Adventures in Cold Stratification

I grow all kinds of plants, but my favorites are mostly native plants, wild plants, and medicinal herbs. These are exactly the kinds of plants that tend to require special treatments, such as cold stratification. I’ve tried a number of different methods of cold stratification, and I’ve written about it here before. I normally stratify my seeds in the refrigerator in a very controlled way. I like the fridge method because it gives me very good results, and it’s predictable. This year, my fridge was too full of other stuff, and there was no way to fit trays of seeds in there. Constraint breeds creativity, and I was pushed to find another way to cold stratify my seeds. Much to my surprise, I think I like the new way even better!

In past years, I haven’t had good results from outdoor cold stratification. My little soil cubes dried out very fast in the cold winter wind, and I was unable to keep the seeds moist enough to germinate. When I sowed out in the open field, many of the seeds washed away or were lost in a sea of weeds. When I tried sowing in the garden, labels got lost, and by springtime I couldn’t remember what I planted where.

I don’t know why this didn’t occur to me earlier, but larger quantities of potting soil hold moisture much longer than small cubes of potting soil. I had a bunch of gallon sized plastic nursery pots lying around. It seemed like those would hold moisture reasonably well. I filled them up with potting soil and sowed my seeds thickly on top. I placed them outside on my back patio in December. I checked on them every few days, but they rarely needed water. I watered on less than ten occasions all winter, and it was overall a very minimal effort.

seeds gold stratifying in black gallon nursery pots

The results seem very satisfactory so far! I am already seeing little sprouts popping up in my containers of jewelweed, raspberry, purple giant hyssop, New Jersey tea, and blue vervain, and others. Hopefully the sprouts I see are the seeds I planted, and not weed seeds that blew in! I labeled the pots in several different ways, but the only labels that reliably endured were my recycled soda can labels.

I also tried a similar method called “Winter Sowing”. Although technically, sowing any seed in winter could be called winter sowing, there is a group of people who have become very proprietary about the term “winter sowing”, so I will use their definition here. This method involves a partially covered mini-greenhouse planting with at least three inches of potting soil. The most common container is a gallon jug, such as the kind used for packaging distilled water or milk. Holes are drilled in the bottom for drainage, then the gallon jug is sliced most of the way around the middle, but a sort of hinge piece remains attached. The potting soil is added to the bottom, seeds are planted, the seam is taped together again, and then the uncapped jug is placed outside in a similar way to my gallon nursery pots. In the water jugs, the soil stays a little warmer because the top of the jug acts as a little greenhouse. I am also seeing very good germination with this method. I chose seeds that didn’t need as much cold stratification for this method, thinking that the seeds that needed 60, 90, or 120 days of cold stratification might prefer to be out in the open to experience the full range of cold weather fluctuations. Some seeds, like onions, don’t need any cold stratification but are extremely hardy, so I planted them this way as well.

Winter Sowing Milk Jugs
sprouts germinating inside a winter sowing milk jug