Frost and Freezing in Fall Vegetable Crops – Part 2

Vegetable crops can be protected from late-season freeze events.

 

Wind machines and heaters are expensive pieces of equipment used by large commercial orchards to combat spring frosts and freezes, but may not be economical for some vegetable growers. Luckily, many of the same techniques for extending the early season can be used for extending the late season for these crops.

A “water-soaked” appearance is a
common identifier of freeze damage on
fruiting vegetables. These decorative
gourds will still harden off, but
the discoloration is permanent.

How to tell if you have frost-damaged vegetables

If you see that your land has some frost on it, it’s best to check the plants and fruit twice after the sun warms them up. Freeze-killed leaves will at first turn brown and look somewhat transparent as they thaw – a term generally referred to as “water-soaked.” Once dry, they may curl up and become brittle. Check once more well after thaw to see if more leaves have turned brown as this is a sign that the vascular tissue of the plant was freeze-damaged as well. After thaw, the marketable part of the plant may also show signs of damage if it received any.

A “water-soaked” appearance is a common identifier of freeze damage on fruiting vegetables. These decorative gourds will still harden off, but the discoloration is permanent. Photo credit: Ben Phillips, MSU Extension

The list below is adapted from Purdue Extension Bulletin HO-203 and describes what to look for in freeze-damaged vegetables. For positive identification of suspected freeze damage, find an expert like a Regional Horticulture Specialist with the University of Missouri. Taking a photo and sending by e-mail is a real time saver these days.

•Beets: External and internal water-soaking; sometimes blackening of conducting tissue.

•Broccoli: The youngest florets in the center of the curd are most sensitive to freezing injury. They turn brown and give off strong odors upon thawing.

•Cabbage: Leaves become water-soaked, translucent and limp upon thawing; epidermis separates.

•Carrots: Blistered appearance, jagged length-wise cracks. Interior becomes water-soaked and darkened upon thawing.

•Cauliflower: Curds turn brown and have a strong off-odor when cooked.

•Celery: Leaves and petioles appear wilted and water-soaked upon thawing. Petioles freeze more readily than leaves.

•Cucumbers: Transparent, water-soaked appearance in cross section, just under the skin.

•Garlic: Thawed cloves appear grayish-yellow and water-soaked.

•Lettuce: Blistering; dead cells of the separated epidermis on outer leaves become tan; increased susceptibility to physical damage and decay.

•Onions: Thawed bulbs are soft, grayish-yellow and water-soaked in cross section; often limited to individual scales.

•Peppers: Dead, water-soaked tissue in part or all of pericarp surface; pitting, shriveling, and decay follow thawing.

•Potatoes: Freezing injury may not be externally evident, but shows as gray or bluish-gray patches beneath the skin. Thawed tubers become soft and watery.

•Pumpkins: Water-soaked spots on upper surface of fruit that soften the rind. Badly damaged fruit will eventually collapse in on itself.

•Radishes: Thawed tissues appear translucent; roots soften and shrivel.

•Squash: Water-soaked spots on upper surface of fruit. Ornamental and winter squashes may still harden, but others will soften and rot.

•Sweet corn: Reduced ear size and weight with shriveled kernels. Ears can take a “bar-bell” shape if they are still developing.

•Sweet potatoes: A yellowish-brown discoloration of the vascular ring and a yellowish-green, water-soaked appearance of other tissues. Roots soften and become very susceptible to decay.

•Tomatoes: Water-soaked and soft upon thawing. In partially frozen fruits, the margin between healthy and dead tissue is distinct, especially in green fruits.

•Turnips: Small water-soaked spots or pitting on the surface. Injured tissues appear tan or gray and give off an objectionable odor.

Mitigating freeze damage on vegetables

One of the amazing properties of water is that as it freezes, it releases a small amount of heat to its surrounding environment. So, one active measure that can be taken to reduce freeze damage on plant tissue is to apply consistent canopy irrigation at a low spray rate to keep a light film of water on the plants during the time of freeze potential. As the water freezes, it will keep the plant tissue from dipping below the freeze point.

A preventative freeze method can be to irrigate the soil around the plants during the day to absorb heat from the sun. This works best in dark-colored, low-porosity soils with high water-holding capacity (something positive regarding Missouri clay for once!). The heat held in the water between soil particles will slowly radiate up into the atmosphere, keeping the plants above freezing overnight. Further, supported or floating row covers made of clear or black polyethylene or polypropylene plastic can be used to create an insulated barrier overnight, which can protect vegetables down to 28 degrees Fahrenheit, depending on the weight of the row cover.

However, important research in strawberries offers some valuable crossover information. In a trial across 20 freeze events in Florida, strawberry fields treated with a combination of canopy irrigation and row covers experienced a higher survival of flowers and greater fruit production than fields treated with just canopy irrigation or row covers alone.

References

•Effects of Cold Weather on Horticultural Plants in Indiana, Purdue University Cooperative Extension Service

•Understanding Frost (pdf), Cornell Cooperative Extension

•Frost protection: fundamentals, practice and economics (pdf), Food and Agriculture Organization of the United Nations

•Irrigation Method and Rowcover Use for Strawberry Freeze Protection, Journal of the American Society for Horticultural Science

•Row Covers for Commercial Vegetable Culture in Florida (pdf), University of Florida Extension

(This article was published by Michigan State University Extension. Adapted by James Quinn for University of Missouri, October 2013)