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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 sown the internal mechanisms that inhibit their ability to germinate.  The seeds will germinate when temperature 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, columbine, globeflower, masterwort, and garden phlox.  It’s also very useful for starting seeds of many fall-ripening trees and shrubs, such as barberries, dogwoods, lilacs, species roses, and viburnums. 

You can use four-inch plastic pots or flats to sow the seeds.  If you have used the containers before, clean them first with a solution of one part 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 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 receive enough moisture which is a necessary prelude to the chilling period.

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 slower rate.)  If you opt for a cold frame, lay 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.

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 unheated greenhouse where they receive bright but indirect sunlight.  Keep an eye out for slugs and snails, which can devour young seedlings overnight.

When the seedlings have grown their second set of true leaves, thin or transplant them to prevent overcrowding.  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 ¼ 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.

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 afterwards 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.

Information for this article was adapted from Horticulture Online.

By Sherri Sanders
County Extension Agent - Agriculture
The Cooperative Extension Service
U of A System Division of Agriculture

Media Contact: Sherri Sanders
County Extension Agent - Agriculture
U of A Division of Agriculture
Cooperative Extension Service
411 No Spruce Searcy AR 72143
(501) 268-5394
ssanders@uaex.edu

 

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