Spring in NJ this year has certainly been a whirlwind of conditions. While a few short weeks ago many were battling freezing temperatures, we now face extreme heat. Heat damage typically occurs when temperatures are sustained above 86℉ for long periods. With temperatures in the upper 90s and potentially exceeding 100℉ in some locations this week, you should take extra measures to protect your crops from heat damage. Newly planted transplants and young seedlings with shallow roots are particularly sensitive to extreme heat.

Symptoms of heat stress and heat damage including wilting, cupping of leaves, scorching on leaves and stems, and leaf drop.

Heat damage on romaine lettuce. Photo: RF Smith and MD Cahn, U of CA

Increasing irrigation is the best mitigation for most systems. Evaporative cooling allows plants to release heat in the form of water vapor through open pores or stomata on their leaves. When plants are low on water, they will close their stomata to retain turgidity, which in turn prevents evaporative cooling. To compensate for this, increase irrigation to prevent desiccation.

Planting on black plastic mulch should be avoided in the summer, as it will cause overheating. White plastic or metalized mulches are effective alternatives to control weeds, increase soil water retention, and reduce soil temperature. If black mulch is used, a foliar application of kaolin clay can be used as a reflective layer. Work conducted by R.J.G. Taveras at Virginia Tech found that applying 20-40 lb/A of Kaolin clay could maintain soil temperatures 10℉ cooler than black plastic alone.

Shade cloth can be placed over stakes or hoops to reduce soil and air temperature. Work by Dr. Emmalea Ernest at the University of DE found 30% shade cloth optimized benefits of cooling without reducing sunlight too drastically for the majority of vegetable crops.

Finally, avoid fertilizing or applying pesticides when temperatures are consistently above 86℉ to prevent burning the crop. Consult the product’s label for more specific guidance.

Contact your County Agent with concerns or questions. Stay cool!