In response to last week’s “Gardening Corner,” fellow Master Gardener Kathy Gierlak, emailed me about winter sowing, a method she likes to use:
“Many native plant seeds need cold, moist stratification to sprout. With this method you plant seeds in plastic milk jugs (I happen to use the clear plastic salad boxes) and just put them outside for the winter. You put drainage holes in the containers so water can get in and out. The containers act as mini greenhouses, the seeds germinate and are protected from animals until it’s time to plant. There’s a lot of info online about this. Easy to do and I like repurposing my salad boxes! I have raised some of the plants that I have donated to our [Master Gardener] plant extravaganza this way.”
Many perennial, biennial, and annual flower and vegetable seeds require stratification, or exposure to cold, moist conditions in order to break down the hard seed coat and allow the embryo to emerge from dormancy. In nature, seeds with a hard coat undergo stratification in the soil during the cold, wet winter months from the constant freezing and thawing of the soil.
Stratification can involve mechanically abrading the seed coat with a file or sandpaper; soaking the seeds in tepid water; or placing them on a moist paper towel in a plastic bag in the refrigerator until they begin to sprout.
Winter sowing is a simple, inexpensive technique that mimics the natural process of stratification in the soil, but offers some protection for the germinating seeds from harsh winter conditions; also, you avoid having plastic bags of potting soil in your refrigerator all winter.
The equipment and supplies necessary for winter sowing make use of items most of us have around the house: empty 2-liter soda bottles, clear or translucent 1-gallon milk jugs or juice bottles, and take-out or salad containers with clear plastic lids. You can convert these items into miniature greenhouses.
Some gardeners prefer the translucent milk jugs because they filter the sunlight and prevent too much heat buildup in case of a stretch of warm, sunny weather. Other gardeners like clear plastic or have no preference. The same holds true with whether to use the jug or bottle cap or discard it. One source mentioned that leaving the cap off allows rain to enter and excess heat to escape on warm days. A second source prefers to keep the container capped, removing it when the days grow consistently warm.
Cut carefully around the middle of a jug just below the handle, leaving a ½ to 1 inch corner section intact as a hinge. Punch several drainage holes in the bottom of the container using a Phillips screwdriver. Heat the tip of the screwdriver with a lighter to make punching through the plastic easier. Cut a few slits in the top of the jug or bottle for ventilation. If you use plastic salad containers, follow the same procedure.
Place 2 to 3 inches of lightweight potting media, such as half-and-half peat moss and perlite into the bottom of the container. Moisten, but don’t drench, the potting mix. Sow your seeds and pat them gently into the soil. Cover the seeds with soil to the recommended depth.
Tape the containers shut with duct tape, and set them outside in a sunny spot that receives rain and snow, but is protected from harsh winds. Don’t forget to label your containers.
The natural winter cycles of freezing and thawing will gradually stratify the seed coats, allowing them to germinate. Look for tiny seedlings as the days begin to grow warm, but the nights are still cold. On days when the temperatures are above freezing, you can open the containers for a few hours, and water the seedlings if the soil is dry. Retape the containers before nightfall. As the nights grow warmer, you can leave the containers open to the air. When your seedlings are ready to plant, doing so on a cloudy day will reduce transplant shock.