Successful Winter Seed Sowing
Posted: January 14, 2017
What is “winter sowing”? Winter sowing is a method gardeners might choose if they want to get a head start on gardening by sowing seeds during the winter. Winter sowing is for gardeners who don’t have the space or lighting available to sow seeds indoors.
Instead, this method uses recycled containers which act as miniature greenhouses to sow seeds of plants that would normally survive winter either as perennials or as reseeding annuals. The containers protect the seeds so more survive to germinate in the spring. They receive the same weather and temperature as those directly sown in the ground. The uncapped spouts allow rain and melting snow to keep the soil moist.
Generally, only a few varieties of seeds can be successfully sown indoors this early. But one can begin winter sowing any time after the first day of winter and as long as the nights are still chilly.
To determine which seeds can be winter sown, most plant descriptions will have notations about a seed's germination requirements or will have a few clue-in phrases such as: pre-chilling, freeze, refrigerate, stratify, colonize, self-sows. In addition, look for terms like “sow outdoors in early spring or while frosts may still occur”, “sow early autumn”, “hardy”, “withstands frost”, “direct sow early”, “wildflower” or “weed” in the name. Also search for common names indicating a natural environment such as plains, mountain, field, river or an origin in a temperate climate such as Siberian, Orientale, Canadensis.
Recommended containers are recycled clear or opaque containers such as plastic milk or water jugs, two-liter soda bottles, deep foil pans with domed clear lids, and quart dairy containers. Any of these can be used keeping in mind two important requirements: if a container is tinted plastic, your hand must be visible through the plastic for sufficient light, and it must be deep enough to allow 3-4” of potting soil plus additional headroom for the seedlings.
Throw away the caps from the jugs and bottles and remove labels. Wash the containers in hot soapy water, rinse and drain. To convert a two-liter soda bottle or milk jug into a miniature-greenhouse, use a utility knife to cut a horizontal slit half way up the side. Then use scissors to cut the container in half, but leave a 1" hinge. To provide drainage, place the bottle on a cutting board so your work surface isn’t marred. Use a pointed object to puncture 3/4-inch diameter holes in the bottom of each container.
Another very important step is labeling. One can use vinyl mini blind slats cut into 5-6” lengths, but make sure they are lead-free. Using a permanent marker or china pencil, write the seed’s name, the mature size and planting instructions.
Use packaged potting soil marked as "soil-less". Do not use soil mixes that are prone to harden, states "weed-free", contains fertilizer or water crystals. Pre-moisten with warm water until the soil is slightly damp so the soil will wick up the water. Fill containers with 3 to 4” of soil, then tamp the container softly on a hard surface to settle the soil and add more if necessary.
Sprinkle seeds directly on top of the soil and gently press down into the soil, making sure there is good seed-to-soil contact. For large seeds, which require a depth of an inch or more, press the seeds into the soil just below the surface, sprinkle soil over the seeds to just cover and lightly press down. Insert the label inside the container along the side.
If your container has a clear lid, poke 4 to 5 holes in the lid to provide air transpiration. If the lid is not clear, cut out the inside of the lid rim to create a holder for clear plastic or cover with clear plastic and tape the plastic to the sides of the container with duct tape. Puncture 4 to 5 quarter-inch holes in the plastic.
Tape the top of the milk jug or soda bottle to the bottom with a 4-inch piece of duct tape. Because these are miniature greenhouses, it’s possible to overheat the seeds if the container is not properly vented. To water your greenhouse, set the container in several inches of water overnight to soak up moisture.
Take the container outside to a protected area such as a deck, uncovered porch, against a fence or in a flowerbed. Don't put the containers against the house or under the eaves. Sunny locations work best so the air inside the containers warms up enough to create condensation. Condensation is the indicator of sufficient moisture. If the containers do not show condensation on a sunny day above 32°F, water the seeds by dripping water slowly down the sides of the container or setting the containers in water. If condensation still does not show, the ventilation holes in the lids may be too big. Try taping over some of the holes. If your containers are holding too much water, the holes in the bottom may be too small, and you will need to enlarge the holes.
The hardest part is waiting. When the weather warms, the containers will thaw and freeze repeatedly as winter gives way to spring. This action of freezing and thawing helps loosen the seed coat. Amazingly, just when winter is about to break and you’re still getting nightly freezes, the first signs of germination will appear. The seeds know when it's safe to germinate because it's part of their genetics.
As your seedlings grow and the temperatures rise, make slits in the tops of the soda bottles/jugs or widen the slits in the covers. When you notice “true” leaves, make the slits a little bigger once every week or two depending on the temperatures until there are more open areas than covered. Now you can transplant the seedlings into your garden because they are also hardened off!
The main requirements for outdoor Winter Sowing are selecting the appropriate seeds for your plant zone, and providing both adequate drainage and air transpiration. Follow these three principles and you will have success.
Prepared by Karen Brown, Penn State Master Gardener