With a collection of over 8,000 species of plants from all over the world, there are currently some which are basking in the prolonged, hot weather and others which are not so happy.
Our Head of Horticulture, Sally Petitt talks here about the impact of the prolonged dry weather on the University Botanic Garden; explains what precautions we are taking and why and provides advice about what gardeners can do during one of the driest summers since 1976.
Visitors to the Botanic Garden will notice some areas of the Garden looking dry and brown and others thriving in the heat. How do we decide what to water and what to leave and why?
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 our plant collections which are here to support research and teaching. Practically we can’t have sprinklers watering everywhere – so like most gardens in Cambridge, we’ve left most of our grassy areas to go dry as lawns are hardy and will recover and we’ve concentrated our resources on watering plants that really need water to survive.
So which are our priority areas for watering and why? Should gardeners also adopt the same approach in their own gardens?
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. We are also watering new turf around our Systematic Beds as we laid 4,000m2 of new turf in the spring and without water this would quickly die completely in its first year. We are also watering these areas because we are having a battle with chafer grubs: affected lawns require a nematode treatment which has to be applied into moist soil to remain effective
Where do we get our water from; how do we water and what time of day do you recommend people water?
We use our own water source which is a borehole and we water using a mixture of methods: sprinkler cannons; seep hoses; and by hand with a lance or watering can. We can have six cannons on overnight and they are working at capacity; we also 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.
Which plants are suffering the most and which are thriving?
One of the first to show signs of drought 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. The lawns we do irrigate are selected for good reasons being our two high impact lawns – the Main Lawn and Cory Lodge Lawn as mentioned earlier.
The plants which are thriving and may think they are back in their native environment in this weather are those in the Mediterranean Beds – 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 unfortunately they’ve gone over very quickly this year.
If visitors want to see colour in the Garden or get inspiration for what they should be planting in their own gardens for drought where do you recommend they visit?
I suggest they head to our Mediterranean Beds and our Dry Garden. 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, a wonderfully 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?
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 is a good example 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?
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.
What are your top tips for water conservation and irrigation in this weather?
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.
What other 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 we’ve decided to rope off a few areas of the Botanic Garden for safety reasons based around potential summer branch drop. This has been done where species are known to be vulnerable to summer branch drop, and which have the greatest concentrations of visitors
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.
Little is known about this phenomenon, which may 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 mean 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.
We have had no rain for over 7 weeks now. Although we have roped off areas of greatest concern, there is no guarantee that summer branch drop won’t occur in other areas of the Garden.
Which areas in the Garden should visitors expect to see roped off for this reason?
We’ve roped off some of the large trees along the Main Walk and the Middle Walk, and will also rope off some of the Old Pinetum, as understandably visitors may choose to picnic or seek shade under the trees. Activities underneath trees such as this are not recommended during or just after a drought – not just here in the Botanic Garden but in our own or other gardens at home or elsewhere, or in woodland. In other areas we have moved benches from beneath tree canopies, and ask that visitors don’t move any benches, particularly at this time.
This is an exceptionally dry year and we hope our visitors appreciate the constraints we’re working under. It’s about getting the balance right and safety has to be our priority as well as the health of our plant collections.