The extreme heat of summer can scorch and stress all but the very toughest of plants.
Preparing your garden with shade, mulch and an increased watering routine can help protect your plants when the heat hits.
Understanding the impact of heat
It's normal for plants to wilt in hot weatherWilting is often the first sign that a plant is being stressed by the heat.
During the day, most plants constantly lose water through evaporation from tiny holes or pores known as stomates on the undersides of their leaves. This is part of a natural process called transpiration, which is as important to plants as breathing is to us.
But when the weather is hot or windy, plants lose a lot more water than they usually do. When there’s not enough moisture in the soil or potting mix to replace it, plants begin to wilt and droop because they have lost water pressure in the cells of their leaves and soft stems.
Wilted plants usually recover quickly after a deep watering, but if they’re left to dry out completely they can become browned and scorched.
Plants can also become scorched or sunburned when the sun is so intense or the temperature so high that it causes physical damage to the plant. This is more likely when the soil or potting mix is dry and the humidity is low – especially when it’s also windy.
It's worth knowing how much heat and sun your plants can tolerate. Japanese maples, especially the fine-leaved “dissectum” forms, are well known for having their leaves brown off on the edges in hot weather. And while tropical plants might come from hot places they lose lots of water through their big, broad leaves, so you have to keep the water right up to them in summer.
Keeping soil moist is the best way to protect your plants from the heat. When you know it’s going to be hot, give your more vulnerable plants a good, deep watering in the morning to prepare them for the day ahead. Run the hose slowly and for longer to allow water to get right down into your garden soil and plant pots.
Here are some other tips to make your watering more effective:
Light watering grows weak plants. It encourages plants to develop more roots at the surface, where they will get cooked in extended hot weather. This makes it harder for your plants to recover from heat stress.
Using a shower-head style hose wand makes it easier to deliver a deep, gentle watering without blasting away soil and exposing vulnerable roots.
In heatwave conditions potted plants can need daily watering
If you don’t have an irrigation system, get a soaker hose and lay it out across your garden bed upside-down so no water is lost to the air.
Don’t water every square inch of your garden. Water around the bases of your plants, where the roots are. Try to keep water off the leaves of your plants – if water evaporates off the leaves it’s wasted, and if it stays on the leaves it can encourage fungal disease.
Old garden soil and potting mix can repel water. Regular applications of granulated or liquid soil wetter can improve water penetration so your plants can make the most of the water you’re giving them.
Don’t forget your potted plants. In heatwave conditions pots can need daily watering.
There are other ways you can help your plants when extreme hot weather is forecast.
Have some garden-cover shade cloth on hand to drape over vulnerable plants. Garden-cover shade cloth blocks out 30 to 50 per cent of UV rays, rather than the 70 to 90 per cent block-out of the shade cloth you would put over your entertaining areas.
Move potted plants into shady spots, especially seedlings and cuttings. You can make this easier by putting larger pots on pot roller bases or stands.
Apply spray-on polymer-based plant protection products to foliage. These reduce the impact of heat and slow down moisture loss.
Design for the climate
The key to a resilient garden is in the planning, whether you’re starting from scratch or just adding a new space or garden bed.
Plant to create cool zones and shadeDifferent plants suit different conditions. A ginger that thrives in the tropical north won’t cope in the cooler south, and a Rhododendron that wows visitors to gardens in Victoria's cool and leafy Dandenong Ranges doesn’t stand a chance in hot and humid Cairns.
When selecting plants, always consider your local climate and its extremes as well as the micro-climates in different parts of your garden.
Reducing hard surfaces, such as paved or concreted areas, helps keep your garden cooler.
Lawns can be 30 per cent cooler than hard surfaces, and they don’t retain heat. Hard surfaces can make a heatwave worse by retaining heat and radiating it overnight when your plants really need a break. Consider adding cool lawns or installing shade sails over hard surfaces, even if you remove them after summer.
If you have a vegetable garden or a bed with other potentially heat-sensitive plants consider adding posts around them or a pergola above them so you can attach shade cloth or shade sails during heatwaves.
Raised garden beds and retaining walls with plants up top can cop a double whammy if their faces are exposed to direct sun – the soil is heated up and dried out from the side as well as the top. Avoid this by planting in front of them to provide shade.
Mulch retains moisture and insulates against temperature extremes. Keep a layer of mulch about 50mm deep. Always remember to water before and after mulching and if installing drip irrigation pipes put them under the mulch in contact with the soil.
Check out these guides for more gardening advice and let us know if we can help: