Submitted By: Leah Fronk, Penn State Extension Horticulture Educator
According to the US Drought Monitor on July 25, 2024, at least 75% of Franklin County is classified “moderate drought” and the remainder as “severe drought.” At least 50% of Bedford, Somerset, and Fulton counties are labeled as “severe drought” on the same map. For those of you tending gardens and watching the field corn, this comes as no surprise. It has been a hot, dry summer, and our crops are feeling the heat.
The ideal temperature for most crops is 60-85°F. When temperatures above 90°F are sustained for several days in a row, plants will show signs of heat stress. Symptoms of heat stress will vary depending on plant type and maturity. In general, plant growth is slowed down because the basic plant functions of photosynthesis and transpiration are slowed down under high temperatures. However, plant respiration continues at higher temperatures which depletes the plants of carbohydrates. This is why a prolonged period of high temperatures can cause plants to die from emptying their food reserves.
Symptoms of heat stress include leaf rolling and cupping, wilting, and dry leaf edges. However, there are other symptoms of high heat you may not be aware of:
⦁ Blossom and fruit drop: tomatoes, peppers, eggplant, beans and summer squash can experience flower abortion brought about by hot day and night temperatures. You may notice flowers on the vegetable plants, but no fruit is set. This goes back to the plant depleting its energy stores and not having enough energy to set fruit, or it may be due to the next reason.
⦁ Poor pollination: Bees (and other insects) often take it easy on hot days and pollinate less. This can be pronounced in crops such as cucumbers and squash crops that require pollination for good fruit set. In high humidity conditions, pollen is heavier and stickier, and not as easily blown by wind. Poor pollination also results in misshapen or deformed fruit.
⦁ Ripening issues: tomatoes can have “yellow shoulders” due to poor fertility, but heat is another factor. The production of the red pigment in tomato (lycopene) stops at temperatures over 85°F. Heat can also cause the inside of the tomato to have areas of white flesh even when ripe, referred to as “gray wall.”
⦁ Leaf scorch: This condition is often seen when there is a combination of high heat, low soil moisture, and wind. It is first noticed along the outer edge of the leaves and looks like dry, crinkled plant tissue that eventually turns brown. Leaf tissue that is scorched will not recover. As weather conditions improve, the plant will produce new growth and scorched growth may be removed if desired.
⦁ Sunburn/sunscald: When the fruit surface becomes heated beyond the air temperature, you may notice a discolored or bleached spot on the exposed surface. Good leaf cover can help to reduce sunscald by keeping the fruits shaded. Many crops can suffer from sunscald: melons, peppers, tomato, cucumber, and even raspberries can show white drupelets under heat and sun exposure.
⦁ Bolting: Premature flowering of plants before they are harvested is known as bolting. This is the plant’s defense mechanism to produce a flower stem and reproduce. This is also known as “going to seed.” It is more common in cool-season vegetables such as lettuce, spinach, broccoli and cauliflower, and cilantro.
While we cannot eliminate heat stress in garden plants, we can reduce it. Sufficient water is important during dry, hot spells. In the absence of rain, aim for at least 1” of water per week. In general, 1-3 deep waterings each week is better than frequent, shallow waterings. Container gardens and young transplants may require watering every day until their root systems become established. Early morning watering or early evening watering is a good practice, and even better is using a soaker hose or drip irrigation to target water directly at the roots and reduce evaporation.
Providing temporary shade can reduce temperature and sunscald on your plants. Use tomato stakes or bamboo stakes to provide structure to drape row cover or shade cloth during prolonged times of heat. Securing a beach umbrella over plants struggling in the heat can provide temporary relief. However, do not allow the shade cloth to touch the plants to avoid causing leaf burn. Using mulch such as straw can help to reduce surface radiation and conserve moisture. If you have young plants ready to transplant, wait until the heat has passed. The hot soil can damage the stem at ground level. Furthermore, transplanting is stressful on plants without the added worry of high heat. Another tip for gardeners who use synthetic fertilizers and fungicides, or insect sprays is to postpone these applications until the weather cools down. Some sprays can cause a phytotoxic response in plants, and using fertilizer during this time is not recommended. Weeding should not be postponed in high heat. Weeds compete with crops for nutrients and water, and some weeds can harbor insects and diseases.
