Planting seeds outside during the winter is essential for success with many plants, ideal for others, and just plain convenient for some.
Seeds that need cold treatment (cold stratification) include the ones with protective coatings, native wildflowers, and cold-hardy perennials. The instructions for seeds such as Pulmonaria and Achillea say to try to start them at 60-degrees but if that fails, chill the containers. Or, chill them at the beginning. Go to the website http://tomclothier.hort.net for a list of seeds’ requirements.
Three cold-treatment methods that work include: 1) Plant in recycled containers that are monitored outside; 2) Pre-chill the seeds in the refrigerator; and, 3) direct sow the seeds on prepared beds that are either open to the elements or mulched.
Flowers such as Poppies, Larkspur and Nigella are planted directly on top of prepared soil now. These and other deeply rooted flowers rarely do as well if they are transplanted from containers.
Clear the bed and prepare the soil first. I usually put a thin layer of vermiculite on top and/or potting soil under winter-seeded areas so I can monitor them for rain or animal damage.
Green vegetables such as spinach, chard, broccoli, and kale can be direct-sown in the ground now and mulched. They come up when the soil and air temperature are to their liking. Some cool weather vegetables, such as Alaskan peas, can rot in the ground if they are planted this early because they are two-inches deep where the soil stays cold and wet.
Sowing seeds in recycled plastic is an ideal method for most perennial plants. The containers provide protection from birds, squirrels and neighborhood cats as well as reducing weather damage.
To make a min-greenhouse garden, collect clear plastic bottles from milk, juice, etc. Poke drainage holes in the bottom with scissors or something like a heated ice pick. Cut containers horizontally at least 4-inches from the bottom and discard the bottle caps.
Gather your seeds and write the plant’s name and date planted on the container with a permanent marker. Also put a marked tag, such as a Popsicle stick inside each container.
When planting begins in the spring it will reduce the confusion if your labels include a hint about where they go in the garden (sun or shade, wet or good drainage, etc.).
Fill the bottom of each mini-greenhouse with loose soil such as commercial potting soil. Many people mix their own out of garden dirt, sand, compost and perlite.
Wet the soil and let it drain. Tiny seeds that need light to germinate are pressed into the top of the soil and seeds that need dark to germinate are pressed into the soil.
Secure the top of the greenhouses with tape. It does not have to be a perfect seal but you want it to hang together through rain, freeze, warm days and wind.
Put the containers in a sunny spot where rain and snow can enter through the top. During periods of no rainfall, bottom water the greenhouses by putting them into a plastic and pouring water - not freezing cold water - into the pan. When the soil has absorbed enough water to be moist not wet, drain the saucers.
If plants emerge on warm days you can open the greenhouses but re-seal them again before freezing temperatures return.
To pre-chill seeds indoors, wet 1/3 cup garden sand with 2-teaspoons water. Add seed and refrigerate in a labeled plastic bag. Mark the bag with the ideal start and end dates of cold treatment. Check the seeds weekly for sprouting. Immediately plant into containers any seedlings that emerge.
Wait another month to start seeds of tender perennials and annuals.