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Watering Plants During a Heat Wave May Seem Like a No-Brainer. But It’s Not.

plant reacts to extreme heat
Wilted, curled leaves are an early sign of heat and drought stress.

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 upon 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. But it’s not that simple.

As a plant starts to defend itself from extreme heat, its ability to take up water and nutrients diminishes. This makes irrigating it tricky. Too little water and the plant withers. Too much and it drowns.

Most plants, especially those in your vegetable garden, go into defense mode when daytime temps remain above 90 degrees (F) and nighttime temps hover around 80 (F). Plants have mechanisms that allow them to slowly acclimate to higher temps as the season progresses, so by August they’ve adapted to intense sunlight and high heat. But a great deal of damage can occur early in the season when a heat wave hits because the plant isn’t ready 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). This 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. However, this interruption can set the expected fruit and vegetable harvest back weeks and sometimes end it completely.

How to water plants during extreme heat

1. When a heat wave is predicted, deeply water every plant 24-48 hours before the onset.

Give the root zone around each plant a thorough soaking. Top off mulch around root zones to three inches, especially in vegetable beds and around new transplants. If plants and soil are dry going into the heat wave, they’ll have little chance of surviving.

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2. Every day of the heat wave, watch for signs of stress.

First, the leaves will curl as the plant begins to conserve water; next, the plant will start losing its green color as photosynthesis declines; then, the plant will begin to droop, as it has exhausted its energy reserves 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.

3. Check the moisture in the soil every day.

Lift up the garden mulch, and even if the soil 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 for 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.

 Buy on Amazon: Control Wizard Soil pH and Moisture Meter 

Why plants have a hard time taking up water during a heatwave

Watering is tricky during a heatwave – 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. Get a soil test to learn what type of soil you have.

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 in the root zone. Water slowly and repeatedly at these times.

The best time of day to water plants in extreme heat

You also have the additional problem of a high evaporation rate during a heat wave, so the less water that’s in the air the better. That means never use sprinklers when the heat is scorching.

Water is best utilized by plants in the 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).

watering tomatoes with watering can
A watering can is the best way to water during a heat wave.

The best way to water plants is with a watering can

Controlling irrigation is essential during a heat wave, as large amounts of water can be lost to evaporation and runoff. The most controlled way to water, outside of drip irrigation, is with a watering can.

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 to a depth the length of 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.

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If you have a large garden, this, of course, would be an all-day task. You can adapt this method to watering with a hose, but concentrate on the root zones, not overhead watering. Drip irrigation is a fine alternative if using a watering can isn’t practical.

Watering plants in containers during extreme heat

In the case of container plants, heat dries out and shrinks the potting soil, leaving 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 potting soil and even less by the plant’s roots.

The most efficient way to water plants in containers during a heat wave is from the bottom-up. Sit the container in something like a big plastic bin. Pour water into the bin and watch as the soil and plant roots take the water up through the bottom of the pot and the water slowly disappears. 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. It also helps, of course, to put your container plants in shade to cool the container and the plant.

Even with this careful and deep watering, the soil may be dry again later in the day, because plants will use a remarkable amount of water just to survive 12 hours. You’re basically providing life support for the plants until the next significant rainfall or until the heat wave breaks.

**And by the way, never ever, ever fertilize plants during a heat wave. First, a lack of nutrients isn’t the problem. Second, the plants can’t take it up, so the fertilizer either runs off or collects in the soil, which may burn the plant when it resumes its normal functions.

2 thoughts on “Watering Plants During a Heat Wave May Seem Like a No-Brainer. But It’s Not.”

  1. this is a very helpful post, especially for a new gardener like myself. thanks 🙂

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