Extend the harvest with simple frost protection

Fall’s warm days and cool nights boost cool-season veggies. Learn how to protect crops to extend harvests and grow fresh food into winter.

The warm sunny days and cool nights of fall are excellent for growing vegetables. With protection, many cool-season crops can be grown well into winter. Simple tools such as floating row covers, cold frames and plastic-covered tunnels extend the growing and harvesting season so you can enjoy fresh food all winter. These systems vary in cost and the amount of protection they provide. Likewise, different crops require different levels of protection. Match your frost protection strategy to different crops and group plantings accordingly.

Frosts and Freezes

Some vegetable crops are sensitive to frost damage, while other crops benefit from freezing temperatures. The flavor of kale, for example, is improved by a light frost and the sharpness of horseradish matures only after several hard frosts. Cool season crops can be grouped based on their frost tolerance:

Hardy vegetables tolerate frosts below 28°F: spinach, onions, garlic, leeks, rutabaga, broccoli, kohlrabi, kale, cabbage, collards, Brussels sprouts, arugula, radish, mustard greens and turnip.

Semi-hardy crops tolerate frosts between 28–32°F: beets, carrots, parsnip, lettuce, chard, Chinese cabbage, endive, radicchio, cauliflower, parsley and celery.

Although many root crops die back to the ground after a freeze, the roots themselves are protected below ground. These can be left in place for winter storage and harvested as needed. Mulch rows for extra insulation.

Root crops: Beets, carrots, parsnip, rutabagas, turnips, radishes and Irish potatoes.

Protecting Crops with Row Covers

Late fall and early winter in Oklahoma tend to be mild, with occasional frosts interspersed with cool, sunny days. Keep crops like radishes, brassicas and leafy greens productive by covering them when freezing weather is expected.

Floating row covers are the cheapest option for season extension but provide the lowest level of protection. These products, made from spun polyester or polypropylene fabric, are called “floating” because they are laid directly over plants without a support system. Floating row covers allow water, sunlight and air to pass through, and don’t have a large effect on temperature. However, they do protect crops against light frost by slowing the formation of ice crystals and trapping heat being released from the soil.

Low tunnels are more expensive to build but provide better heat retention, allowing gardeners to not only extend the harvest season, but grow vegetables and herbs well into winter. Low tunnels are essentially miniature greenhouses that utilize a structure to support sheets of clear plastic over vegetable rows. They rely on the sun to warm that air and soil during the day, which is trapped inside by the plastic covering.

Cold frames offer a third option, priced somewhere between floating row covers and low tunnels. They also function like a miniature green house and can be built from scrap material to cut costs. Cold frames are basically a box with a transparent cover (called a sash), commonly made from an old window or storm door. They are often built into the ground to provide added installation, with the sash angled toward the southern sun.

Floating row covers are easy to remove and replace according to the weather, but cold frames and plastic tunnels require a bit of management. On warm, sunny days, heat builds up inside these closed systems. Be sure to include a system for venting cold frames (prop open sash) and low tunnels (roll up sides) as part of your design. Winter crops also need irrigation, so plan accordingly. All these products can be used to start crops earlier in spring and protect against late spring freezes, providing even more opportunity for season extension.