Plants grow best in temperatures ranging from 56 to 86 degrees Fahrenheit. When there are sustained temperatures of 90 degrees or above, plants grow more slowly and begin to show signs of stress.
Extreme air and soil temperatures combined with low humidity and wind cause a slowdown of chemical activity and plant growth. It is important to recognize signs of plant stress so you can adapt the garden environment or adjust irrigation.
The following are signs of plant stress:
Dry leaf edges are a survival mechanism used by some plants that cause the outer edges of the leaves to dry up while allowing most of the leaf to function. It is most common in large leaved plants like pumpkins and squash but can be found on other plants.
Wilting occurs when there is a lack of water pressure in the plant. A plant that wilts in the hottest part of the day but recovers in the evening and early morning is suffering from heat stress. This is most often found in non- woody annuals and perennials.
Leaf rolling and cupping is a heat coping strategy found in corn and tomatoes. The leaves roll and cup to minimize the leaf surface and the stomata (microscopic openings in leaves that allow movement of moisture and gasses) close. This reduces moisture loss in the plant.
Blossom and fruit drop is a strategy plants use to conserve resources for the parts of the plant necessary for survival. Blossom and fruit drop occurs in peppers, cucumbers and squash when high temperatures persist.
Bolting is a premature flowering of the plant before it would be typically harvested. Cool season plants like cilantro, broccoli, cauliflower, lettuce and spinach should be harvested before the arrival of a heat wave.
Sunscald or sunburn on fruits is caused by exposure to the hot sun during high temperatures. Sunscald looks different on various fruits and sometimes resembles a watery area, discoloration, or blister on the skin of the fruit in addition to dried-out fruit, or sunken hardened areas on the fruit surface. Damage occurs on the side of the fruit exposed to the sun.
Blossom End Rot is a common problem in tomatoes due to deficient calcium. It becomes more of a problem with high temperatures and potential sun scald. Avoid letting the plant completely dry out before watering.
Heat Wave in the Garden: How to Identify and Prevent Heat Stress in Plants https://extension.oregonstate.edu/gardening/flowers-shrubs-trees/heat-wave-garden-how-identify-prevent-heat-stress-plants
Steps for Reducing Heat Stress in Plants
The following steps will help to prevent damage to plants during a long period of excessive heat.
Watering in the morning using a soaker hose or drip irrigation is ideal. Deep watering is preferred over shallow watering. In high temperatures, if there is wilting on a plant, it is better to water the plant as soon as possible to relieve plant stress.
Know your plants and determine the frequency of watering based on the depth of their root systems. Young plants or shallowly rooted plants will dry out more quickly than older, established plants that have an extensive network of roots.
Hanging pots or container plants dry out quickly and may need watering twice a day.
Do not forget to water your mature trees as they can suffer from heat and drought stress as much as smaller plants.
Mulch will conserve the moisture of the soil around the plants and keep the area cool. Two to three inches of mulch (bark chips, straw or compost) will help your plants through a period of excessive heat.
Provide shade for plants in the ground that receive sunlight during the hottest part of the day. Installing shade cloth on a temporary structure can prevent scorching of the leaves and blossoms. If that is not possible, an umbrella (like a beach or picnic umbrella) not typically used for gardening can be used to protect plants in the hottest part of the day. Remember to remove the umbrella for a portion of the day so that the plant has some exposure to sunlight.
If you have plants in containers, consider moving them into the shade for the duration of the heat wave.
Provide humidity – The leaves of plants take in small amounts of water through the stomata or tiny openings in the epidermis of the leaves. It is not typically recommended to mist or water the leaves of plants as it can increase risk of plant disease via a pathogen. But when excessive temperatures occur, plants can benefit from an increase in humidity, which may outweigh the risk of disease.
A couple of steps for increasing humidity safely are misting potted plants and lightly overhead watering plants in the early hours of the morning. Early morning watering of leaves will allow drying throughout the day.
During periods of high temperatures, you should not do the following:
- Don’t plant or transplant – wait until cooler temperatures
- Wait to fertilize – new growth is not ideal in the heat
- Don’t apply chemicals – always check the label before using any chemicals
- Don’t prune trees or shrubs – it can lead to sunburn of the newly exposed canopy
- Do make sure to eliminate weeds that compete with plants for water and nutrients
Even when you have taken steps to support the health of your plants after several hot days, some plants may begin to look extremely bad. Burned leaves does not mean that your plant is dead. Patience and observation are needed.
You can cut off dead flowers but resist the urge to remove all of the damaged leaves. Even though they looked crinkly and dry at the edges many of the leaves are still functional and photosynthesis is taking place.
Do not heavily prune your plants. Pruning will encourage new growth which is susceptible to burning if high temperatures return. Foliage will grow back unless the plant is too damaged to recover.
Berries can be pruned lightly but keep to your usual pruning schedule to remove the fruiting canes.
How To Care for Heat Damaged Plants
https://extension.oregonstate.edu/news/how-care-heat-damaged-plants
Selecting Different Plants
Plant selection is key. If you find that you are tending each year to a plant that is stressed by high temperatures, maybe it’s time to make a new selection for that area of your garden. There are a wide variety of plants that will tolerate hotter soils and drier air temperatures.
Trees and Shrubs by Drought Tolerance
https://extension.oregonstate.edu/collection/trees-shrubs-drought-tolerance
Northwest Native Plants for a Sunny Border https://extension.oregonstate.edu/gardening/techniques/northwest-native-plants-sunny-border
Click here for a printable pdf of this article. Written by Cheryl Borden July 2024. Photos via Canva.
Clackamas County Master Gardener™ Association 