7 Tips for Watering Plants in a Heat Wave

Watching plants wilt during a heat wave is one of the most stressful experiences for any gardener. Even when you water in the morning and everything looks fine, by afternoon, your garden could be collapsing. Regular watering schedules that work perfectly during normal weather can become completely inadequate when temperatures climb above 85°F (29°C) for days on end.

Getting watering right is always critical, but even more so during a heat wave. Plants that survive extreme heat with proper hydration will bounce back quickly once temperatures drop. Unfortunately, those that suffer severe water stress may never fully recover, even if you manage to keep them alive. Here are some tips to water plants in a heat wave to avoid that sad end.

Deep Watering Beats Frequent Watering

The biggest mistake you can make during heat waves is switching to light but frequent watering sessions. I know it seems logical: if plants are stressed, give them small drinks throughout the day. But this approach encourages shallow root systems that make plants even more vulnerable to heat stress. Plus, that shallow water evaporates quickly, making the extra effort not worth it in the end.

Deep watering encourages roots to grow down into cooler, more stable soil layers. Shallow watering keeps roots near the surface where soil temperatures can exceed 100°F (38°C) and moisture evaporates rapidly. A plant with deep roots can access water reserves that surface-rooted plants can’t reach.

To water plants in a heat wave, soak the soil several inches deep during each watering session. Take your time, or set up irrigation systems that provide consistent moisture over time with limited loss. The deep watering principle also applies to potted plants, although they will still need water more frequently throughout the day.

Sandy soils drain faster and may need daily deep watering, while clay soils hold moisture longer and can go 2-3 days between sessions, depending on conditions.

Timing is Critical

Normal weather gives you flexibility with watering times during the day. Heat waves eliminate that luxury. The window for effective watering narrows dramatically when temperatures spike.

Early morning watering allows plants to hydrate before the day’s heat hits. This timing also minimizes evaporation losses and gives plants the best chance to cope with afternoon temperatures.

Evening watering can work, but only if you water late enough for plants to absorb moisture overnight. Watering in the late afternoon when it’s still hot means most of the water evaporates before plants can use it. Wait until after the sun dips when temperatures start dropping, or stick with early morning sessions.

If you are opting for evening watering, don’t get any moisture on the foliage. This can encourage disease and pest problems. Instead, focus on the soil.

Midday watering is generally wasteful due to evaporation, but there’s one exception. If plants are showing severe stress and wilting badly, a quick drink can prevent permanent damage. Just don’t make this your regular strategy.

Mulch, Mulch, Mulch

Exposed soil can reach extremely high surface temperatures, heating plant roots and causing rapid moisture loss. A thick layer of organic mulch can reduce soil temperature by 10 degrees and dramatically slow moisture evaporation.

Fresh mulch applied right before a predicted heat wave can make the difference between plants surviving or succumbing to heat stress. Even newspaper or cardboard covered with a thin layer of grass clippings works if you don’t have other organic mulch available on short notice.

For container plants, consider placing containers inside larger pots and filling the space between with mulch or moist peat moss. This insulation protects roots from overheating and reduces water loss.

Increase Water Volume, Not Just Frequency

Plants under heat stress need significantly more water than normal growing conditions require. A tomato plant that usually needs 1 inch of water per week might need 2 or 3 inches of water during a heat wave. Leafy vegetables like lettuce and spinach need even more water relative to their size because they lose moisture rapidly through their large leaf surfaces.

Container plants are especially vulnerable because they have limited soil volume to store moisture. Pots that normally need watering every other day might need daily watering, or even twice daily for small containers. Large containers fare better than small ones because they hold more moisture reserves.

Mature plants with extensive root systems can access more soil moisture, but also lose more water through transpiration. Young plants with small root systems need more frequent watering, but in smaller quantities.

Don’t Rely on Plant Appearance Indicators

Relying on how plants look to tell you when to water can lead to both overwatering and underwatering during extreme heat. Many plants naturally wilt during the hottest part of the day as a protective mechanism, even when soil moisture is adequate. This temporary wilting doesn’t necessarily mean they need more water.

Stick your finger 2 inches into the soil. If it feels dry at that depth, you can water. For in-ground beds, you might need to dig down a few more inches to check moisture levels before you water plants during a heat wave.

Soil moisture meters take the guesswork out of the equation, especially for larger garden areas. These tools give you accurate readings at different soil depths and help prevent both drought stress and overwatering.

Learn to distinguish between heat stress wilting and moisture stress wilting. Heat stress wilting often occurs even when the soil is moist, and the plants usually recover in the evening. Moisture stress wilting is more severe and doesn’t improve until plants receive water.

Prioritize Vulnerable Plants

During severe heat waves, you might not be able to keep everything perfectly watered. Focus your efforts on the plants that matter most and are most likely to survive with proper care.

Newly planted trees and shrubs need priority watering because their root systems haven’t established yet. These plants can die from just a few days of severe water stress, and replacing them is expensive.

Within the vegetable garden, prioritize crops that are close to harvest or particularly heat-sensitive varieties. Container plants need more attention than in-ground plants because they have no access to groundwater and limited soil volume for moisture storage.

Established trees and shrubs with deep root systems can usually survive a few days of water stress better than shallow-rooted plants. Native plants adapted to your climate are also generally more heat-tolerant.

Create Temporary Shade

Sometimes, even perfect plant watering in a heat wave isn’t enough. Temporary shade can reduce plant stress and cut water needs significantly for short periods.

Shade cloth (with around 30% to 50% shade) works well for vegetable gardens. Even old bedsheets or umbrellas can provide enough relief for small areas or individual plants. For container plants, moving them to naturally shaded areas during the worst heat can prevent heat stress.