Advice from Cambridge University Botanic Garden on what will survive future heatwaves

Gardeners looking at their brown flower beds and dusty shrubs have been offered advice from Cambridge University Botanic Garden about what to plant in their borders that will survive future heatwaves.

Sally Pettit, head of horticulture at the garden, has been giving tips on which plants thrive in dry conditions, inspired by the Mediterranean beds at the site which don’t need supplementary watering once established.

With the heatwave breaking, the hopes are rising for the return of green gardens – but some will need completely overhauling after the ravages of this summer.

Sally said: “Here at the Botanic Garden, we are trying as much as possible to water overnight to minimise the amount of water lost through evaporation, but although we normally try to adopt best watering practices, we have recently resorted to watering during the day, using water from our borehole, to ensure that we can give our collection the best possible chance of surviving. Of course, it is virtually impossible to provide all our 2,000 trees with adequate water, but we are watering newly planted trees individually to encourage their establishment. When watering, we try to ensure that plants and plantings have a good dowsing, rather than applying little and often, which is far less effective.

“Our collection contains 8,000 species from around the world, ranging from high altitude alpines to tropical plants, so it is a constant challenge to ensure that we can grow and maintain them in our garden and glasshouses. Due to our local conditions of low rainfall, well-drained soils and mild winters, we are able to grow Mediterranean species such as lavender, cistus and phlomis well, and these are enjoying the current hot, dry weather in our Mediterranean beds and in our dry garden.

“Other species, such as our handkerchief tree (Davidia involucrata) prefer moist conditions and these have struggled in recent weeks, defoliating much earlier than normal in an attempt to preserve themselves. Others, including magnolia sprengeri ‘Diva’ have had their foliage scorched in the extreme heat. We are very conscious that plants from wetter regions, eg Asian woodlands will be at far greater risk than Mediterranean plants. Some plants will defoliate or die back prematurely, but many are more resilient than we tend to think and will bounce back next spring providing rainfall returns to normal levels over autumn, winter and spring.”

As climate change models project less summer rainfall and higher summer temperatures in the coming decades, water will become even more precious. The Botanic Garden regularly reviews its watering and planting strategy, ensuring they are able to grow plants needed for conservation, research and display, while using water as efficiently as possible. All water for the outdoor plants comes from the garden’s borehole and mains water is limited to the glasshouses.

Sally advises that the types of plants best suited to dry conditions include:

• Succulents: These fleshy plants store water so they can survive long droughts. Some have swollen stems or leaves which are small or missing entirely, while others have thick, juicy leaves. Many survive for several years without rain. When fully hydrated a cactus can consist of up to 94 per cent water.
• Plants with scented leaves: Many plants that live in dry places have strongly-scented leaves to discourage grazing. We use many of these plants, such as thyme and rosemary, to flavour food.
• Plants with thin leaves: All plants lose water through their leaves. Lots of water can escape from the large, thin, floppy leaves of ‘normal’ plants. In most drought-tolerant plants, the leaves are modified to minimise water loss.