Have you ever noticed your perfectly healthy plants wilting at the end of a hot, sunny day? This is the plant’s way of protecting itself from the elements, much like our bodies adjust to temperature changes. Plants routinely gather moisture and nutrients from the soil through their roots and move it up their stems and leaves. Moisture is then emitted back into the air through pores on the underside of the leaves, called stomata. This process is called transpiration. On an especially hot day, the leaves will begin to emit more moisture than the roots can supply, which triggers the pores to close. As a result, the leaves lose rigidity and wilt but the plant is protected from further loss of moisture. While all plants transpire, broadleaf plants such as cucumbers, squash, hydrangea and many annual flowers are especially susceptible to heat wilt.
To help your plants recover:
- If the soil is moist about an inch under the soil line, the plant will likely recover on its own in the cooler night air. If it is dry, water the plant at the soil line and check in the morning; again, the plant should have perked up. If the coming day will be another scorcher, consider a bit more morning water to avoid further late day stress.
- Be gentle around the limp foliage, it is stressed and can easily break.
To reduce the potential for heat wilt:
- Add 2″-3″ mulch around plants, stopping at the outer edge of the plant so water can be absorbed easily.
- Promote healthy roots by routinely watering deeper a few times a week versus a daily short spritz.
- When adding new plants, loosen soil deeply and add organics such as compost to aid in drainage and root development.
- Consider location when planting. Afternoon sun is very likely to wilt plants that need only partial sun. Also pair plants needing the same water and light conditions together.
- Add succulents and plants requiring less water to your garden. They will thrive during the dog days of summer.
Transpiration fun facts and resources:
- An acre of corn can transpire 3,000-4,000 gallons of water a day.
- Transpiration is to thank for the comforting shade under large trees. Eighty percent of the cooling effect of a shade tree comes from the evaporative effects of transpiration.
- Much more on the transpiration process can be found on CMG Garden Notes 141 and at http://water.usgs.gov/edu/watercycletranspiration.html
Written by Linda McDonnell, a Denver County Master Gardener