Plants for Heat Waves

A decade ago, the thought that we could regularly have summers with temperatures in the 30s would have seemed ridiculous, but here we are and despite best efforts, so many plants are seriously struggling to cope. As a gardener, it's time for some difficult decisions - how far can we go in helping these plants limp through difficult conditions? At some point, we have to take a deep breath and remove some of our more sensitive plants. What can we grow instead that will stand up to extreme levels of summer heat and dry soil?


When it comes to plants that might have to go, I'm noticing large leaved plants with relatively shallow roots, such as Foxglove, Primrose and Brunnera are struggling. Those large leaves are great for catching lots of sunlight - an advantage in shady areas, but a clear disadvantage in all this harsh sunlight - they're looking frazzled! The other group of plants that's struggling is those multi-stem plants, such as Rudbeckia, Lysimachia and Michaelmas Daisy or other Asters. These plants have shallow, creeping roots and are well adapted to cope in moist or even wet soil. Many have been seriously wilting following July's heat wave, in our baked, dry soil.

Sadly, these hot summers are likely to be a feature for some time at least. One possibility is to try moving any plants like those I've described above into shadier areas, especially shade that falls in that really hot and harsh middle part of the day. A handily placed tree or a bed sited where something casts shade from the mid-day sun, is going to be helpful here. Beyond that, we either need to take steps to build soil health and drought resilience, or we need to change our planting choices to reflect these new conditions. Or both!

I've written before about building more drought proof soils for vegetable gardens and many of the principles there are relevant for all gardens. Here I'm going to talk about the features of more drought proof plants and will offer up lists of good plants to seek out, along with tips for finding your own choice of future-proofed plants.

Plants that can cope with hot, dry conditions have a number of characteristics that you can look out for. It's helpful to have in mind how a plant uses water, so you can get a clear picture in your mind's eye of how these features are helping the plant handle the heat. Plants have pores all over the surface of their leaves and - basically put - these pores sweat out moisture, just like the pores in our skin. This is a crucial action in plants as it is this sweating - properly referred to as 'transpiration' - that pulls water up from the soil in a continuous column - through the roots, up the stems into the leaves and out through those pores into the air. Without transpiration, that water would stay put in the soil and the plant would stagnate and eventually die.

When it's super hot, however, plants may transpire so quickly, the soil can't supply all the moisture they need to keep filling up those columns of water running up through their stems and leaves, so the plants begin to completely empty of moisture, leaving them wilted at first and if things go on much longer, they become brown and frazzled, first at the leaf tips, then over the whole of the leaves and ultimately down the stems and into the roots too.

But thankfully, lots of plants have developed some pretty clever ways of dealing with this problem and these are the plants we could do with looking out for. Deep roots can forage for more moisture, so deep rooted plants are going to be a key feature of a drought-proof garden. But some of the cleverer tricks are all to do with cunning leaf adaptations that reduce transpiration and help the plant hold on to whatever moisture it is able to find. If you take a look around the garden at the plants that are handling really hot weather, it's very likely that they're going to be using some of the following five key techniques.

1. PALE LEAVES

Some plants have really pale leaves - grey-green or even really close to white. These reflect sunlight, reducing the amount of heat that soaks into the plant and so they keep the plant a little cooler, slowing down transpiration. Plants that do this include Russian Sage, Lavender, Cotton Lavender, Curry Plant and Artemisia

Lavender has a number of tricks for surviving the heat, the first of which is its pale, reflective leaves.


2. SMALL LEAVES

Others have developed really tiny leaves or really finely cut leaves - less leaf surface means less transpiration, so again, these plants are much better at holding on to moisture on a hot day. Plants like this include Yarrow, Thrift, Fennel and Mexican fleabane, shrubs such as Box, Heather, Common Rue, and Rosemary and trees, such as Tamarisk, Juniper, Mimosa and Yew.

Heather, with its tiny leaves, is adapted to hold on to moisture in its home territories - exposed, windy hilltops.


3. HAIRY LEAVES

Some plants have a trick for increasing the humidity of the air around their leaves. This weakens the pull of the sun's heat on that column of water running up through these plants. The sun can pull that water more strongly out of a plant if it's pulling it out into dry air, rather than trying to effectively pour moisture into air that is already moist. So these plants create a layer of moist air around them using a really simple, every-day trick - they grow hairy leaves! Leaf hairs trap moist air all around the leaves and make a significant difference to the ability of the leaves to hold on to moisture - think Cardoon plants, with their massive leaves. Although they also have the advantage of grey leaves and a deep root system, the hairs all over the surface of these leaves double up their defences, allowing these huge leaves to remain plump and moist, even in the hottest of conditions. Mine barely flinched during our 30+ degrees weather recently. Other hairy leaved plants include Lamb's Ears (Stachys), Lady's Mantle and Shrubby Mallow (Lavatera)

Cardoon to the right, is perfectly adapted to dry conditions, with deep roots, pale and hairy leaves.


4. WAXY LEAVES

Other leaf adaptations include tough waxy leaves, with the waxy surface both act as a barrier to moisture loss and as a reflective surface, to deflect the sun's heat and keep the leaves cool. Plants with this adaptation include Ceanothus, Laurel, Globe Thistle, Euphorbia/Spurge and Sea Holly.

Globe thistle, with its tough, waxy leaves, is a great choice for the dry garden


5. SUCCULENT LEAVES

Another leaf-based trick is to have succulent leaves and stems, both of which actively store moisture, to be used during hot weather. Succulent garden plants that can be grown here in the UK include House Leeks and Sedums.

Sedums are a native plant that really works hard in the garden - drought tolerant, nectar rich and with great winter stems


My lists here are far from exhaustive and there are lots more plants that are really good during dry weather. Hopefully though those five leaf-based tricks will help you to find more plants that will be able to cope during hot weather, so your garden can remain a haven of lovely, providing you with beautiful vistas and your visiting insects and other garden wildlife with plenty of goodies, whatever the weather may throw at us.

If you have your own favourite plants for dry conditions, please do share them below in the comments. If you'd like to read my post about supporting a more drought tolerant soil, have a read here. And if you'd like some help and advice in setting out more drought resistant flower beds, get in touch!

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