- The first category are perennial seeds. These can be collected the growing season before or purchased in early winter. Most perennials need stratification and sowing them using this method is the easiest way to get them started. This is an excellent method for common milkweed, butterfly weed, and swamp milkweed.
- The second category are cool season annuals, vegetables, and herbs. Classic examples are calendula, annual bachelor's buttons, snapdragons, sweet peas, and larkspur. Read the seed packet carefully. If it says "sow outside 4-6 weeks before your last frost, the seeds are a candidate for winter sowing in milk jugs. Note that some seedlings don't transplant easily- poppies, carrots, and parsnips are examples that are much better direct sown in the ground in the early spring.
What happens next?
I can take many weeks, even months, for these seeds to germinate outside. Be patient and don't lose faith! One day you will look down through the hole in the top of the jug and you will see little baby seedlings popping up! Let them grow for a while, until they get at least one or two sets of true leaves. When they look lush and vigorous, remove the tape, slowly easy the clump of soil filled with seedlings out of the jug, and place it in a shallow saucer or basin of water. Have flats of 3 or 4" pots ready, filled with high quality potting soil. Poke a hole in the center of each pot and carefully tease a seedling out and replant it in the pot. Water and move your babies to a sheltered, partially shady spot for a few days to harden off and then, gradually, move them into the sun to grow on until they are large enough to transplant into the garden. If you are very careful and experienced in transplanting seedlings, you could also try moving them directly into a prepared garden bed, spacing them according to the directions on the seed packet. Be sure to shade them for the first few days as they adjust to their new home. I often create a tent with hoops and floating row cover.