Drought Tolerant Does Not Always Mean Heat Tolerant
GARDENING FOR TOMORROW
Marcus Bergin
8/20/20268 min read
Drought Tolerant Does Not Always Mean Heat Tolerant
"Drought tolerant" has become one of the most desirable phrases you can find on a plant label.
It suggests something that copes with less water, survives long stretches without rain, keeps going when more demanding plants start to give up. With hotter, drier summers and the odd hosepipe ban on the horizon, drought tolerance understandably feels like part of the answer.
Then an unusually hot summer arrives, and some of those supposedly drought-tolerant plants end up scorched, faded or thoroughly crispy anyway.
It's easy to conclude the label was wrong — sometimes a plant genuinely has been oversold, or "drought tolerant" interpreted a little generously. More often, though, the real difficulty is that drought tolerance and heat tolerance simply aren't the same thing. A plant can cope perfectly well with relatively dry soil and still be damaged by intense heat.
We tend to think of a hot, dry summer as one condition. From a plant's point of view, several pressures are happening at once — less moisture in the soil, higher air temperatures, intense sunlight, possibly a drying wind. Leaves get heated directly by the sun and surrounded by paving, gravel or walls busy absorbing and reflecting warmth. Separating those pressures matters, because pouring on more water won't necessarily fix all of them.
Drought tolerance is about how a plant manages when water's limited — small, narrow or silvery leaves, waxy surfaces, fine hairs, deep roots, or simply slowing growth to cut water loss. Heat tolerance is a different thing entirely: the ability to keep functioning when temperatures get unusually high. A plant can have all the water it needs and still not manage to keep its leaves cool enough, or replace moisture as fast as it's losing it. Which is why a plant can be perfectly well watered and still look cooked.
One of the more telling things from difficult summers has been how many gardeners checked the ground around a struggling plant and found it wasn't especially dry. Roots had plenty of moisture, but the leaves and flowers were scorched anyway. Hydrangeas, acers, heucheras and ferns have all shown damage with damp soil sitting right underneath them. The problem wasn't thirst.
Forecast temperatures don't tell the whole story either. Official readings are taken under standard shaded, ventilated conditions. A plant growing in full sun, against a south-facing wall or surrounded by paving lives in a much harsher environment than the number on the forecast suggests. Leaves are among the surfaces taking the direct hit. A gardener can step into the shade, have a drink, go indoors. A plant rooted beside a wall has no such option — it sits there through the hottest part of the afternoon while the wall and the paving carry on radiating heat around it.
That's why "full sun" needs a bit more interpreting than we usually give it. Morning sun in an open border is a different thing entirely from the same hours beside a west-facing wall. An ordinary summer's sunshine is a different thing from repeated afternoons of exceptional heat. The label can correctly say the plant prefers full sun. It has no way of knowing what full sun actually means in the exact spot we've put it.
Wind complicates things further. A warm breeze feels pleasant enough to us, but moving air speeds up how fast moisture leaves both foliage and soil. A hot, dry wind can damage leaves even with reasonably moist roots underneath — young hedges often stay healthy along most of their length and scorch precisely where wind funnels through a gap. The plant's trying to draw water up fast enough to replace what's being lost above ground, and once that balance fails, the edges of the leaves start to dry out.
Humidity plays its part too, in ways that don't always run the way you'd expect. Ferns and other plants suited to cool, moist air can be perfectly well watered at the roots and still wither in hot, dry surroundings, because their natural habitat was never defined by soil moisture alone — nearby foliage, cooler air and less exposure were always part of the deal. It might explain why a mature fern in established dry shade copes for weeks and then browns suddenly during the most intense heat. It tolerated the dry ground fine. What it couldn't tolerate was heat, low humidity and continued water loss from the fronds, all at once. At the other extreme, high humidity combined with heat brings its own problems, reducing how well a plant can cool itself and encouraging certain diseases. Either way, "drought tolerant" tells us very little about how a plant will actually respond.
Containers add another layer of difficulty. We usually worry about the compost drying out, but the roots can get seriously hot too — a dark pot sitting on paving soaks up heat, and the small volume of compost gives far less insulation than open ground. A pot can be watered daily and still be an unsuitable place for its roots, foliage baking in the sun while the root system is effectively being heated from the outside. Moving that pot into temporary shade can sometimes do more good than another can of water.
None of this means water doesn't matter. Drought and heat commonly turn up together, and a plant already short of water is far less able to protect itself from the heat on top. The first step is still to check the soil — push a finger or a trowel beneath the surface rather than judging by appearance. Dry several inches down, and the plant probably needs a proper watering. Moist underneath but scorched on the exposed side, and heat and sunlight are more likely the real cause.
