Extend the Season in Your Vegetable Garden
Many of us grow fall greens such as collards, turnip greens, and lettuces to harvest as the season cools and transitions into winter. By selecting appropriate plants varieties, selecting the appropriate site, and using cold protection, you can extend your harvest, help crops survive the winter, and harvest earlier in spring. Many greens and roots will get sweeter with cooler temperatures, making cool-season crops an enjoyable addition to the table.
Season extension begins with selecting the appropriate species and most cold-hardy varieties available. Spinach and arugula are among the most cold-hardy greens. They can often survive periods when outside temperatures into the single digits. Other greens that may be suitable for cold-season harvest include lettuces, kale, collards, and mild mustards. With these greens, it is important to select those varieties bred for cold-hardiness. This will improve their ability to survive deep freezes and increase the harvestable yields. Of the kales, Red Russian and Winterbor are more cold-hardy. Mustards such as Chinese Thick Stem, Osaka Purple, and hardy Pac Choi are excellent candidates for winter production. Lettuces are often bred for tolerance to heat or cold- select the cold hardy types for fall and winter growing. Some examples include Winter Density, Forellenschluss, Rouge D’Hiver, and Schweitzer’s Mescher Bibb.
Parsley, cilantro, dill, and chervil are good herbs for the cool season. Root crops such as turnips, beets, parsnips, and carrots can tolerate light frost without damage to the roots. However, some will also establish them in fall and leave them in the ground to harvest throughout the winter. Protect them with a layer of straw mulch or row cover to protect the roots once hard frosts and freezes set in, and remove to harvest as needed. You can also grow less hardy varieties for early fall harvests. Plan to provide them with ample protection if you hope to harvest beyond regular frosts and freezes.
It is important to establish your winter crops early enough so that they are well-established before cold temperatures set in. Plan to sow or transplant at least 4-6 weeks before frost, or even earlier if you desire a longer harvest period during the fall.
Choose a well-drained, south-facing sunny site. Light conditions are limited in winter, so your goal should be to maximize the light available. Where possible, plant in a warm microclimate such as next to a brick wall with a windbreak to the north. Keep plants watered but avoid overwatering during periods of cloudy weather. If you plant a winter garden each year, rotate crops or sites to break disease cycles.
While many cold-hardy crops survive cold temperatures, the harvestable portion may be damaged in frost and freeze events. Gardeners can protect them through a few different techniques to preserve the harvest and protect crops in very cold conditions.
Low tunnels are made from polyethylene or fabric covers covering a series of hoops. The covers are secured to the ground with different weights or fasteners. Cover materials vary in weight and light transmittance, as well as management approach. Polyethylene covers act like a mini-greenhouse, providing good frost protection and the best light transmittance. However, they do not allow water penetration. Gardeners must remove them to water plants regularly. They can trap lots of heat on sunny days, which can burn leaves and damage plants, so may need to be removed or opened on sunny days and especially warmer sunny days.
Cold frames are a glass or polyethylene ‘sash’ set on an angled frame. Polyethylene and glass structures offer good light transmittance and frost protection but must be removed or opened for watering and on warm, sunny days. When placed on top of straw bales as support, the straw will offer insulation for the plants as well.
Row covers, sometimes called Remay, are made of fabric of varying weights. Like the flexible polyethylene or plastic covers, these are placed on top of hoops which elevate the cover above the crop canopy to keep it from touching the plants. Lightweight covers provide little frost protection- often up to 4 degrees F- but have better light transmittance. Heavier covers provide more protection but block more light. If you invest in lighter covers, you can use a double layer for additional cold protection.
Secure cover edges and minimize holes to keep wind from lifting and ripping the cover. Simply remove the weights and lift when you want to harvest. Remove them during warmer days and periods to allow more light to reach the plants and promote hardiness among the plant tissue.
Row covers do allow water to penetrate, and they can hold in some moisture, which can be beneficial during the germination period. In addition, lightweight covers can be used to protect young plants against insects as they get established in late summer.