In the Garden
Ralph Purkhiser, Purdue University Master Gardener
Winter sowing sites have taken over my computer! Many of the posts tout winter sowing as a new phenomenon is the gardening world. Nothing could be further from the truth. It takes only a few minutes of reading the posts to determine that this very helpful planting method is misunderstood by many.
Winter sowing is not the same as greenhouse or hotbed sowing. Both of those methods are intended to provide a modified environment by using some form of heat, thereby causing seeds to germinate earlier than they would if planted outside. After germination, a heated environment is necessary to keep the young plants alive.
Winter sowing is also not the same as starting plants in your house in a sunny window or under grow lights. These methods are also used to produce transplants for the garden earlier than one could by sowing the seeds directly in the garden in the spring.
Winter sowing will not produce transplants for early planting. Instead, it is meant to provide a more controlled environment and to isolate seedlings that would be sprouting when nature warms enough for germination.
In a natural setting, a plant produces seed at the end of a growing season. As the weather cools in fall and winter, those seeds drop to the ground or are carried by birds and other wildlife. Some of those seeds germinate in the cool weather of autumn, but many seeds lie on the soil until the following spring. The seeds of many plants actually require a cold period before they will germinate. Winter provides that cold period, and when the weather warms in the spring, the seeds germinate and the new plants grow. Of course, not every seed becomes a new plant. Some are eaten my wildlife. Some do not make good contact with the soil and may dry out too much to germinate. Some may be blown onto pavement or other inhospitable environments. Rains may wash away some seeds, and some may be trapped under huge piles of leaves. Winter sowing simply removes many of these variables.
The best candidates for winter sowing are the plants that naturally self-sow. If you purchase seeds for coneflowers, milkweed and other wildflowers, the packet may instruct you to chill the seeds in the refrigerator for several weeks. You may skip that step if you are winter sowing. This scarification will occur naturally, just as it does when the plants drop seeds in a meadow.
Most winter sowing instructions tell you to plant in clear or translucent plastic jugs, such as water jugs. You will need four inches of soil for the best results, so the jug should be cut to leave a container about six inches tall. Cut some drainage holes in the bottom. I find that a hot-glue gun with no glue stick is good for this. Fill the container with about four inches of moistened planting medium. I suggest using a sterile soilless mix, so you do not have to worry about weed seeds. Spread your seeds and cover them at the depth recommended on the packet. Use masking tape to tape the top of the jug back on. If you are doing more than one kind of seed, use a nursery marker to label the jugs. Regular markers may fade.
Place the jug in a sunny location outside. When it warms enough for germination, the tiny plants will grow. You may then need to open the jugs to add water. If the plants touch the top of the jug, you will need to transplant them to the garden or into individual containers.
While this method may be new to some, it is essentially the same as growing in a cold frame. It does not give you plants earlier than direct sowing, but it allows you to control the soil and moisture, and it isolates the plants from others that would be growing in the garden. It is a great way to get more of the plants you need to fill a pollinator garden.