Heat Stress

Sep 19, 2022

Heat Stress

Sep 19, 2022

By Julie Pramuk

In a recent UC Master Gardener workshop at our new Las Flores Learning garden we focused on dealing with the recent heat wave and its effects in our gardens.  We outlined several topics to consider, with in- depth demonstrations and examples of:

  1. Healthy Soil components
  2. Water needs: too much or too little
  3. What does heat stress look like
  4. Right Plant, right place

This post will focus on the "Right plant, Right Place” aspect of the workshop and tips for avoiding heat stress. If I were to ask a landscape expert to shine a light on our most common gardening and maintenance mistakes, siting a plant in the wrong place would win the blue ribbon. If we think of ourselves in relation to plants, we share many of the needs to be healthy. The first consideration is where the plant begins its life and the process of growing into a mature plant. When we are too warm, we can move ourselves into a more favorable environment. We can go into our homes, turn on air conditioning, get a cool glass of water or draw the shades in a cooler room. Plants, of course, cannot get up and move. They rely on us to give them water and be planted in an optimal setting for sun or shade according to the plant's specific needs. Plants also have immune systems like we do, and like humans, these defenses do not work well under stress. Here are a few suggestions for helping your plants in extreme temperatures.

Know the microclimates within your own yard. We live in a mediterranean climate in California. However, our individual gardens have their own specific needs depending on the shade or sun during the day. When deciding on a plant, check the plant tab to see what the plant's needs are. For example, a plant tag which says,” full sun” means at least 6 hours of sunlight a day. Some of our summer fruits and vegetables needing more like 8- 10 hours a day. Part sun/part shade may mean 3-6 hours of sun a day and shade being closer to 3-5 hours a day. During our heat waves, the temperature can jump to 9-10 degrees higher putting our plants under severe stress. This is the time at which you may notice severe wilting, foliar collapse, or sunburn on your trees and plants, vegetables and fruits.

Consider watering needs by locating plants needing more water closer to your house where you can monitor their water needs. Plants that can get by with less water could be placed further out on your property. Use separate valves for different watering needs. Water judiciously to avoid wasting water. Provide temporary shade for your plants with shade cloth, burlap, or some other covering for severe day or night temperatures. Using umbrellas or chairs is a quick and easy way to shade plants. Don't prune off dried or dead parts of your plants during a heat wave. Wait until summer ends and you feel safe to prune. The dried sunburned parts of plants act to shade the leaves underneath. Green parts of sunburned leaves will help the plant to photosynthesize and generate new leaves.

Plant in the fall or early winter when, hopefully, there is a promise of rain and plants can establish a good root system. Skip fertilizing in a heat wave. Plants can't spare the energy to take up fertilizer and the fertilizer may burn the roots. Remember to mulch, mulch, mulch your garden. Adding a 3-4 inch layer around your shrubs and trees will help keep the soil several degrees cooler and prevent your soil from drying out. Keep mulch 6 inches away from the trunks of trees. Finally, there are times when our gardens suffer for various reasons: an irrigation system breaks down or you are away for a time and you wonder if a plant or tree be revived. Give your tree or plants a chance to respond. After a freeze, for example, a tree or plant may need a season to recover. Be patient. Different plants have their own way of responding and healing just like humans.

Napa Master Gardeners are available to answer garden questions by email: mastergardeners@countyofnapa.org. or phone at 707-253-4143.  Volunteers will get back to you after they research answers to your questions.

Visit our website: napamg.ucanr.edu to find answers to all of your horticultural questions.

Photo credits: Jane Callier

The UC Master Gardeners of Napa County have a series of documents of useful garden practices in their Healthy Garden Tips collection, including microclimate documents listed below.

Information Links:

Healthy Garden Tips

Right Plant, Right Place https://ucanr.edu/sites/ucmgnapa/files/153367.pdf

Cool Season Gardens for Napa County https://ucanr.edu/sites/ucmgnapa/files/153368.pdf

Creating Microclimates in the Garden https://ucanr.edu/sites/ucmgnapa/files/153509.pdf

Climate Zones Map of Napa County https://ucanr.edu/sites/ucmgnapa/files/254550.pdf


UC Sonoma County-Right plant right place



UC IPM-dealing with thermal injury or high temperatures