The short version: A heatwave hurts a vegetable garden in three ways: it dries the soil faster than roots can drink, it cooks exposed fruit and leaves, and it stops tomatoes, beans and capsicums setting fruit. The response is the same every time the forecast turns red. Water deeply the evening before it arrives, then early each morning. Throw 50 percent shade cloth over the leafy beds and the fruiting crops. Mulch anything bare. Move pots into shade and group them. Pick everything ripe or nearly ripe before the heat. Do not feed, prune or plant until it breaks. And look after the gardener: work early, drink, wear a hat, and treat confusion or dizziness in the heat as an emergency.
Australian summers arrive in bursts. Three or four days above 38 degrees can undo a season's work in a bed that was thriving the week before, and the damage is often invisible until the flowers drop and the fruit scalds a week later. The good news is that a garden prepared for heat comes through it with very little loss, and preparation is mostly water, shade and timing.
What heat actually does to vegetables
The first casualty is water balance. On a hot, windy day a plant loses moisture through its leaves faster than the roots can replace it, and it wilts by afternoon even in moist soil. That is normal and it recovers overnight; a plant still wilted at dawn is genuinely short of water. The second casualty is pollination. Tomato pollen fails once days climb into the low to mid thirties and nights stay above about 21 degrees, capsicum flowers drop over a slightly wider range that runs into the high thirties, beans abort flowers from around 29 degrees, and cucumbers stall above about 35; the figures come mostly from overseas extension research, but the pattern is familiar to every Australian gardener whose January flowers never became fruit. The third is scorch. Fruit suddenly exposed to full sun develops sunscald, a pale, leathery patch on the sun-facing side, and leaves brown at the edges when the soil cannot keep up. And the fourth is bolting: as a general rule, lettuce runs to seed after several days in the low thirties, coriander and rocket follow, and a bed of greens turns bitter in a week. Our guide to vegetables that thrive in an Australian summer covers the crops that shrug most of this off.
The day before: water deeply
The single most effective step is a deep soak the evening or morning before the heat arrives. Soil that is wet 20 to 30 cm down holds a plant through a hot day in a way that a light sprinkle never does, and well-hydrated leaves cope with heat far better than stressed ones. Scratch back the surface and feel the soil at about 5 cm to judge whether you have done enough; a dry crust often hides moist soil beneath, and vice versa. Then water early each morning of the heatwave, at the base of the plants, so the roots have a full day's supply. Late afternoon is the second-best time. One persistent belief is worth weighing: the physics says a water droplet cannot focus enough sunlight to scorch a leaf, and most leaf scorch traces to dry soil rather than wet leaves, though some Australian gardening authorities still advise keeping water off foliage in full sun as a precaution. Water at the base and the question does not arise, and if a plant is collapsing at midday, water it. Our guide to deep watering explains why depth beats frequency.
Shade cloth: which, where and when
Shade cloth is the cheapest insurance in the garden. A cloth of about 50 to 60 percent suits most vegetables, and 70 to 90 percent is for the truly heat sensitive or for a nursery bench; the fine white netting sold as windbreak gives only about 20 percent and is not enough on its own. Rig it on stakes, hoops or a simple bamboo frame so it sits well above the plants with air moving underneath, or drape it over trellised tomatoes and cucumbers and peg it down, and take it off again when the heat breaks so the plants are not starved of light for weeks. Leafy crops, lettuce especially, benefit most; tomatoes and capsicums gain protection from sunscald and, sometimes, a truss that sets in the shade when the rest of the plant drops its flowers. A bed under a permanent frame that can carry cloth in summer and bird netting in autumn is one of the best structures a small garden can have.
Mulch, pots and small things that matter
Bare soil in a heatwave can reach temperatures that kill surface roots. A 6 to 8 cm layer of coarse straw, lucerne or sugarcane cuts evaporation sharply and keeps the soil cooler; very fine mulch does the opposite, so use the chunky stuff. Pots are the most vulnerable plants in the garden, because the whole root ball heats through by afternoon. Move them into shade or indoors for the worst days, group them so they shade each other, stand them in saucers of water for the duration, and slip small pots inside larger ones with a gap between for insulation. Water pots in the morning and check them again in the evening. Our guide to watering pots in summer goes into detail.
What to pick, and what to leave alone
Harvest before the heat, not after. Pick every ripe and nearly ripe tomato, capsicum and cucumber, cut the lettuce and herbs you want, and take zucchini while they are small, because, as most gardeners find, heat-stressed plants ripen erratically, fruit scalds and greens bolt. Then leave the plants alone. Do not fertilise during a heatwave, which pushes soft growth that scorches, and do not prune, which exposes fruit that was shaded; Yates' advice is to feed again only once temperatures drop back below about 30 degrees. Do not plant out seedlings or sow into hot beds. And do not panic at the afternoon wilt; judge the plant in the morning.
