Native plants are at home with local growing conditions and don’t require special tools or even a greenhouse to grow from seed. Perhaps you have had ideas of converting your lawn or revamping your garden to accommodate more native plants, in keeping with our growing understanding of the importance of providing space for wildlife and pollinating insects in suburban landscapes.

Native plant seeds can be collected from late fall into the winter months and propagated using the cold stratification technique. Pictured: New York Iron Weed (left) and Goldenrod (right).

It’s an excellent idea—with one substantial downside: finding and buying all of those plants. While native plants are showing up in more nurseries, understanding their “native-ness” (i.e. are they cultivated/hybridized or wild?) and then having the wherewithal to purchase the plants can be daunting.
But it’s winter, and you have time to consider alternatives: starting from seed. Even if you have never tried growing your own plants, there is a technique that is reasonably simple, fun (it can be enjoyed by all ages), and surprisingly inexpensive: winter stratification of native plant seeds.

The term stratification is used in horticulture to define the replication or use of natural settings—in this case exposure to cold and moist winter conditions—to stimulate seeds to germinate. There are at least two ways to employ this technique: direct sowing of seeds in the ground, or in pots over the winter months.
We’re going to take a look at planting collected seeds in simple containers, in this case re-purposed gallon or half gallon plastic milk jugs, over the winter. Imagine your delight in the early spring when you first detect a flush of green emerging from the bottom of that plastic jug that has been sitting outside in the cold. That was my reaction—both amazement that it works and anticipation of truly wild plants for my garden.

Cold stratification, here using milk jugs in a sunny outdoor location, is a simple technique that can be used to generate local native plants to enhance our local gardens.

Many plants employ mechanisms, such as tough outer shells, that prevent their seeds from germinating before the conditions exist for them to survive. In New England our winter days and nights of cold help to break down seed coatings, initiating biochemical changes within the seed. The once-impenetrable outer shell gradually allows water and oxygen to penetrate; swelling further ruptures the seed coat. Hormones signal the seed that conditions are changing, allowing enzymes to break down nutrients and starch that have been stored in the seed to assist with growth and development. A seedling is on its way, and it’s the variations in cold temperatures that have made it possible.

Many good step-by-step guidelines for this growing technique can be found on the internet. Among the best are Maine’s Wild Seed Project, the Yale Peabody Museum, and Native Plant Trust in Massachusetts (see Resources at the end of this article). If you’re interested in getting involved or learning more about wild seed propagation, consider joining Wild Ones, a national “native plants, natural landscapes” organization. The Mountain Laurel Chapter in Connecticut, for example, works in partnership with the Connecticut College Arboretum in New London.

Seeds can be collected from wild plants in fall and into the winter months; the stratification pots can be set outside anytime in the cold weather. Each native seed has its own timetable for germination, which is often variable. A pot that doesn’t germinate initially may even emerge the following year.

If you collect the seeds before you can get them right into their pots, be sure to keep them in a cool, dry place to prevent loss of viability. Paper is best for collecting and storing. Plastic can trap moisture and cause mildew, and for the same reason avoid collecting when there is dew or a recent rain.

Native plants that can be propagated from seed by using cold stratification include Evening Primrose (Oenothera biennis), Joe Pye Weed (Eutrochium sp., pictured left), Goldenrod (Solidago sp.), Milkweed (Asclepias), and Butterflyweed (A. tuberosa, pictured right).

If you decide you want to propagate more of your existing garden plants using this method, be sure to allow some plants to go intentionally to seed next growing season (yes, a few less bouquets). It’s best to collect a few seeds of the same species from as many different healthy-looking plants as possible to promote genetic diversity and approximately the same amount of seeds from each plant.

Here are a few additional helpful rules of thumb:

• Sow seeds densely, roughly 1/8- to 1/4-inch apart, and the depth should be the thickness of the seed.
• Ensure good drainage (a few slits in the bottom of the jug will do). Use clean containers to prevent mold, or if conditions are dry (and windy), spray some water into the opening.
• Ensure good light (especially for tiny seeds). East exposure works well.
• Emerged seedlings can be transplanted carefully when the first true leaves appear (after the initial “baby” leaves). Clump three to ten seedlings per pot (unless it’s a tree species) to assist with cross-pollination and ensure some genetic diversity to produce viable seeds when they mature.
• Some native seedlings can stay in the original pot (or flat) for the first growing season; transplant them either later in the summer or overwinter one more season.

The greatest challenge, and a seemingly unlikely pitfall, is labeling. It is alarmingly easy to get confused between collecting bags and planted containers. Tags and tape using permanent markers invariably are too faint to read by spring. I have finally had success taking a photo of the initial outdoor arrangement of pots (when the marker label on the pot is new) and printing it out with notes.
One last suggestion: It’s a good idea to get permission from a property owner before collecting any plants or flower heads from private property. And because the seed resource is important to wildlife over the winter, don’t take more than you need. If you end up with more plants than you can use, share them—especially with neighbors, children, or others who may not know why native plants are important and worth growing and planting.

The most delightful aspect of this simple and inexpensive technique is the joy that comes from seeing plants germinate, and ultimately having truly native plants in your landscape that can provide the resources—food, shelter—at the right time for the other native species in a functioning landscape. One yard at a time, one neighborhood, one community: collectively we can make the difference.

Judy Preston is a local ecologist active in the Connecticut River estuary.