The pattern of damage offers clues. Drought stress tends to show as general wilting, slowed growth and leaf loss as the plant conserves water. Heat and sun scorch usually shows worst on the exposed side — bleached patches, brown leaf margins, flowers fading and crisping unusually fast, sometimes within a single very hot afternoon on a plant that looked fine that morning. These aren't perfect categories — drought and heat interact constantly, and the symptoms overlap — but the point isn't to attach a precise label to every brown leaf. It's to stop assuming every bit of summer stress can be solved with more water. Once a leaf's scorched, no amount of watering brings that tissue back green. It can help protect what's left and support new growth, but the damaged bit is finished, and that distinction alone can save a lot of wasted water.
Temporary shade often does more good during extreme heat. Pots can be moved, grouped together, kept out of the fiercest afternoon sun. A parasol, some shade cloth, even a chair angled the right way can protect a vulnerable plant through a short spell of exceptional weather. Greenhouses may need shading and extra ventilation. Plants against walls can be screened for a while, particularly if they're newly planted and haven't built much of a root system yet.
New plants need special attention, because drought tolerance is mostly something that develops as a plant establishes. A mature lavender or salvia with roots running well out into the surrounding soil is nothing like a plant that went in a few weeks ago and is still entirely dependent on its original root ball. That one may need regular watering right through its first season. "Drought tolerant once established" is an important qualifier — it just leaves us to find out how long establishment actually takes in our own soil.
Mulch cuts moisture loss and moderates soil temperature. Denser planting shades the ground. Shrubs and small trees filter both sunlight and wind. None of that makes an individual plant more heat tolerant on its own, but together they build a garden where fewer plants are left fully exposed to the worst of it. Perhaps we need to stop treating shade as a problem to be solved and start treating it as a form of protection. That's not an argument for turning every garden into woodland — just for recognising the value of afternoon shade, dappled light and a few layers of foliage. A south-facing garden is desirable enough, but unbroken sun from morning to evening gets harder on plants and people alike.
Even gardens in genuinely hot countries lean on shade. Mediterranean plants aren't necessarily growing in the most exposed spot going — plenty grow on slopes, among other vegetation, or where rocks and neighbouring plants are protecting their roots. Some tolerate blazing sun happily. Others do noticeably better sheltered through the hottest hours. "Mediterranean plant" is no more precise a phrase than "British plant" — it covers species from all sorts of habitats with very different needs.
Which is why lists of plants that survived one difficult summer deserve some scepticism. Gaura, perovskia, salvias, agastache, sedums, roses and hardy geraniums might get reported as outstanding performers in one garden, while in another the salvias are stunted and scorched. One variety of phlox stays fresh right beside another that turns brown. A plant praised in Cornwall on free-draining soil can fail entirely on clay in Lincolnshire. All those observations are useful — but only alongside the conditions they came from. Was the plant established? What was the soil? How much rain fell locally? Was it shaded from the afternoon sun, watered, exposed to wind? Surviving a cool, wet summer in a northern garden doesn't prove the same plant will handle a south-facing wall through drought. We need local evidence more than universal winners.
Neighbouring gardens can be surprisingly useful here — plants thriving in the same soil and weather have already passed a more relevant test than anything on a general recommendation list, even if the differences between gardens, and between positions within the same garden, never fully go away. The most useful list, in the end, is probably the one each gardener builds for their own ground: what survived with almost no help, what needed watering, what scorched despite moist soil, what came right again once it was moved into shade. Worth tracking varieties as well as species. Patterns get clearer over a few summers.
One exceptional season shouldn't decide the fate of every plant on its own — it can expose real vulnerabilities, but it may also reflect conditions the garden rarely sees. A plant that's performed well for twenty years doesn't necessarily need pulling out because it struggled once. Repeated damage is a different matter. If a plant scorches every summer, needs constant intervention and spends months recovering, it's probably in the wrong spot. Moving it into afternoon shade might be enough. If there's nowhere better for it, another plant may genuinely be the better choice.
The answer isn't simply to replace everything with plants from hotter countries, either — those still have to get through winter, particularly on cold, wet clay. A drought-tolerant plant that rots before spring hasn't made the garden any more resilient. We need plants that can cope with the whole range of conditions a garden presents, not just its most dramatic week in August.
Which brings us back to the limits of the plant label. "Drought tolerant" is useful, but it's only one part of the story. We also want to know how a plant handles heat, reflected sun, drying wind, humidity and winter wet — how much care it needs while establishing, and whether it bounces back after stress. No label can describe every garden it might end up in. Some of that knowledge only ever comes from watching, and getting it wrong once or twice.
Maybe the most useful change is in the questions we ask. When a plant turns brown, don't assume straight away it needs more water. Check the soil. Look at which side's damaged, what's around it, when the symptoms actually showed up. Think about the pot, the wall, the paving, the wind, the hours of direct sun it gets.
Don't just ask whether it was thirsty. Ask whether it was simply too hot.
Happy Gardening.
Marcus
