When a heat wave hits, it can affect your plants. Most plants grow best in a temperature range of 59°-86°F. Prolonged temperatures above 90°F slow plant growth and some plants begin to show signs of stress. Above 104°F, many plants will survive but will show various signs of heat stress depending on the plant type, plant maturity, and factors associated with the high temperatures, such as drought and wind. Extreme air and soil temperatures slow plant chemical activity and growth.
What are the signs of heat stress in plants?
Leaves may curl or cup. Corn and tomatoes are among the plants whose leaves curl or cup inward in response to heat. The leaf's surface area is minimized and stomata (microscopic openings in the leaf that allow water and gases to pass through) close. This combines to reduce the plant's water loss.
Wilt
Wilting occurs when the plant's water content decreases and water pressure within the plant is insufficient. It is common in non-woody annuals and perennials, but many plants will recover when temperatures drop. The longer a plant remains wilted, the greater the chance of permanent damage. Plants that wilt during the hottest part of the day and recover in the evening or early morning may be experiencing heat stress.
Dry leaf ends
A survival mechanism used by some plant species is that the outer edges of the leaf dry out but the leaf remains viable. This is more common in plants with large leaves, such as squash and pumpkins, where the leaf has a larger surface area and can remain functional. Dry leaf edges can appear as a symptom of disease.
Damage caused by ozone
Ozone damage can also be confused with disease: in tomatoes, for example, dry brown spots between the veins resemble bacterial leaf spot (which has a yellow “ring” that is not present in ozone damage). In cucumbers, squash, and pumpkins, leaves damaged by ozone turn white and appear very dry.
Flower and fruit fall
Many ornamental plants will lose buds and flowers or drop off if exposed to high temperatures for extended periods, allowing the plant to conserve resources for those areas it needs to survive. Peppers, squash, and cucumbers often lose flowers and drop fruit during extended periods of high temperatures. Most plants will return to normal production once the heatwave has passed.
Bolt tightening
Bolting is when a plant blooms earlier than it would normally be harvested. Warmer temperatures cause cool season vegetables to bolt. A heat wave could mean the end of many cool season plants, including cilantro, broccoli, cauliflower, lettuce and spinach. For best quality and flavor, harvest these foods when a heat wave is in the forecast.
Sunburn
Many fruits, from apples to tomatoes to melons, can burn in high temperatures, mainly on the sides of the fruit that are most exposed to the sun. Sunburn looks different depending on the type of fruit and can include watery spots on the fruit, discolored or blistered skin, dried fruit, or a pitted, hard surface on the fruit.
Blossom rot
Calcium deficiency, a common problem with tomatoes, is exacerbated by sunburn. Watering regularly will help minimize blossom end rot. Be careful not to let your plants dry out completely between waterings.
How to reduce heat stress in plants
Watering
Recommendations for the ideal timing of watering vary, but deep watering is preferred over shallow watering. Morning watering is often recommended when temperatures are warm, but wilted plants should be watered as soon as possible to minimize the time they remain wilted.
Be aware of your plant's roots. Shallow-rooted plants dry out and die more easily.
Taproot plants are able to draw water from deeper in the soil. Young plants, which do not yet have fully developed root systems, need to be watered more frequently than older, more mature plants that have extensive root networks to draw moisture from the soil.
Some important items to consider include:
Mulch helps retain moisture in the soil by slowing evaporation and minimizing temperature fluctuations. Young plants with shallow roots are most affected when the top few inches of soil are hot and dry. Mulch can be as simple as a layer of grass clippings about an inch thick around each plant.
Create shade: Move containers to a shady location. If possible, provide shade cloth or other means of shading. Heat stress is made worse if other factors that stress plants are also present, such as drought, wind, or strong sunlight. If you use shading devices not typically used in gardening, such as beach or picnic umbrellas, consider adding them late in the morning and removing them at the end of the day to allow plants access to direct sunlight. Be prepared for plants in deep shade for extended periods to “stretch” toward more light.
Provide humidity: Leaves absorb small amounts of water through stomata, or leaf openings. In small gardens and home greenhouses, many plants benefit from additional humidity. Intentional foliar watering is not usually recommended in gardens. Leaf moisture, combined with the presence of pathogens and moderate temperatures, increases a plant's susceptibility to disease. However, during prolonged high temperatures, the benefits of a humid environment may outweigh the risk of plant disease. Strategies for increasing humidity include light overhead watering of the leaves and crowns of greenhouse plants in the morning. Outdoor potted plants can be misted en masse with a mist sprayer. Pressurized mist sprayers make this easier when there are many plants or the plants are large.
Do not plant or transplant. If soil temperatures are too high, young seedlings will die or develop tumors where the stem meets the soil. Transplanting will stress the plant even though high temperatures will not cause problems. Postpone planting or transplanting until temperatures cool.
Wait to fertilize: During periods of extreme heat, encouraging new growth is not ideal, so wait until cooler weather to fertilize.
Do not use chemicals. Many treatments for plant diseases and pest problems can damage plants if temperatures consistently exceed 80°F. Check the product label for details before use. Some plants, such as conifers and succulents, can be further damaged by any treatments (including soap and neem oil) applied in extreme heat.
Reduce weed competition: One garden task you shouldn't put off during hot weather is weeding. Many weeds are adapted to harsher conditions than ornamental plants, and they compete with garden plants for water and nutrients. Again, adding mulch around all your plants will help reduce weed growth.
• If you have any gardening questions, contact the Master Gardener Clinic at 509-574-1604 or email gardener@co.yakima.wa.us.
