Managing microclimate in low tunnels for optimum crop performance

Now that the risk of frost damage has passed, low tunnels should be managed to avoid heat injury, tunnel removal shock and poor pollination.

With the risk of frost damage almost over for most growing areas, vegetable growers using low tunnels should know that appropriate management of the low tunnels remains critical for optimum crop performance. Low tunnels affect the microclimate around the crop with temperature being the most important factor. At this time of year, low tunnels should be managed to avoid heat injury, tunnel removal shock and poor pollination.

Avoiding heat injury

When outside temperature is consistently high, it is recommended to open low tunnels for ventilation. Temperature inside low tunnels can get extremely high and injure most crops. Under a sunny day with calm winds, we have observed about 20 to 30°F increase in temperature inside low tunnels when compared to outside. Therefore, even with an outside temperature of 70°F, the heat can easily reach 100°F inside clear, perforated, polyethylene low tunnels. The situation could be even worse if the plastic is not perforated (a practice not recommended at this time of year).

Avoiding low tunnel removal shock

Most growers are aware of the process of transplant hardening prior to field establishment as a practice to avoid transplant shock. Low tunnels create a greenhouse effect and provide near optimum conditions for plant growth in the field. Therefore, seedlings inside the low tunnels are tender and have fragile tissue. If the low tunnels are removed at once, these seedlings may undergo a stress similar to “transplant shock.” This stress hereby called “low tunnel removal shock” may result in significant damage to the crop. This damage will be exacerbated in situations of extreme weather conditions like high winds, heat, water stress and high solar radiations. To avoid removal shock low tunnels should be opened progressively. After the removal of low tunnels, crops like tomato and peppers should be staked immediately when they reach appropriate height.

Avoiding poor pollination

Many vegetables, especially cucurbits, require insects (mainly bees) for pollination. Low tunnels present physical barriers for insects including bees. Even when the low tunnels are perforated, most insects normally stay outside. It is therefore, important to open low tunnels before flowers are open to facilitate pollination and proper fruit set. A two- to three-day delay in low tunnel opening was shown to cause significant yield reduction in slicing cucumber.

Dr. Ngouajio's work is funded in part by MSU's AgBioResearch.

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