How to Overwinter Plants Outdoors
As we march deeper into the cold, it’s now time to turn attention to cleaning up and putting the garden to bed. There are dead plants to clean up, soil to amend, cover crops to sow, garlic and flower bulbs to plant… but the first project to tackle in this area is to prepare crops that can survive the winter outside for the cold months ahead. The toughest crops will happily spend the winter outdoors and regrow next spring for eating or seed production. A few are ready as they are, others need a little help. Here are a few suggestions on how make sure they are ready:
Vegetables: Cool-weather-loving crops like broccoli, Brussels sprouts, cabbage, and cauliflower don’t mind a few frosts. A few frosts will concentrate plant sugars in the leaves and spears and they will use warm days to grow, albeit slowly. Use a row cover to add a few degrees of warmth and to extend the season even further. These crops won’t, however, make it all the way through winter. Harvest for as long as possible, then remove and send to the compost pile.
Parsnips, carrots, garlic, scallions, and hardy greens (such as kale, collards, lettuces, spinach, and mache) are winter survivors. They not only taste sweeter after a few frosts, but can be harvested on an as-needed basis through the winter and some into next spring. Protect them with six to eight inches of mulch to prevent the soil from heaving. Hardy greens benefit from a row cover with hoops, a plastic cover, or a cold frame, which will keep them warm and growing (slowly) and will keep snow off for easy harvesting.
Perennial vegetables and herbs: Asparagus, rhubarb, and other perennial vegetables as well as perennial herbs such as chives, sage, thyme, oregano, tarragon, chamomile, arnica, anise hyssop store energy in their roots over the winter and grow anew from the base each spring. So, after foliage dies back in late fall, cut these plants down to a couple of inches from the ground or leave them until spring (some beneficial insects benefit from an "untidy" garden). If you can, top-dress perennial beds with compost, manure, and mulch – the nutrients will seep into the soil over the winter and be accessible for the plants in spring.
Flowers: Now is also an ideal period for sowing flower varieties whose seeds require cold stratification before germinating. Poppies, Milkweed, Johnny Jump-ups and Echinacea are all flowers that need a winter-like environment before germinating.
Biennial seed crops: Some plants take two seasons to produce seed. In our region, the majority of biennial seed crops need to be taken out of the ground and stored over the winter to keep them warm enough for the next growing season, but hardier varieties like parsnips and leeks can overwinter right in the ground. Tucking them in for winter with mulch or straw and putting a row cover over them will help them survive the coldest periods. And, snow will actually help insulate them. For more information on growing and storing biennials for seed production, take a look here.
Potted plants: Most container plants will need to be brought inside in order to make it through the winter because containers keep the roots more exposed to cold than plants in the ground. However, there are a few tricks for keeping potted plants bundled up and warm enough outdoors. For a thorough overview on overwintering plants in containers (and great suggestions for ornamentals), we recommend reading Margaret Roach’s, A Way to Garden blog, a wonderful gardening resource.