It starts with the leaves curling. Every gardener has seen this when temperatures soar. To defend itself and conserve energy, it’s as if the plant starts to fold up on itself. Then comes a loss of “green-ness”, as color starts to drain, sort of like when people get the flu. Then comes the drooping, a sort of “going-down-for-the-3rd-time”. After that, well, there really is no “after that”, because the drooping, caused by an absence of water, means the plant is on its way out.
It may seem like a no-brainer that plants should be watered regularly during a heat wave. However, as a plant starts to defend itself from incendiary temperatures, its ability to take up water and nutrients diminishes. This makes irrigating tricky. Too little water and the plant withers. Too much and it drowns.
Most plants, especially those in your vegetable garden, go into defensive mode when daytime temps remain above 90 degrees (F) and nighttime temps hover around 80 (F). In the vegetable garden, many plant’s optimum temperature – that at which it produces best, is between 50-75 degrees (F), according to Penn State University’s College Of Agricultural Sciences. Plants have mechanisms that allow them to acclimate to higher temps as the season progresses. That’s why so much damage is wrought by early season heat waves, because the plant is in no way prepared for it – temperatures of 100 (F) in June are far more damaging than in August.
How a plant cools itself
A plant uses water to cool itself via transpiration – it pulls water from the soil through its roots, which moves through the plant distributing nutrients, eventually evaporating through pores in the leaves. High temperatures speed transpiration, but as the surface soil dries, soon there isn’t enough water for the plant to carry on its normal functions. This is where deep root growth is of benefit, as the plant will search for water in the deepest soil possible, soil which is cool and still holds moisture.
When a plant isn’t acclimated to extreme heat, its first response is to curl its leaves and stop stem elongation (growing), which minimizes evaporation and conserves energy. If the heat persists, the plant will also drop blossoms, and stop fruit, vegetable, and flower production. When temps return to normal, these functions will resume.
Build the plant’s defenses and be vigilant
When a heat wave is predicted, deeply water every plant 24-48 hours before the onset. Top off mulch to three inches everywhere, especially in vegetable beds and around new transplants (straw is best). If plants and soil are dry going into the heat wave, they’ll have little chance of surviving.
Every day of the heat wave, watch for signs of stress: The first will be leaves curling as the plant begins to conserve water; next, the plant will start losing its green color, which means that photosynthesis is declining; then, the plant will begin to droop, as it has exhausted its energy supply and is able to take up less and less water through its roots. The time between these phases will depend on the type of plant, the intensity of the daytime heat and how cool nights are.
Check the moisture in the soil every day. Lift up the mulch, and even if the surface appears wet, plunge your index finger into the area around the roots (you can also use a soil moisture meter for this). If the soil feels wet the entire length of your finger, there’s no need to water. If it’s damp or less, water right away. Never let the soil dry out completely.
Getting the water into the plant
Watering is tricky during a heat wave – or more specifically, getting the water into the plant. This largely depends on the ability of your soil to hold water. Clay, loamy, or sandy soils will all perform differently.
Soil must be holding water to attract more water, as water molecules bind to each other. So if your soil is completely or almost dry, a large amount of water will run off before it accumulates.
You also have the additional problem of a high evaporation rate during a heat wave, so the less that’s in the air (no sprinklers), the better. Water is best utilized by plants in early morning, before the sun and heat ramp up. Watering in the early morning instead of the evening also allows time for foliage to dry, reduces loss to evaporation, and reduces the chance of fungal infections (but if heat stress is apparent midday, by all means water).
Use a large watering can and pour a small amount of water around the root zone. Stop. Most of it will run off if the surface soil is dry. Wait about 10 minutes and pour a small amount of water again. Less should run off this time. Wait another 15 minutes, and then water sufficiently to make the soil wet as deep as your index finger. This actually isn’t as hard as it sounds, because by the time you get to the last plant, it will be time to start again with the 1st plant.
In the case of container plants, heat shrinks the soil in the container and leaves a gap between the hot container wall and the roots. It’s remarkable how much water will run right through the container when this happens, and little if any is absorbed by the roots. The most efficient way to water plants in containers during a heat wave is bottom-up. Sit the container in something like a big plastic bin. Pour water into the bin and watch as the plant roots make the water slowly disappear. There’s no runoff or waste, and if done in a shaded area, little loss to evaporation. The plant will absorb only as much as it needs as long as it sits in the water.
Even with this careful and deep watering, the soil may be dry again the next day, because plants will use a remarkable amount of water just to survive the heat wave. You’re basically providing life support for the plants until the next significant rainfall.
And by the way, never ever ever fertilize plants during a heat wave. First, lack of nutrients isn’t the problem. Second, the plants can’t take it up, so the fertilizer ether runs off or collects in the soil, which may burn the plant when it resumes its normal functions.