Q&A: How Do I Cold Stratify Perennial Seeds?

I've ordered some perennial seeds that need to be cold stratified. How do I do it?
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Starting Seeds That Need Cold

Many plants that are native to temperate regions of the world have seeds that require a moist, cold period before they sprout. You can grow these seeds by sowing them in pots and setting the pots outdoors in late fall or winter. Known as "stratification," this treatment replicates the natural conditioning the seeds would undergo in their native habitat, breaking down the internal mechanisms that inhibit their ability to germinate. The seeds will germinate when temperatures rise in the spring.

This rather low-tech method, which is also called moist-chilling, works well for starting seeds of many perennials, including bleeding heart (Dicentra spp.), columbine (Aquilegia spp.), globeflower (Trollius spp.) masterwort (Astrantia spp.), monkshood (Aconitum spp.), penstemon, garden phlox and turtlehead (Chelone spp.). It's also very useful for starting the seeds of many fall-ripening trees and shrubs, such as barberries, dogwoods, lilacs, species roses and viburnums.

1. Sow the seed

You can use four-inch plastic pots or flats for this project. If you have used the containers before, clean them first with a solution of one part household bleach to nine parts water. Prepare a potting mix of equal parts builder's sand and a peat-based potting mix. The coarse texture of this mix will allow plenty of air to circulate around the seeds and the growing roots. Moisten the mix and fill your containers, tamping each down to within one-half- inch of its rim. Sow the seeds about a half-inch apart and cover them with a thin layer of mix. Topping the pots with a thin layer of very fine gravel or coarse sand will prevent the rain from washing out the potting mix and the seeds.

Label each container with the plant's name and the date. (Use a permanent marker so it will survive the elements.) Water the pots well to ensure that the seeds imbibe moisture, which is a necessary prelude to the chilling period.

2. Place the pots in a cold place

For the winter chilling period, choose a location out of direct sun, such as an unheated porch or shed, or an uncovered cold frame on the north side of the house. A place where temperatures remain between 35 and 45 degrees Fahrenheit is ideal. (Higher and lower temperatures will produce changes in the seeds at a slower rate.) If you opt for a cold frame, line it with a layer of moist sand so you can bury the base of the pots to help keep them upright. Covering the pots with wire screen will keep out rodents and birds.

3. Wait for germination

Check your pots frequently and water if the top of the soil mix feels dry. As spring approaches, you should begin to see signs of growth. Different species will germinate at different rates. Some may appear in very early spring, others in late spring, while a few kinds may take up to a year or more.

As the seedlings sprout, move the pots to a nursery area or an unheated greenhouse where they receive bright but indirect sunlight. Keep an eye out for slugs and snails, which can devour young seedlings overnight.

4. Thin or transplant

When the seedlings have grown their second set of true leaves, thin or transplant them to prevent over-crowding. If you don't need many plants, thin to one seedling per pot by cutting off the extras with small scissors. If you want a lot of plants, transplant the seedlings, one per container, into 2 1/4- or 3-inch-wide plastic pots filled with moist, peat-based potting mix. Use a fork to gently lift the seedlings out of their original containers and tease them apart. Handle the seedlings by their leaves to avoid damaging their stems.

Water the seedlings and place them out of direct sun for a few days to recover. After a week, put them in a place where they receive morning sun. Begin feeding the seedlings once a week with liquid fertilizer diluted to half strength.

5. Plant the seedlings in the garden

By late summer or early fall, many of your seedlings will be large enough to set into the garden. Transplant them on a cool day, and water carefully for a few weeks afterward until they are well established. Some slower-growing species may not be large enough to move yet. Keep these in a cold frame over winter and set them out the following spring.

Alternative Cooling Methods

If you live in a warm-winter climate, or if you want to plant in the spring or summer, you can provide a period of moist-chilling by placing already sown pots of seed in the refrigerator for about six weeks. Alternatively, stratify the seeds between layers of damp sand in a plastic bag in the refrigerator. Sow the seeds as soon as they sprout, or after six weeks.