Oct 13, 2016

Protect late-season vegetables from freeze damage

Depending on crop tolerance, a killing frost can result from canopy temperatures dropping 2-5 degrees below freezing for 5-10 minutes, or from a sustained temperature 31.5–32 degrees Fahrenheit lasting 3-5 hours. Fall vegetables have a range of temperature tolerances, reflecting their area of origin. Vegetables that come from flowers, such as vine and solanaceous crops, okra, sweet corn and beans, have largely been cultivated and bred from tropical and subtropical plants, and are easily damaged by a light frost (28-32 F).

When freezing occurs, water expands and can burst cell walls. However, leaf and root vegetables are generally more capable of withstanding hard frosts (less than 28 F), and have more room to spare in their tissues for water expansion and internal ice-formation.

Hard frost hardy (less than 28 F)

• Collards
• Endive/escarole
• Kale
• Kohlrabi
• Lettuce
• Mustard
• Onion (sets and seeds)
• Pea
• Potato
• Rhubarb
• Rutabaga
• Spinach
• Turnip

Light frost hardy (28–32 F)

• Beet
• Broccoli
• Cabbage
• Carrot
• Cauliflower
• Celeriac
• Celery
• Chard
• Onion (plants)
• Parsnip
• Radish

Light frost susceptible (28–32 F)

• Cucumber
• Edible beans
• Eggplant
• Muskmelon
• Okra
• Pepper
• Pumpkin
• Squash, summer/winter
• Sweet corn
• Sweet potato
• Tomato
• Watermelon

How to tell if you have frost-damaged vegetables

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. The marketable part of the plant may also show signs of damage.

The list below is adapted from Purdue Extension Bulletin HO-203, “Effects of Cold Weather on Horticultural Plants in Indiana,” and describes what to look for in freeze-damaged vegetables. For positive identification of suspected freeze damage, contact your regional Michigan State University Extension educator.

• Beet: 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.
• Carrot: 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.
• Cucumber: 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.
• Onion: Thawed bulbs are soft, grayish-yellow and water-soaked in cross section; often limited to individual scales.
• Pepper: Dead, water-soaked tissue in part or all of pericarp surface; pitting, shriveling and decay follow thawing.
• Potato: Freezing injury may not be externally evident, but shows as gray or bluish-gray patches beneath the skin. Thawed tubers become soft and watery.
• Pumpkin: Water-soaked spots on upper surface of fruit which soften the rind. Badly damaged fruit will eventually collapse in on itself.
• Radish: 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 potato: 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.
• Tomato: Water-soaked and soft upon thawing. In partially frozen fruits, the margin between healthy and dead tissue is distinct, especially in green fruits.
• Turnip: Small, water-soaked spots or pitting on the surface. Injured tissues appear tan or gray and give off an objectionable odor.

Methods for protecting frost-sensitive crops

Depending on what materials are available, as well as what crops are being protected, there are several options growers can use to extend the productive season. Commercial growers often rely on passive or heated high tunnels, greenhouses, hoop houses or cold frames to offer several degrees of protection for light-frost susceptible crops in the fall. These structures can also be used to protect hard and light frost hardy crops deeper into the winter months, long after the internal temperatures have dropped below what is appropriate for light frost-susceptible crops.

Commercial growers and home gardeners also rely on floating row covers and other protective covers as a low-cost way to protect sensitive crops from frost. These covers are supported above the crop using wire or metal hoops, or bent PVC hoops. Material edges are commonly weighted with sand bags or simply buried with soil to prevent loss due to wind. Lightweight covers come in varying sizes and weights, providing different levels of frost protection.

The table below provides a few examples of row cover options that provide frost protection. It must be noted that as covers grow heavier, the light transmission drops, meaning less photosynthetic activity will occur unless covers are removed. A notable exception is that of greenhouse film (plastic), which provides significant frost protection while still allowing substantial light transmission. The primary drawback, however, is that this material is not self-venting, meaning growers must remove the cover on sunny days to prevent overheating.

Hoop house and greenhouse structures can be more effective when used in conjunction with interior floating row covers. This double layer of protection creates a microclimate at plant level that can be significantly warmer than exterior temperatures. In areas with relatively mild winters, a lightweight row cover can be effective and does not need to be removed for ventilation or to allow solar exposure. In colder climates, multiple layers of lightweight covers or heavier covers can be used to protect cold-hardy crops throughout the winter months. These covers are typically removed on sunny days to warm the soil, allow plants to thaw or photosynthesize, as well as ventilate and exchange air to discourage disease.

For further reading on low-cost season extension options for commercial growers and home gardeners, visit the MSU North Farm’s Resources page, including the Low-Cost Season Extension Skill-Seeker workshop presentation.

— Ben Phillips and Collin Thompson, Michigan State University

Source: Michigan State University Extension