Winter sowing seeds in milk jugs or similar containers is a low-maintenance, effective way to start flowers and vegetables for your garden. The sealed milk jug acts like a mini greenhouse, causing seeds to germinate as soon as conditions allow, usually several weeks ahead of direct sowing in the ground. Because they are sown outdoors in an unheated environment, there is no need to “harden-off” plants by gradually exposing them to outside temps. Seeds sown in this method also undergo several freeze/thaw cycles that soften or break the seed coat (known as stratification).
Though milk jugs are most commonly referred to for this method, a number of types of plastic containers can be used. Containers should:
- Be semi-transparent (not opaque)
- Allow for at least 2″ of soil depth and, ideally, 4″ or so of room for plant growth (small butter tubs are not ideal for this use, for example)
- Not easy to tip over (2 liter bottles are not ideal for this reason)
Prepare the milk jug by punching drainage holes in the bottom, 4 holes around 1/4″ each are usually sufficient. You can use a screwdriver heated over a flame, an awl or a small power drill. Starting at the handle, cut the milk jug horizontally, at least 2″ up from the bottom, almost all the way around, leaving approximately 1″ connecting the handle to the base to act as a hinge. Note: You do not need the milk jug lid; the top has to stay open to allow moisture in and out. If using a container that was previously used for something other than food (for example a jug of windshield wiper fluid) use caution, thoroughly clean the container with a diluted bleach solution and do not use the jug to plant anything edible.
Be careful and thoughtful when labeling – nothing is worse than having a dozen blank containers at the end of the season and having no idea what each one contains. Many seedlings and small starts are very difficult for even well trained gardeners to recognize. It’s recommended that you label jugs with plant markers and cover the writing with a strip of packing tape. Permanent markers also work if covered but might fade when exposed to the sun. When using a container with a removable lid such as a yogurt container, DON’T only label the lid, but label the container instead or in addition to.
Fill the container to a minimum 2″ soil depth with slightly damp (but not sopping) soil mix or use dry mix but be sure to mist the top of the soil lightly to secure the seeds in place and add moisture for stratification and later germination.
Plant seeds paying close attention to package instructions regarding soil depth and light requirements.
Seal the cut seam around the milk jug to the best of your ability. It does not need to be 100% air tight but do the best you can. Packing tape seems to work best. Avoid masking tape and painters’ tape.
Check regularly when day time temps remain around or above 40 degrees. (Remember, the temps inside the milk jug may be as much as 10 degrees warmer.) Make sure the soil is kept moist.
Once seedlings have emerged, you may wish to cover the milk jugs with a blanket if temps drop below freezing at night (optional) but make sure you keep checking to ensure the soil hasn’t dried out.This generally isn’t a problem unless enough moisture didn’t get in through the winter or the drainage holes are too large.
Pay close attention to daytime temps. When they approach 50-60 degrees, especially on sunny days, it is time to unseal the containers and flip back or remove the tops so as not to “cook” your seedlings.
Don’t forget to cover things back up at night. You do not need to reseal anything at this point, just flip the lids back on.