In July 2019, CUBG recorded the highest ever temperature in the UK.
As the extreme weather warning for England and Wales is extended by the Met Office until Tuesday and we are warned that parts of the UK could hit 40c for the first time next week, CUBG Director, Beverley Glover (BG) and Head of Horticulture, Sally Petitt (SP) talk about the impact of such extreme weather on our world class plant collection; how we manage and mitigate for such temperature highs and provide advice on drought tolerant planting and watering sustainably during hot weather.
How do you feel about CUBG holding the highest temperature recorded in the UK?
BG: We can’t help but feel dismay at the high temperature recorded in 2019 and the predicted high temperatures for the weekend and next week and the very real implication that our local climate is getting hotter. This has inevitable consequences for people, plants and animals around us. This intense heat, and the summer storms we’ve been experiencing over recent years, highlights how dynamic the climate is.
CUBG records the weather for the Met Office – how is this data used?
BG: We’ve been recording the weather at our weather station here in the Garden since 1904. This long history of data is used by the Met Office and was verified by them in defining the scale of the 2019 heatwave, when the highest ever temperature in the UK – 38.7 degrees Celsius – was recorded here 25 July 2019.
This data set is also used by researchers analysing climate change. However, recording these high UK temperatures serves as a serious reminder that we all need to be taking climate change and its impacts seriously.
In what ways do these extreme high temperatures affect the Garden’s plant collection and what are your concerns around this?
BG: We are concerned about the potential impact of hotter, drier weather on our living collection, which we grow for teaching and to support scientists and their research worldwide. It’s this research which is looking to solve some of the world’s greatest challenges such as climate change and the supply of food and medicines, so it’s vital our collection is well maintained and looked after.
Our collection consists of over 8,000 species of plants from all over the world, from the Arctic to the Himalayas to the Tropics. Some are basking in this hot weather and others are not so happy. Our diverse collection also includes many rare and endangered species. We rely on the expertise of our horticultural staff to maintain this valuable collection which is incredibly challenging, but we have a careful system of planting to maximise drought tolerance and are thinking hard about what species of plants will be most likely to thrive in this area in the decades ahead. This may mean a change of what we are able to successfully hold in our collections in the future, and this in turn will have an impact on the research which is able to be done using it.
We have plants whose natural habitats range from the alpine areas to the tropics. If high temperatures persist, how will this affect what we are able to hold in our future collections?
BG: Climate change and associated extreme hot temperatures are important considerations for our current and future collections. At present, our goal is to steward all the plants in our collection and to ensure plants of high conservation value are practically prioritized. Moving forward, we seek to make informed decisions on plant acquisitions. This means understanding how our climate is projected to change, observing how plants in our current collection are responding to extreme weather events, and building resiliency into our collection through acquiring plants that are best suited to our climate now and moving forward. It also means collaboration with and input from other botanic gardens, the scientific community, the local community, and our horticulture and curation staff.
How likely are we to see a changing landscape here at the Botanic Garden as a result of what we’re able to grow and maintain if this climate trend continues?
Due to the nature of a living collection, a changing landscape at the Botanic Garden is inevitable. Through accurate record keeping and adaptive horticultural approaches, we can effectively curate change that safeguards the world’s plant diversity and drives the pursuit of excellence in all areas of research and teaching where access to plant diversity is essential.
Visitors to the Botanic Garden will notice some areas of the Garden looking dry and brown, while other areas are thriving in the heat. How do you decide what to water and what to leave and why? Which areas are thriving and which are suffering?
SP: With 40 acres of Garden to look after and manage, we have to be sensible in our approach to what we water and are trying to be as water conscious as we can. We’re aware our visitors have come to enjoy the Garden and they want to see something that looks nice too, so it’s about striking a sensible balance with keeping areas looking nice and looking after the needs of our plant collection.
One of the casualties of the hot weather is our grass. Visitors will notice much of it is a desiccated, buff yellow colour, like most garden lawns. These will quickly spring back to life once we have sufficient rain. Practically we can’t have sprinklers watering everywhere so we concentrate our resources on watering plants that are valuable to the collection and really need water to survive.
The plants which are thriving and may think they are back in their native environment in this weather are those in the Mediterranean Beds and Dry Garden – they are loving the dry heat. The lavender which surrounds the Rose Garden is plentiful, purple and gorgeous – on the other hand, the roses like lots of moisture and heavier soil than we have here at the Botanic Garden anyway and as a result have gone over very quickly this year.
How else do the horticultural team negate the effects of the heat on the plant collection?
SP: Our Glasshouse collection consists of many rare and endangered species and it can reach up to 50 degrees C on the balcony in the Tropics House. Our Glasshouse team open all the vents in the Glasshouses and keep the collection well-watered in the morning before opening to the public. One of the most important jobs they do is to ‘damp down’ regularly – wetting the floors of the Glasshouse Range to bring the humidity up and the temperature down. If the temperatures get too hot, we sometimes have to close them for the health and safety of our visitors and staff.
If visitors want to see colour here at CUBG or get inspiration for drought tolerant plants they could plant in their own gardens, which areas of the Garden do you recommend they visit?
SP: Our Mediterranean Beds, Dry Perennial Meadow and our Dry Garden are good examples of drought tolerant planting. The Dry Garden is essentially designed for the weather which is causing so many of our garden plants to struggle. It was developed to demonstrate low water or no water gardening – it hasn’t had any supplementary irrigation since it was planted 20 years ago. Visitors will find there all kinds of foliage, textures and colours. For example, the vivid blue sea holly (Eryngium bourgatii ‘Picos Blue’); dried-out pom poms of allium seedheads; ornamental oregano (Origanum laevigatum ‘Herrenhausen’); Potentilla fruticosa ‘Snowflake’, a native British hardy and very tough; an Angelica tree –Aralia elata which is a happy woody species; as well as herbaceous plants, succulents, trees, shrubs and bulbs all chosen to withstand the conditions we are having now.
