Winter sowing is a technique for planting seeds outdoors in winter to give them a head start in spring. While the idea of starting seeds early is very old, winter sowing is becoming increasingly popular.
How Seeds Overwinter
Sowing seeds in the fall or winter is an old technique used worldwide. You may have tried it yourself in the vegetable garden, sowing spinach and corn salad (mache) in the late fall for harvesting soon after the snow melts. The seed lies dormant all winter and is ready to sprout the first chance it gets in the spring.
Any gardener who has ever cursed an effusive self-seeding plant knows that some seeds do very well when left outside in the cold all winter. You may even get a better germination rate than you would if you were starting the same seeds indoors. There are seeds that need to experience cold, damp conditions, either because they have hard shells that are softened by the freezing and thawing or because they are triggered by the change in temperature to sprout. This is called stratification.
Sometimes certain seed packets, like sage and salvia, will tell you to place your seeds in the freezer for a few weeks for quicker germination. The reason for this is to simulate stratification. You can make it even easier by simply winter sowing your parsley seed.
How Modern Winter Sowing Was "Invented"
The phrase "winter sowing" is attributed to Trudi Davidoff, a resourceful gardener who had more seeds than indoor space. Ms. Davidoff sows seeds in covered containers (she used take-out containers with foil bottoms and plastic tops) and then moves the containers outdoors. The containers act as mini greenhouses, allowing the seeds to experience the chill of winter in a controlled environment. When the temperature warms enough, the seeds germinate and start to grow on their own. By the time the soil in the planting beds has warmed, the seedlings are ready to transplant out.
This may all sound like common sense, but we can thank Ms. Davidoff for reminding us and calling it to our attention. The concept has caught on, thanks in part to her website Winter Sown, and in 2006, the USDA recognized the viability of the technique by adding the term to the National Agricultural Library Thesaurus.
You can either direct seed or start seed in containers to be transplanted later. Direct sowing will not give you as much of a head start as sowing in containers, because the soil in the ground will not warm up as fast as the soil in the plastic container. However, it will save you the time and effort of transplanting. Sowing in containers also helps protect the seeds from wildlife.
Winter Sowing Using Containers
Before getting started, collect your equipment:
- Recycled containers; some of the most popular include milk jugs, plastic liter bottles, and foil-and-plastic take-out containers (be sure to use a container that can handle the freezing and thawing caused by winter weather)
- Potting mix
- Scissors or Exacto knife
- A location where your plants can be safely kept outdoors
Choose seeds that are hardy. Seeds of tropical and tender plants will die in the cold. Some good choices for your first efforts at winter sowing include:
- Flowers: Alyssum, butterfly weed, calendula, coreopsis, cosmos, foxgloves, hollyhocks, petunia
- Vegetables: Beets, broccoli, cabbage, chard, carrots, kale, mache, radishes, spinach
How to Create a Greenhouse for Winter Seed Sowing
- Make sure you have drainage holes at the bottom of the container and air vents in the top covering. Keep the vents small, to begin with, and widen them as the weather warms in spring.
- Use at least 2–3 inches of potting mix, preferably 3–5 inches. This will help insulate the seeds and prevent them from being heaved to the surface.
- Sow more seeds than you think you'll need. Germination rates will be lower than if grown in ideal conditions. However, don't sow too thickly, or the seedlings will crowd each other out when they begin to grow.
- Label your containers. You think you will remember, but chances are good you will not.
- Many self-seeders can be sown in the fall for early spring germination or in very early spring for a later bloom. Starting your seeds at staggered times will give you a longer growing season.
- Expect to lose a few.
- Once the seeds have two sets of true leaves, it's time to transplant from the containers. Wait for the weather to warm, and gently separate your seedlings to plant in the garden, if you've started them in containers.