by Marie Hofer, HGTV.com
Have you ever seeded perennials in the spring and wondered why some of them never came up? If the seeds were viable and you supplied the right soil coverage and moisture, the problem may have been that they'd never been chilled.
Call it Mother Nature's little insurance policy. In an effort to make sure that seeds have the best chance of making it to adulthood, she gives them a dormancy that needs to be broken in order for the seeds to germinate. To break the dormancy, the seeds have to undergo the same conditions they would experience in nature. Although there are many types of dormancy breakers, in many cases it's chilling, the length and degree of which varies depending on the species of plant. The required chilling period, called "stratification," protects the seed from accidentally sprouting after, say, a brief warm spell in late fall.
Only some perennials, including some wildflowers and prairie plants, need to be chilled. (And the seeds of most woody trees and shrubs also need chilling.) If plants self-sow or you plant them in the fall, the chilling is automatic (if your winters meet that particular plant's cold requirement, that is). If you intend to plant in spring, however, give them the appropriate amount of refrigerator time before you take them into the garden.
Seed companies usually put the instructions for chilling on the packets--for example, to chill for 7 to 10 days before planting. Or, they simply advise planting in the fall--from that advice, you intuit that the seeds need a cold period. If you harvest your own seed or share with friends, you'll want to do a little research before planting.
Usually the seeds need a moist chill: mix the seed with a moist medium such as sphagnum, peat moss or sand and store in a sealed plastic bag. Once the chilling period has passed, you can plant the seed, with the medium, in the garden. Don't wait too long to plant; once dormancy has been broken, the seed begins deteriorating if it's not given the chance to germinate--that's why seed companies don't moist-chill the seeds before packing.
Some of these plants require chilling; for others, a cold period simply ensures a higher percentage of germination. Here's a partial list:
Hardy ageratum (Eupatorium coelestinum)
Astilbe (Astilbe x arendsii)
Butterfly weed (Asclepias tuberosa)
Peruvian lily (Alstroemeria)
False indigo (Baptisia australis)
Canada tick-trefoil (Desmodium canadense)
Bergenia (Bergenia cordifolia)
Bleeding heart (Dicentra spectabilis)
Columbine (Aquilegia spp.)
Garden phlox (Phlox paniculata)
Globeflower (Trollius spp.)
Goat's beard (Aruncus dioicus)
Great bellflower (Campanula latifolia)
Corsican hellebore (Helleborus argutifolius var. corsicus)
Lavender (Lavandula spp.)
Lavender cotton (Santolina chamaecyparissus)
Lewisia (Lewisia spp.)
Perennial lobelia (Lobelia cardinalis)
Masterwort (Astrantia spp.)
Mediterranean sea holly (Eryngium bourgatii)
Penstemon (Penstemon barbatus)
Purple coneflower (Echinacea purpurea)
Snapdragon (Antirrhinum majus)
Solomon's seal (Polygonatum)
Turtlehead (Chelone spp.)
Pansy (Viola x wittrockiana)
Wallich spurge (Euphorbia wallichii)
White baneberry (Actaea pachypoda)
Willow amsonia (Amsonia tabernaemontana)
If you have any doubt about whether a seed needs chilling or not, ask your local extension agent or consult one of the references listed below.
Resources Manual of Herbaceous Ornamental Plants
Steven M. Still
Stipes Publishing Company; 4th edition (December 1993)
Order this title from Amazon.com
Manual of Woody Landscape Plants: Their Identification, Ornamental Characteristics, Culture, Propagation and Uses
Michael A. Dirr
Stipes Publishing Co.; 5th edition (August 1998)
Order this title from Amazon.com
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