Multiple days of excessive heat can be hard on vegetable crops, especially ones known to be cool-season. Brassicas, lettuce, carrots and spinach are particularly vulnerable to bitterness, wilting, bolting, deformities and coloring that affect the quality of the produce.
Warm season crops like tomatoes and peppers are also at risk of poor flowering and pollination, poor nutrient availability, and overall plant stress in the heat which leads to poor growth, yield, and fruit quality issues in the future. While we cannot control the weather there are a few strategies growers can implement to help vegetable crops survive during these long dry summer days.
Here are a few strategies:
Consistent Watering Schedule: Watering at the right time is critical when plants are experiencing heavy heat. Not only will watering help cool the plant through uptake but it will also help keep the soil moist and cool. The best time to water is early in the morning. Plants will be well hydrated in the morning before the heat and sun. Regularly water your crops as needed. What is needed will depend on the prevailing temperatures, soil type, soil organic matter, and the size of the plants. When temperatures are high plants may need water at least once a day. Sandier soils need more frequent watering in small amounts to avoid water loss and nutrient leaching. As plants grow larger, their water needs also go up. An analog tensiometer (Irrometer®) or digital soil moisture sensor (Watermark®) is a good investment to know soil moisture and decide when to irrigate. Here are two videos that show installation and interpretation of soil moisture sensors:
https://www.extension.iastate.edu/vegetablelab/how-install-soil-moisture-sensor
https://www.extension.iastate.edu/vegetablelab/reading-and-interpreting-soil-moisture-sensor-data
Irrigating crops in the morning is preferred. Evening/night watering puts plants at risk for diseases, insects and pests as the foliage and roots will stay damp into the evening. Mid-day and afternoon watering is also not recommended as more evaporation will take place and much of the water will be lost before the plant takes it up. It is important to note, recently transplanted crops need to be watered frequently for a few days to help mitigate transplant shock. It is also equally important to avoid overwatering as this will prevent the roots from getting oxygen and put them at risk for diseases and pests. Also excess water will move fertilizer and nutrients below the root zone and will ultimately leach nutrients, especially N.
Shade cloth
A shade cloth is a lightweight cloth that is placed on a wire hoop frame several inches over the crop to help shade the plant. Shade covers help the soil and plants stay a few degrees cooler while also reducing the amount of moisture lost. Shade clothes are also great for newly transplanted crops that are particularly vulnerable to intense field conditions.
Shade clothes come in various densities that can block different levels of light. For most vegetable crops, a 30 to 50 percent shade cloth is recommended. It is important to note, the shade cloth should stay above the plant and not come in direct contact to avoid retaining heat and potential leaf burn. Shade clothes are particularly effective for cool season crops like broccoli, kale, lettuce, spinach, cauliflower, and swiss chard.
Mulching
Mulching is a tool used to keep soil temperatures down and retain moisture around the plant’s root zone. Straw, hay, grass clippings and other forms of organic matter are good options for mulching purposes. They also can be effective in keeping weeds down around the plants. Growers utilizing plastic mulch could use a white-on-black instead of black plastic mulch to regulate soil temperature. When compared to the black plastic, the white plastic mulch can reduce temperature by 2-4°C and also reflect additional light to the plant canopy.
Plant Heat Tolerant Crops and Varieties.
Breeders continue to advance varieties of cool season crops that show some heat tolerant traits. These varieties have been selected to tolerate heat and have fewer quality issues than others. Look for these traits when buying your transplants and seeds for the summer months. Some plants are naturally more heat loving than others. Sweet Potatoes, melons, eggplants, and cucumbers are all heat lovers that are good options for crops during the heat of the summer.
Authors:
Rachel Perry & Ajay Nair
Department of Horticulture
Iowa State University
rperry3@iastate.edu & nairajay@iastate.edu
-Acreage Living Newsletter, Iowa State University Extension and Outreach Small Farm Sustainability