Heat injury with plastic mulches

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.

Between May 28 and 29 the weather favored a clear and sunny sky with maximum temperatures exceeded 90°F in many parts of the state. These conditions in early season are conducive to heat injury with plastic mulches because the crop canopy is not yet large enough to cover an important portion of the mulch. Black plastic mulches are widely use in vegetable production to control weeds and to warm the soil in early season. However, under hot and sunny days they can cause significant injury to the crop. The injury is usually related to heat and is either direct or indirect or both.

Direct crop injury

Black mulches allow little light wave lengths to pass through. They first absorb the light that is then converted into heat. The heat is finally transferred to the soil. For greatest efficiency it is usually recommended to have close contact between plastic and soil. Under a clear and sunny day, the black mulch surface temperature can exceed 130°F. This results in injury or desiccation of most plant parts (root, stem, leaves and fruits) in direct contact with the mulch.

Indirect crop injury

Crop injury can still occur even if the crop is not in direct contact with hot plastic mulch. During a sunny day, hot air builds up between the plastic and the soil. If the plastic is loose, its flapping (even under a light wind) creates movement of hot air and funnels it through crop holes. This causes stem girdling and transplants desiccation, a problem commonly observed with young pepper transplants.

Implications for crop management

Before the injury: Reduce the risk. The risk of heat injury from plastic mulch can be minimized by improving the contact between the mulch and the soil. Plastic mulches are either slick (smooth surface) or embossed (with diamond-shaped pattern on the surface). Embossed films are preferred since they have reduced expansion and contraction under fluctuating temperatures that can loosen plastic from the bed.

Hardened transplants will tolerate heat injury better than young and tender ones. Crop holes on the plastic should be round, cut evenly and large enough so that there is no direct contact between the plant and the plastic. The transplant should be centered in the middle of the hole.

Infra-red transmitting (IRT) mulches maintain a cooler surface temperature than black mulches under hot and sunny days. But they are also more expensive than black mulches.

After the injury: Minimize the effects. Depending on the importance of the injury growers might consider replanting. If injury is extensive and occurs when the transplant is very young, replanting might be a good option. When replanting is not possible, it is better to minimize the effects of other stresses (drought, diseases, nutrients) on the plant. For peppers, the plants become very susceptible to wind damage and should be staked and tie as early as possible.

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

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