Heat Survival Kit: How to Water Your Garden in a Heat Wave

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Over the past few years, we have seen two different weather patterns: hot and dry for a few weeks followed by intense rainfall. These weather extremes are becoming more common. As we head into July, heat waves and drought periods will likely get longer and more intense. Growing resilient plants in weather extremes begins with wise planning (tips for how to design a weather-smart garden). But there are steps you can take this season to grow a successful garden when temperatures soar.

wilted leaves of hydrangea

Wilted leaves are one of the first signs of water stress. Hydrangeas are more drought sensitive than most shrubs (N.C. Cooperative Extension, 2014).

Quick science lesson

Plants are cooled by water evaporation from leaf surfaces. This process is called transpiration. During a heat wave, a plant loses more water because there is more transpiration. At the same time, the plant goes into “defense mode” by slowing its growth and fruiting, and doesn’t take up as much water. This makes watering during a heatwave tricky. Signs of heat stress are curled leaves, loss of green color, and wilting. The plant may drop flowers and fruit to survive.

Before a heat wave

Prepare your garden for high temperatures by keeping plants well-hydrated. Water 24-48 hours before the onset of heat. This can lower the air and soil temperature around plants. Apply 3-4 inches of straw or leaf mulch to reduce evaporation and conserve water. Organic mulches shade the soil surface, keeping soil near plant roots cool and moist.

During a heat wave

Watering at the right time, right way, and right amount is crucial for plant survival and productivity during a heat wave. Flowering to harvest is the most critical period when plants need sufficient water.

Right time

It is tempting to water when plant leaves wilt, but this doesn’t mean it is time to turn on the hose. Plants can wilt even when the soil is wet if the roots are unable to take up water. This can be caused by root damage, disease, or even too much water because roots need oxygen too!

Check the soil to decide when it is time to water. Insert your index finger into the soil 1-2 in deep. If you feel moisture beneath the surface, wait to water. On average, gardens require 1 in of water per week. However, daily watering may be needed during periods of excessive heat. Keep your soil type in mind: sandy soil which drains rapidly will need more frequent watering than heavy, clay soil.

drip irrigation system

Drip irrigation system for tomato plants (Yadkinville Community Garden, 2019)

Right way

“Long and slow” is the key. Apply water directly to the soil (not on plant leaves) within the dripline where fine roots are present. This is best done with a drip irrigation system or soaker hose. Avoid overhead watering to prevent leaf wetting and reduce water loss from evaporation. Morning is the best time of day to water. Watering early reduces the risk of foliar disease due to leaves remaining wet overnight.

Right amount

Apply enough water to moisten the soil in the plant root zone. This depends on the plant type; trees and shrubs with deep roots will require longer watering than small seedlings. Check the soil moisture about 1 hour after watering by digging a 6-8 inch deep hole to ensure water has reached this depth. Avoid shallow and frequent “sprinklings” which encourage roots to grow near the soil surface.

Tomato Tip: watering consistently can prevent plant disorders like blossom end rot.