Recent conditions in some areas (soaked soil, fog- and dew-filled mornings, high daytime humidity) can give a different impression about the season so far than weather data at https://www.oardc.ohio-state.edu/weather1/ and various forecasts. Temperature, rainfall, and other data are collected around the clock at OSU vegetable (and other) research sites in Fremont, Celeryville, Wooster, and Piketon and have been for decades. So far in 2021, these four locations have accumulated less precipitation and more growing degree days (GDD) than their historical averages. Also, climate and weather authorities reported on June 11 that the Upper Midwest, including Ohio, is set to experience hot, droughty conditions. Most agree that a dry year is less problematic than a wet one — provided irrigation is possible. However, it can be difficult for vegetable growers to escape the unwanted effects of excessively high temperatures. A way to separate potentially minor, moderate, and severe heat stress, example effects of moderate-severe heat stress, and main strategies for mitigating heat stress during production are summarized below.
Five Major Factors Influencing Whether Heat Stress is Minor, Moderate, or Severe
- Crop and variety (sensitivity 1). All crops and varieties have a range of temperature in which they perform best. A crop’s genetic past (i.e., heritage/Center of Origin) and level of improvement through breeding matter. Individual crops and varieties are thought or proven to be relatively heat tolerant or intolerant.
- Timing (sensitivity 2). When high temperatures occur in the crop cycle is key. Crop plants can tolerate high temperatures more reliably at some stages than others. Even relatively tolerant varieties can be impacted by temporary spikes in temperature at the “wrong” time.
- Intensity. The extent to which actual temperatures exceed the crop’s and variety’s optimal range is important … 5 degrees? 15 degrees?
- Duration. The length of time the temperature was consistently above optimal. Short periods of intense stress can be problematic although the effects of prolonged moderate stress typically accumulate.
- Mitigation: were steps taken to lessen the stress?
Combinations of these five factors represent common scenarios. For example, for vegetables for which pollination is required, excessively high temperatures lasting only hours can disrupt pollination or trigger flower or fruit drop or interruptions in normal developmental patterns. The result can be loss of a “set” (dip in production) and/or malformed or misshapen units to be harvested (e.g., pods, fruits, roots, stems, leaves, tubers). Longer periods of above-optimal temperatures can speed (e.g., bolting) or delay (e.g., prolonged vegetative state) maturity depending on the crop and when they occur in the crop cycle. Heat stress is also implicated as a contributing factor in fruit ripening and physiological disorders (e.g., blossom-end rot). Above-optimal temperatures can also trigger changes in the chemical composition of plant tissues, possibly affecting the color and/or taste of marketable units. Similarly, prevailing temperatures can influence a crop’s tolerance to typical inputs and protectants.
Irrigation and shading are among the most common strategies for mitigating the effects of excessively high temperatures in field and high tunnel vegetable production. Irrigation is essential for the obvious reason that evapotranspiration is the crop’s primary means of cooling itself. A warm period or season calls for the best irrigation (scheduling) practices, not just pouring water on because, as we know, excessive irrigation (soil moisture) disrupts water uptake, compounding the heat stress problem. Circumstances allow some growers to shade the crop (e.g., in high tunnels) as they attempt to reduce the temperature around it.
At this time, 2021 has not earned the label as a “hot or heat stress” year. Let’s hope that remains true even as we remain aware of factors contributing to heat stress and ways of addressing it. In addition to proper irrigation, shading (if possible), and careful application of inputs and protectants, consider tracking variety performance closely to aid in variety selection going forward.