I finally got around to trying the approach of sowing seeds in plastic milk jugs a few years ago, and I think it’s a terrific option for seeds that require or benefit from exposure to a natural chilling period or alternating warm and cold seasons. You’ll often hear this called “winter sowing,” but it works fine in other seasons too, as long as you keep the jugs out of full sun in summer. It is terrific for seeds that tend to be slow to sprout, including many perennials and woody plants. You can also use it for annuals that need or benefit from cool conditions for germination, and for annuals and perennials that easily self-sow. (Starting them this way saves valuable indoor space for seeds that really need supplemental light and heat).
Seeds sown in repurposed plastic jugs are easy to move around, are less likely to get too wet or to dry out, and are less exposed to mice, birds, and other pests than seeds sown in pots or in nursery beds. Really, the only problem can be having enough jugs on hand if you want to do a lot of sowing. Put the word out to friends (particularly those with kids), or make a serious commitment to increasing your milk consumption, and you can quickly get plenty to work with.
Poke a few drainage holes in the bottom of a quart, half-gallon, or gallon jug; cut 3/4 of the way around the middle with a craft knife or heavy scissors; fold back the top; and add moist growing medium (I like to use 2 or 3 inches’ worth). Sow the seeds as usual, add a label, and then rejoin the top and bottom with a piece of duct or masking tape. I also like to write the name and sowing date on the outside of the jug with a permanent marker. Don’t put the tops on the jugs.
Then you can just carry the jugs outside and leave them there. They’ll get some moisture through the open top but are pretty well protected from hail or heavy rain and from critters too.
I put my finished jugs in an empty planter or plastic crates (with drainage holes) to keep them from getting knocked over. Beginning in early March, I start peering inside each one every few days to see if anything is sprouting. Here in my southeastern Pennsylvania garden, the most germination action is usually from late March to late April. By mid-May, I put the rest of the jugs in an out-of-the-way spot where I can check them occasionally and water if needed, and leave them out until at least the next spring in case the seeds need extra time to sprout.
Once the seeds start sprouting, remove the tape and cut off the top of the jug. The removed tops can be handy for protecting newly planted-out seedlings in the garden, and you can reuse the bottoms for other seed-sowing projects.