Hothouse flowers: Keep your plants cool and stress-free during heatwaves

As temperatures heat up once more, some plants struggle to survive without special attention

As humans we have various efficient ways of coping with heat and intense sunshine. We sweat (the body’s way to quickly cool itself); stay out of direct sun; drink plenty of liquids; wear light, loose-fitting protective clothing, and hats and sunglasses; shield our skin with sun-cream; use air conditioning; take cooling showers; and go for a refreshing swim if and when we can.

But spare a thought for the plants in our gardens, which have a tougher time of it.

Although we’ve devised many clever ways to overcome the potential discomfort of a summer heatwave, those same heatwaves can cause what horticulturists sometimes refer to as “plant stress”. The result is disruption to essential plant processes such as photosynthesis and problems with germination, pollination, floriferousness, crop productivity and a plant’s ability to set seed as well as its general health and wellbeing.

Not that plants haven’t devised their own ingenious ways of dealing with less-than-ideal growing conditions. They have, from modifying the shape, colour and surfaces of their stems and leaves in order to reduce water loss to developing deep, fleshy tap roots to access water deep in the soil.


But faced with unusually hot, dry weather, their root systems inevitably come under pressure to replace the water lost to transpiration, as the plant does what’s required to regulate its “body” temperature (a plant’s version of sweating). That’s not so bad when the plant is a healthy, well-established one growing in fertile, healthy soil. But it can have potentially calamitous results if, and when, those same root systems are still young and fragile (for example, seedlings) or not yet fully established (for example, container-grown-plants that have only recently been planted out into the garden).

Plus it’s not just root systems that come under pressure. Although they have a remarkable ability to produce their own version of sun-cream (using protective molecules that block ultraviolet-B radiation), in very intense sunlight a plant’s delicate leaves can also suffer from leaf scorch, the plant equivalent of sunburn.

Even strong, apparently healthy, well-established plants aren’t entirely immune to these challenges. They too can suffer the long-term combined effects of extreme heat and water shortages

Flowers also fade much more quickly, and flower production slows right down in dry, very hot conditions as plants try to conserve their energies and resources, prioritising survival over productivity. Viable pollen and nectar production (necessary as food for visiting pollinating insects and for the production of fruit and seeds) is also inhibited by very high temperatures and low humidity.

Those plant priorities become obvious if you’ve ever left a hanging basket or window-box unwatered during a hot spell. First to go are the flowers and flowerbuds, which quickly droop and drop. Next in line are the leaves, which start to roll at the edges, then wilt and drop as the plant fights to keep its all-important root system alive. Much like an army rallying its forces to deal with a big battle on one war front, these plants then become more vulnerable to attack elsewhere in the shape of pests and disease. You can often see this with neglected, malnourished or underwatered plants that have become overwhelmed with aphids, a sign that their natural defence systems are compromised.

Even strong, apparently healthy, well-established plants aren’t entirely immune to these challenges. They too can suffer the long-term combined effects of extreme heat and water shortages, especially if their root systems have already been silently compromised by previous droughts, repeated heatwaves and soil compaction caused by building works.

Plants growing under cover of a glasshouse, polytunnel, conservatory, cold-frame or cloche are especially vulnerable, as these sheltered structures amplify the challenges presented by extreme heat. If it’s 25-30 degrees outdoors, for example, it can be anywhere up to 10-15 degrees hotter in a polytunnel — searingly high temperatures that will have huge consequences for the plants growing there if they’re not kept well-watered and ventilated and some form of shading isn’t provided.

This is because plants, just like people, have a certain spectrum of temperature within which they are most comfortable, and above and below which they struggle. That optimal temperature varies according to the particular species as well as according to each stage of a plant’s life cycle. But once the temperature goes above 30 degrees, many species will start to exhibit signs of plant stress.

Here in Ireland we’re been lucky that our gardens haven’t suffered from the same intense heat and severe drought as other parts of Europe this summer. But with repeated heatwaves predicted to become yet another consequence of climate change, what can we do as gardeners to keep our gardens happy?

Much of it is obvious.

For example, by keeping vulnerable plants sufficiently hydrated and properly watered, and always directing the water very gently to around the base of the plant, where most of its roots are located.

Watering in the cool of the evening rather than the heat of the day will also maximise the benefits and allow thirsty roots and leaves sufficient time to properly drink that precious water in. Similarly, placing shallow trays filled with fresh water beneath containers and grouping them together will provide shelter and slow down water loss.

Throwing open doors and windows of covered structures, installing temporary shading netting to shield plants from the searing sunlight and placing buckets of water nearby to cool the air will also do a lot to keep your plants happy. So will using mulches throughout the growing season to protect the soil structure, shade roots, boost soil health and resilience as well as to conserve available moisture in the ground.

We can also support plant health in other, longer-term ways by using natural foliar feeds, soil drenches and soil amendments such as liquid seaweed, home-made compost, Soil Renew and biochar, all of which will help plants to better withstand periods of prolonged heat and drought. Creating pockets of natural shade within our gardens or allotments by growing trees or large shrubs will provide some much-appreciated shelter for both the soil and smaller, more vulnerable plants as well as for garden wildlife. Similarly, by prioritising long-lived, deep-rooted, climate-appropriate woody and perennial species over short-lived annuals and biennials and letting our lawns grow longer, we can take away a lot of the need for regular watering. All of the above are small, but important steps on the road to creating and sustaining climate-resilient, nature-friendly gardens, something that’s becoming more important than ever.

But if you want to save seed from some of your own plants, then don’t deadhead the faded flowers and instead allow them to set seed. This is a great time of the year to start saving the ripe seed of many kinds of annuals, biennials, perennials and shrubby species. Always harvest on a dry, still day, choosing only seed heads that are fully ripe and placing them into labelled paper envelopes before putting them somewhere cool, dry and out of direct sunshine to full dry out.

Dates for your diary

A Month of Dahlias: June Blake’s garden, Tinode, near Blessington, Co Wicklow: A celebration of this beautiful and diverse genus of summer/autumn-flowering plants with a talk/guided tour by its owner and creator June Blake every Sunday throughout the month at 2pm, continuing throughout August.

Fionnuala Fallon

Fionnuala Fallon

Fionnuala Fallon is an Irish Times contributor specialising in gardening