Last year I experimented with the winter sowing method of germinating seeds. I had prepared 2 trays and one bottle for winter sowing. The trays were unsuitable and nothing germinated, but the bottle did quite well. In it was some Elecampane seeds. Only a few seeds to try and I got seedlings which grew large enough, despite my ignoring them, to plant in the garden. I'm pretty sure the trays I used dried out too quickly, so I'm trying the method again, this time only using appropriate containers.
I cut the pop bottle in half and discarded the cap. In essence, this method relies on making mini green houses. It will get warm in there, so ventilation is needed. With a bottle like this, you just take the cap off. Otherwise, you need to cut ventilation holes in the lid. You also need to cut some drainage holes in the bottom. I put in 3 or 4 in the bottom of each bottle.
I also cut some slits around the top of the bottom half of the bottle. They are about 2 inches deep. I took a little wedge out of one as well. This is just to help the top slide on more easily. I found it buckled otherwise. It left a gap, which I worried would let moisture escape too quickly.
I filled the bottom of the bottle with about 3 inches of planting soil. Everything I've read suggests that 3 inches depth of soil is ideal. My trays only had 1 inch of soil, so that was likely a problem as well. I soaked the soil and let it drain - remember those drainage holes? They're important. Then I sprinkled seed all over the top and put just a tiny bit of soil over them to cover. Rule of thumb here tends to be cover with as much soil depth as the seeds are large. These are Red Yarrow seeds and not very big at all.
Then I slid the top of the bottle, over the bottom half. I used packing tape this year. Last year I used duct tape, but I can't figure out where I put the roll of it. I'm hoping the packing tape will hold up to the moisture. I also wrote a label in both marker and pencil. Marker is easier to see but can often fade out with sunshine. The pencil on the other hand is harder to see, but won't fade. It may be overkill, but I'd rather not have to try to remember what is in which bottle in May. I taped over the whole label to help waterproof it and taped it to the bottom of the bottle, below the soil line.
I then took the bottles outside and plunked them in the snow, in an area where I can keep an eye on them. They should get some sunshine, but being in full sunshine can dry them out. The seeds and soil will freeze. As the days get milder, the soil in the bottle will warm up during the day and freeze again at night, causing the seed coatings and casing to wear. When the time is right, the seeds should germinate. As the days grow warmer, you do need to keep an eye on them to make sure there is still moisture in the bottles. There should be condensation on the inside of the bottle when it's warmer. If not, add a bit of water. Using a bottle without a cap not only allows excess heat to escape but allows a bit of moisture in when it snows or rains.
These bottles have Red Yarrow and Icelandic Poppies in them. I want to try some other perennials plants as well, including Gaillardia, Aster, Rudbekia Goldsturm, Golden Marguerite and Phlox. I may try some Woad, Weld and the Osage Orange, depending on how many bottles I can find to use.
I can see this being a great method for perennial seeds. Despite some websites saying annuals will work with this method and suggest it for beans, tomatoes and other warm weather seeds, I don't think I'd try them. Bean seeds won't germinate if the soil is too cold. Tomatoes need a longer season than we actually have here if directly planted, so starting them inside, 8 weeks early, is a good way to ensure they have the chance to fully set and ripen fruit before we get frost. I might try spinach, lettuce or arugula if I thought the ground would be ready for transplanting early enough.