Managing row covers to avoid heat injury and poor pollination
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Row covers are used as a means to modify the microclimate around the crop. Attempts to modify the environment for enhanced crop production dates back to the 16th century. Since the early 1960s, the introduction of plastics in agriculture helped develop new and efficient row cover technologies for open field vegetable production. Row covers are used for a number of reasons such as, season extension (frost protection), insect exclusion, heavy rain and hail protection. Regardless of the primary goal of the row covers, their management should help avoid excessive heat that can injure the crop and also avoid poor pollination that may reduce yield.
Row covers and heat injury
When using row covers, always keep in mind that they are more efficient at increasing temperature, especially during a sunny day, than protecting against frost. Even in the absence of a frost risk, some growers may consider using row covers because most warm season vegetables stop growing at temperatures below 40-50oF. A row cover may increase the temperature enough to promote the growth of these warm season vegetables.
When outside temperatures are high, it is recommended to remove row covers. Temperatures inside row covers can get extremely high and injure crops, especially when row covers without perforations or holes are used.
Under a sunny day with a calm wind, we have observed about 20 to 30 degree temperature increases inside row covers when compared to outside. Depending on the outside temperature, this extreme heat can easily damage certain crops like tomato. Under those conditions, it is recommended to open the row covers for ventilation.
Row covers and pollination
Many crops require insects, especially bees, for pollination. In cucurbits for example, there are separate male and female flowers. For adequate pollination and fruit set, the pollen needs to be moved from the male flower to the female flowers. Row covers present physical barriers for insects including bees. Even when the row covers are perforated, most insects normally stay outside. In a recent study conducted by our team, we observed a significant reduction in slicing cucumber yield when the row covers were removed two to three days after appearance of the first flowers. It is important to know that flowers of most cucurbit crops remain open for only 24 hours. It is therefore important to remove the rows covers before cucurbit flowers open to facilitate pollination and proper fruit set.