area of ground baked by the sun through the summer, they provide year-round interest and none of the heartbreak of watching unsuitable plants burn and wilt.
To help identify subjects for a dry garden, look for the characteristics that plants have developed in adapting to these harsh conditions:
- Grey or silver leaves which reflect sunlight
- Fine hairs or a waxy covering on the leaves to trap moisture in the air
- Swollen modified stems or leaves, such as succulents, that store moisture
- Small leaves and low growth habit to prevent loss of moisture through transpiration
Almost all further increase their chances in harsh environments by developing long, penetrating root systems which both anchor them in uncompromising terrain and seek out the last drops of moisture.
Dry Garden facts and figures
260 tonnes of gabbro, an igneous glacial rock from Scotland, were transported from the Huntley near Aberdeen for the large boulders. The stone is usually crushed for road hardcore. The largest boulder weighs over 8.5 tonnes.
800 tonnes of hardcore and sub-soil (to create the mounds).
460 cubic metres of soil and sandy grit.
70 tonnes of rounded flint (for mulch), laid at a depth of 40cm (16in).
80 tonnes of crushed concrete sub-base for the paths.
45 tonnes of Cedec (crushed red granite) for path surfaces.
Over 4,500 plants representing more than 740 different species and cultivars.
It took three and a half months to build and three weeks to complete the planting. Some plants were placed and the surface mulch infilled around, others planted into the mulch. There is no geotextile layer under the mulch as its depth should be sufficient to suppress weed growth while allowing a degree of self-seeding from ornamental annuals and perennials.
Selected plant list with country of origin
| Ballota pseudodictamnus AGM Turkey | Beschornia yuccoides AGM Mexico |
| Ceratostigma plumbaginoides AGM China | Crepis incana AGM Greece |
| Diascia vigilis AGM South Africa | Echinops ritro AGM Central Asia |
| Eryngium giganteum AGM Iran | Euphorbia x martinii AGM South France |
| Gladiolus callianthus AGM East Africa | Jovellana violacea AGM Chile |
| Lavandula lanata AGM Mediterranean | Leptospermum rupestre AGM Australia |
| Lotus berthelotii AGM Canary Islands | Parahebe catarractae AGM New Zealand |
| Penstemon rupicola AGM USA | Rhodanthemum hosmariense AGM Morocco |
| Sempervivum ciliosum AGM East Europe | Senecio vivara AGM Argentina |
| Zauscheneria californica ‘Dublin’ AGM California |
Testing tender plants to the limit in Rosemoor’s frost pocket
Christopher Bailes, curator, RHS Garden Rosemoor, finds more and more tender plants are proving hardy with minimal help.
Situated in an area noted for many fine gardens growing a wide variety of tender plants, historically Rosemoor’s frost-prone, valley-bottom microclimate has constrained the range of plants which could be grown. The key factor limiting the plants we can grow appears to be spring frosts, which typically occur at a time when new growth is at its most vulnerable. Oddly, this can affect very hardy plants as well as tender subjects. The former may be able to withstand exceptionally severe conditions while dormant, but if lulled into early growth by our typically mild and moist winter conditions, will put on soft new growth in early spring. This will then be at high risk of damage from late frosts, which occur particularly when there is a northerly or north westerly airflow. During the fourteen years that the RHS has been at Rosemoor (particularly in the early years) we have experienced frosts in April and May severe enough to defoliate susceptible trees over 10m (33ft) tall, and to affect the flowering of mature oaks.
I well remember Lady Anne Berry, Rosemoor’s previous owner, pointing out to me not long after my arrival in October 1988, that I could forget any notions I had of growing many of the tender plants which are characteristic of West Country gardens, as she had tried and been beaten by the garden’s capricious microclimate. However, in common with gardeners everywhere we like to find out for ourselves what is possible and what is not. In the early 1990s we began experimental plantings of tender trees and shrubs on the south-west facing Old Kitchen Garden wall and in the sheltered west-facing beds above the Croquet Lawn. In both instances, while we did not realise it at the time, we were planting at the ideal time to take advantage of the warming trend of the last decade.