After the heat breaks
Water deeply again, take the shade cloth off, feed with a seaweed solution and a light liquid fertiliser, and give the plants a week before deciding what is lost. Tomatoes and beans that dropped their flowers usually set again on the next flush once nights cool. Cut sunscalded fruit off and use the sound parts. Pull bolted lettuce and resow in shade. Plants that were watered before and through the heat mostly carry on as if nothing happened.
Watering rules to know
Most capital cities have permanent water-saving rules that shape a heatwave routine. Sydney allows hand watering with a trigger nozzle, and sprinklers, only before 10 am and after 4 pm, with drip and smart systems permitted any time. Melbourne restricts automatic systems to between 6 pm and 10 am, with a hand-held hose, can or bucket allowed any time. Perth assigns sprinkler days by address, only before 9 am or after 6 pm, and bans sprinklers through winter, dates that are worth checking each year. South Australia and other regions have their own rules, and all of them tighten in drought, so check your water utility's site; a hand-held hose in the early morning is within the rules almost everywhere.
Looking after the gardener
The Bureau of Meteorology issues heatwave warnings up to four days ahead and grades them by severity, and a severe or extreme heatwave is dangerous to people as well as plants. Do the garden work early in the morning, in a hat and long sleeves, drink water before you are thirsty, and stop at the first sign of a headache, dizziness, nausea or heavy sweating, which are heat exhaustion. Confusion, slurred speech, agitation, a very high temperature or collapse are heat stroke and need an ambulance on triple zero (000) while the person is moved to shade and cooled. People over 65, babies and young children, pregnant women, outdoor workers, people who are homeless or socially isolated, and anyone with heart, lung or kidney disease, diabetes or a mental illness are most at risk, and a hot garden is not the place for them at two in the afternoon.
Heatwaves across Australia's climate zones
- Tropical (Cairns and far north Queensland, Darwin and the Top End of the Northern Territory, the Kimberley in Western Australia): heat is constant and humidity is the extra burden, so airflow under shade cloth matters as much as the shade; grow heat-adapted crops through the wet and temperate ones in the dry.
- Subtropical (Brisbane and coastal Queensland, Coffs Harbour and northern New South Wales): humid heat rots as well as scorches; water in the morning only, keep foliage dry by evening and use a frame rather than a draped cloth.
- Arid and semi-arid (inland Queensland, Broken Hill and Dubbo in New South Wales, Mildura in Victoria, inland South Australia and Western Australia, Alice Springs in the Northern Territory): the longest and driest heatwaves; permanent summer shade structures, deep mulch and afternoon shade on every bed are the norm, with pots moved under cover from December.
- Temperate (Sydney and New South Wales, Melbourne and Victoria): short, sharp bursts of extreme heat, often with a hot northerly; the day-before soak and a roll of shade cloth kept by the shed door cover most of it.
- Mediterranean (Perth and southern Western Australia, Adelaide and the Adelaide Plains in South Australia): dry heat and easterly winds pull water from leaves fast; mulch deeply, water early on roster days and give lettuce afternoon shade from November to March.
- Cool and alpine (Hobart and Tasmania, Canberra and the ACT, Ballarat in Victoria, Orange and Armidale on the New South Wales tablelands, and the alpine country of the Snowy Mountains and the Victorian high country): rare but damaging in Hobart and the high country, because plants and gardeners are unprepared, while Canberra averages about five days above 35°C a year; keep shade cloth on hand and water pots twice daily on hot days.
These zone notes are general guidance rather than sourced figures. Whatever the zone, the forecast is the trigger: when the warning comes, water, shade, pick and wait.
Frequently asked questions
Should I water in the middle of a hot day?
If a plant is collapsing, yes, at the base. Morning watering is better because less evaporates, but an emergency drink is never wrong.
Why did all my tomato flowers drop?
Heat at flowering. Once days pass the low thirties and nights stay warm, the pollen fails and the flowers abort. The plant sets again when the weather cools.
What percentage shade cloth should I buy?
About 50 percent for general vegetable use, higher for a seedling bench, and take it off once the heatwave passes.
Can I save wilted lettuce?
If it has not bolted, a deep evening water and shade will bring it back. If a flower stalk has started, it is bitter and finished; resow.
Water the day before, shade the beds, pick what is ready and leave the rest alone, and most heatwaves pass with nothing worse than a few dropped flowers. NSW Health's Beat the Heat advice covers the human side, which matters more than the tomatoes.
Related growing guides
- When the heat sets in: vegetables that thrive in an Australian summer
- Watering Pots in an Australian Summer Without Losing the Crop
- How to water vegetables so the roots go deep
Sow the crops that take the heat: browse our heat-loving vegetable seeds, lettuce seeds and the full range of vegetable seeds.