Climate change plants: warmer
Nick Bailey picks plants that may now thrive in Britain’s warming climate
Neither Nostradamus nor Mystic Meg has been able to predict exactly which direction our climate is headed, but change is underway. During my 26 years as a horticulturalist and garden designer, winters have, almost without exception, become progressively warmer. This, along with other environmental shifts, is cited in the recent RHS Report ‘Gardening in a Changing Climate’.
There are, of course, negative aspects to warmer winters, such as pests and diseases not being killed off, and some of our natives suffering with reduced dormancy periods; but there is also a huge positive. Plants previously unthinkable in our cool temperate climate are now very much in the game. Flora of the Mediterranean, subtropical and tropical climate zones that historically would have been holed-up winter-long under glass are now free to put down roots and establish outdoors. So, in spite of many potential negatives, climate change has the possibility of opening up a huge new palette of plants to UK gardeners and designers.
Throughout my seven years at the Chelsea Physic Garden, I pushed the boundaries of what is now possible to grow outdoors in the UK. This protracted experiment yielded some surprising results and highlighted many potential plants of the future.
Seeds of success
Westringia fruticosa was one of the first successes. I’d known it from my time working in Australia, where I’d noted its almost incessant flowering and capacity to be either clipped as a ball or left as a freeform evergreen shrub, reaching 3m x 2m. It emanates from the baking east coast, but grown in terracotta in a free-draining soil it managed to see off three nights of -4°C in its first London winter, flowering throughout with aplomb. Could it be the silver-leafed replacement for buxus?
Looking south from Australia presents a range of quirky flora from New Zealand. One such specimen is the mirror bush, Coprosma repens. It got a bit frost-burned during its first winter at the physic garden, but with its roots down it has subsequently gone from strength to strength, producing a near mature 2m x 1.5m shrub in just over five years. Its common name comes from its impossibly glossy leaves which bring sparkle to shaded plantings. The straight species is a mid-green with insignificant flowers, but for the colour-hungry there are several bright-leaved cultivars. It’s a sure-fire landscape shrub of the future.
Also hailing from Oceania is Allocasuarina decaisneana. Perhaps its closest aesthetic comparator in Tamarisk. In its native Australia it’s grown as a free-form tree up to 10m or clipped into a formal hedge. It has a coniferous quality to it but is peppered with thousands of small red tufty flowers attached into its wiry cladodes. On free-draining soils in the south east this may just be one of our future hedges.
For pure drama and an elegant architectural silhouette, the Americas provides a palm which currently sits squarely on the edge of hardiness. Butia capitata, although available to buy in the UK, is rare in cultivation. In theory, it can tolerate down to -8°C but that is only at maturity and with a good root run in free-draining soil; -5°C is probably more realistic. It forms a perfect fountain of foliage, up to 6m tall and 4m wide, with a blue/grey tone. Although slow to grow, it maintains a refined form, even through those difficult teenage years.
Introducing species from the tropics into UK cultivation may seems a step too far but both Elettaria cardamomum (cardamom) and Tetrastigma voinierianum can already scrape through our winters in very sheltered gardens so are likely to become more widely grown in the future. Cardomom is a lush foliage plant related to ginger which works well in exotic plantings and releases a delicious scent from its foliage when touched. It does not get as big here as it does in its native India but the verdant foliage lasts from June to November when it retreats underground until spring.
Tetrastigma voinierianum knows no such bounds. Its tendrilled, climbing stems and lush, toothed leaves will easily put on 2m growth a year if positioned in a warm spot, trained against a wall and can cope just below freezing for a few days. It will reach 6m plus given the opportunity.
On a smaller scale (about 1.5 x 1m), Polygala myrtifolia is a South African shrub with pea-like flowers. It has been making guest appearances at RHS Chelsea for the past few years, where it is often clipped into 80cm buns, smothered in electric-pink blooms. Although in theory it can handle -6°C this is only in maturity on a free-draining soil. It is so floriferous over such a long period that it is sure to become a garden staple.
Finally, warmer, drier winters will provide the opportunity to grow a wider range of Mediterranean plants too. Origanum dictamnus is not uncommon but has historically struggled through wet winters. It’s a low sprawling pot or rock garden plant, which grows to 30cm x 60cm, with silver felty foliage and pinky-green hops-like flowers which are sure to see it move into common cultivation soon.
These are but a few of our potential ‘future plants’, but many more are yet to be trialled. So be brave, take a punt and make the best of our changing climate by experimenting with a fresh palette of possibilities.