Early successes led to further experimentation, and ever-increasing numbers of once tender subjects are proving themselves to be hardy with us. It helps greatly that winters have been relatively mild for some time, and that the severity of the spring frosts has eased considerably, as this has allowed plants to establish and grow beyond the especially vulnerable young stage where they can be severely affected by a cold spell. More mature specimens of trees and shrubs can often grow away from frost damage from older wood which is more frost-hardy. Herbaceous perennials, once firmly and deeply established in the soil, can also better tolerate occasional cold spells.
Of course, we ensure that we do everything we can to assist establishment of tender plants. Correct siting is crucial, as is protection in the early stages of establishment, and in some cases delicate plants are swaddled each winter. It is interesting to compare the growth of the same subjects when left unprotected with those which we mollycoddle, as there can on occasion be scarcely any difference! An excellent example of a plant which we now consider effectively hardy is the Japanese hardy banana, Musa basjoo. When we first grew it we would religiously lift the large plants at the end of the summer, pot them into dustbins (the only container that would accommodate them), and heave them into our polytunnels to keep frost-free. Now we swathe the base of the 2m (6.5ft) stems with dry bracken fronds for insulation, leaving the top 60cm (2ft) or so sticking out, and they have survived superbly. Other previously tender groups such as certain ginger lilies (Hedychium) and cannas are surviving well outdoors with minimal or no protection. Rather than bring the latter indoors to ensure their survival, now the main reason to do so is to lengthen their season of display by ensuring we have free-growing plants in spring to steal a march upon the season.
There are numerous examples of plants which we now consider effectively hardy in current winter conditions based upon the last eight years or so of gardening here. Included among these are some which, like the hardy banana and cannas, provide that exotic touch to the garden, such as tree ferns and the Mediterranean fan palm (Chamaerops). Many southern hemisphere plants from Australia have now settled in well with us, including Lomatia (an attractive group of evergreen shrubs), grevilleas in variety, and a wider range of eucalyptus. Careful siting and milder winters have enabled us to establish the Chilean bellflower, Lapageria. At the front of Rosemoor House the 4m (13ft) stems of the tender Dahlia imperialis grow remarkably rapidly in early summer bearing huge pinnate leaves. Unfortunately the flowers tend to be produced too late and are taken by autumn frosts, but as they are insignificant by comparison with the foliage this is no great loss. Our National Collections are also benefiting from the warming trend, particularly dogwoods (Cornus) where the beautiful tender evergreen Cornus capitata has settled down well in several parts of the garden.
The list above gives just a taste of some of the plants we are establishing throughout the garden and we continue to test the previously accepted barriers of hardiness.
Wisley Walled Garden sets the tone for modern gardening with a tropical twist
Jim Gardiner, curator, RHS Garden Wisley, on sub-tropical survivors for permanent planting.
Meteorological records have been kept at RHS Garden Wisley since 1904 and show that temperatures vary between 30ºC (86ºF) and -10ºC (14ºF). The hottest day on record saw temperatures of 35.4ºC (96ºF) in August 1990. In January 1981 -15ºC (5ºF) was recorded, the coldest on record. June and July are the sunniest months, with an average of 199 hours of sunshine. An all time high of 294 hours was recorded in June 1997, with 227 hours during June 2001. The long term average rainfall is 652mm (26in) per year, though in 2000 it was 958.9mm (38in).
Walled Garden (west)
The 3m (10ft) high walls that enclose the Walled Garden (west), create a microclimate enabling tender and subtropical plants to survive year round outside. Over 30 percent of the permanent plants are tender e.g. palms (including Brahea armata), ferns (including Dicksonia antarctica) and bananas (including the half hardy Musa basjoo).
Redesigned and planted in 2001, with the support of Witan Investment plc, this garden illustrates how to mix hardy and tender plants from around the world in a modern setting. All plants are left out over winter although some of the more tender exotics, such as bananas, are carefully wrapped to shield them from the worst of the weather and excessive winter wet.
Bananas
One of the hardiest species to grow outside is Musa basjoo but other bananas also have potential, e.g. Musa sikkimensis and Musa lasiocarpa. Small plants can be brought inside and overwintered in a conservatory or frost-free glasshouse, but larger plants will need to be protected in situ. Form a cage of chicken wire around the stem and pack with dry straw. Top with a cap of polystyrene to keep rain out of the crown. Finally, wrap with fleece to keep out the wind. Unwrap plants by May.