What can gardeners do to create this type of dry garden planting?
SP: To plan, think about what they want to grow, see what’s available and what’s suitable. It’s about “right plant, right place” and the Dry Garden and Mediterranean Garden are both good examples of how this applies. Plants with grey leaves, hairy leaves, slender leaves, leathery or succulent leaves, and bulbs have evolved to withstand extreme heat and long periods of little rain.
At the Botanic Garden we’re on very well-drained soil – so it’s not moisture retentive and so when we plant we mulch. In one bed we have a big layer of gravel to retain moisture.
Which other areas of the Garden offer colour and might interest our visitors?
SP: There are other herbaceous plants that we do water within reason because we are open to the public and we are aware that visitors understandably expect to see some colour.
The Herbaceous Beds have a wonderful succession of colour over the summer; so do the Bee Borders; the Scented Garden is also a highlight and the Stream Garden also has some great colour. It’s also about texture and shade. There are some lovely plants too in the Woodland Garden which provide both.
Which are our priority areas for watering and why? Should gardeners also adopt the same approach in their own gardens?
SP: We are focusing on newly planted areas which were planted up in the spring as they need water to establish. We are also watering herbaceous plants which are likely to flag and lose strength in dry periods, resulting in poor flowering or dieback. Most of these can be found in our Bee Borders and Herbaceous Beds; woody plants under 3 years old as we want them to endure in the future and annuals as we need them to produce seed which we will collect for sowing next year.
We also focus on the Woodland Garden and Winter Garden. The plants growing here are mainly woody and having been pruned in spring to generate new coloured growth for next winter also need plentiful water to ensure there is colour for our visitors in the winter.
We also put a lot of effort into our main lawn areas because we need to ensure that they stay strong and healthy to withstand the impact of our events. The Main Lawn in front of the Glasshouses is irrigated and also the Cory Lodge Lawn.
What are your tips for water conservation and irrigation in this weather?
SP: Timing is important. Trying to water in the morning or evening is better as plants have longer to sit without the sun on them. Water always to the base of the plant rather than use a sprinkler. Avoid watering areas that don’t need it: for example, lawns will recover, as they are extremely hardy. Water newly-planted plants and pots. You can re-use bath water, dishwater or vegetable cleaning water. Think about using a seep-hose where possible as water evaporates in the air from sprinklers. You can also apply good horticultural practice to help the garden retain moisture e.g. add a layer of mulch – either gravel or compost; plant during autumn to help plants establish well at a time of year when there’s plentiful rain; and harvesting rain from roofs in water butts.
Where do we get our water from; how do we water and what time of day do you recommend people water?
SP: We harvest rainwater and use our own water source which is a borehole. We water using a mixture of methods: sprinkler cannons; seep hoses; and by hand with a lance or watering can. We try to water for two hours in the morning before the public come in, by sprinkler and by hand. Sometimes we have to water certain areas during the day simply because we have a duty to look after such a large area. We do appreciate that our supportive visitors realise that we have a lot of challenges right now.
What other garden issues should people be aware of during prolonged, hot weather?
The other potential issue we as a garden are aware of is a condition known as summer branch drop. This is where branches from trees can fall, without warning, so they are potentially hazardous.
We’re really proud that the Garden has one of the finest collections of trees in the region, including champion and red-listed trees, and some old and large trees which date back to 1846 when the Botanic Garden was created. They are and look amazing but sometimes we have to rope off a few areas for safety reasons.
There’s nothing we can do to prevent summer branch drop. We have annual specialist tree maintenance checks so we know all our trees are in excellent condition. However, the problem can arise during prolonged dry spells – not just here but in any garden or woodland.
When and why might this occur?
Relatively little is known about summer branch drop – when it happens and why. However, what is known is that particular species (cedars, pines, oaks, beeches, chestnuts and poplars) are more prone to this problem as well as old and ageing trees. It may also occur in a tree which already has a weakness or fault. It is more likely to happen in the trees listed above, but which show no signs of weakness or fault. It is believed to happen when long periods of drought cause stress to the tree, and one of the mechanisms the tree will adopt to survive is to shed branches. Here in the Garden we have had several instances of summer branch drop over the last 30 years, which have mainly affected mature trees along the Main Walk.
The concern doesn’t diminish once we have rain. In heavy rains the additional water can add a ‘rain load’ i.e. added weight to leaves and branches which can be detrimental. It also means that trees which have been desperately trying to take in as much water as possible during drought are still in this mode and will continue to take water in rapidly, which adds further stress to the internal mechanisms of the tree, and can result in branch drop. Instances of summer branch drop can occur 6 – 8 hours after heavy rain.
Supporting leading scientific research and welcoming 300,000 visitors a year, Cambridge University Botanic Garden is one of the largest University-owned botanic gardens in the world.
The Garden’s living plant collection of over 8,000 species is spread across 40 acres of landscaped gardens. The collection, which includes iconic and endangered trees and plants, supports University research towards meeting many of the world’s greatest future challenges (such as food security, climate change and medicine).
The Garden also inspires schools, the local community and visitors from around the world about the importance of plants and plant science, horticulture and the joy of gardening.