Tree ferns
Hardiness increases with age so young tree ferns are more vulnerable to cold. To be safe, they should be overwintered in a frost-free glasshouse. Most of the plant’s roots are concentrated in the trunk which should be kept moist. Keep the growing point frost-free – fronds can be sacrificed. In mild winters plants can survive outdoors with little protection. As a minimum, pack the crown with straw and cap with polystyrene. Remove after the last frosts. In colder winters also wrap the trunk with fleece and mulch the roots.
Selected plant list
| Canna ‘Rosemond Coles’ | Cordyline australis ‘Red Robin’ |
| Cordyline australis ‘Torbay Green’ | Cordyline indivisa |
| Dicksonia antarctica | Dregea sinensis |
| Eryngium pandanifolium | Erythrina crista-galli |
| Ficus carica | Lomatia ferruginea |
| Musa basjoo | Mutisia ilicifolia |
| Phormium tenax ‘Yellow Wave’ | Solanum laciniatum |
| Tetrapanax papyrifera | Trachycarpus fortunei |
Making a splash with wetland plants at Harlow Carr
Andrew Hart, Curator, RHS Garden Harlow Carr, highlights plants that flourish in boggy ground
RHS Garden Harlow Carr was established in 1950 on mixed woodland, arable and pastureland. It has heavy clay soil, very acid and is criss-crossed by a number of ditches and streams. A steadily increasing rainfall - 736mm (29in) in 1990, 1194mm (47in) in 2000 - has added to problems of waterlogging, resulting in occasional flooding of the lower parts of the garden and the sweeping away of paths.
Autumns here are shorter than further south, spring comes later, often with snow in March and April, and frosts - severe during the winter - are possible into early June.
Such unpromising attributes were the main reason the garden was established here. It was built primarily as a trial ground to test the hardiness and suitability of plants for northern gardens. That role will continue to expand as accelerating climate change makes a wider range of plants available to northern gardeners. In addition, our long experience of cultivating plants that like 'wet feet' will prove invaluable to gardeners across the UK facing more extreme downpours and waterlogging in future.
The Streamside Garden
Extending the length of the garden along the edge of the Woodland is the Stream Garden, which is one of Harlow Carr’s main features. Possibly one of the longest streamside gardens of its kind in the country, it provides an area that remains cool and damp during summer (extensive mulching is employed to retain the moisture) and allows the cultivation of a myriad of moisture-loving plants.
The explosion of colour that marks its passage begins in spring, with the yellow spathes of skunk cabbage, Lysichiton americanus, and the more unusual white ones of L. camtschatcensis. Then through June and July the candelabra primulas, including the Harlow Carr Hybrids, create a patchwork of pinks, mauves, purples and yellows to contrast with the clear blue of Meconopsis. In late July, the lemon yellow, scented giant Himalayan cowslip, Primula florindae, takes its turn providing a contrast to a native of the Falkland Islands, Senecio smithii, with white daisy flowers and dark green leathery leaves. The red, pink and cream ostrich-feather spikes of the astilbes bring summer to a close, allowing fronds of the royal fern Osmunda regalis, sensitive fern Onoclea sensibilis and shuttlecock fern Matteuccia struthiopteris, to come into their own.
Other significant foliage plants thrive in the dappled shade of the streamside: a wide range of hostas together with the ornamental rhubarb Rheum palmatum, and Gunnera manicata, the Brazilian or, more commonly known, prickly rhubarb (although it is not actually a rhubarb) with its giant umbrella-like leaves overtopping all.
Selected plant list
| Acer palmatum cultivars | Arum italicum |
| Clethera delavayi | Dactylorhiza fuchsii |
| Dryopteris (NCCPG) | Embothrium coccineum |
| Fagus sylvatica ‘Purpurea’ | Fritillaria meleagris |
| Fuchsia magellanica cultivars | Iris spuria subsp. halophila |
| Lysichiton americanus | Metasequoia glyptostroboides |
| Oxydendrum arboreum | Polypodium (NCCPG) |
| Primula florindae | Ranunculus aconitifolius |
| Rheum (culinary rhubarb) cultivars (NCCPG) | Senecio smithii |
(NCCPG) signifies the National Council for the Conservation of Plants & Gardens National Plant Collection which is held at the garden.