By Pat Chadwick
Piedmont Master Gardener
Last summer, a generous neighbor gave me some columbine (Aquilegia) seeds from her beautiful garden. Naturally, I was thrilled to receive the seeds, but it will take some extra steps on my part to make them germinate next spring. That’s because columbine seeds need to be exposed to a period of cold temperatures before they can break dormancy. But why?
Fundamentally, a seed is a plant in embryonic form enclosed in a protective covering—the seed coat. To break dormancy simply means that the embryonic plant must literally break through or escape the seed coat so that germination can take place. For that to happen with some plant species, the seed coat needs to be softened or weakened through processes called stratification and scarification.
Many plants that evolved in temperate climates like ours rely on stratification to break dormancy. It occurs when dormant seeds are exposed to a period of cold temperatures followed by warm temperatures and moisture. This softens or weakens the seed coat, allowing the seeds to germinate once growing conditions are suitable in spring. Think of stratification as a survival mechanism that prevents seeds from germinating either too early in spring, when the tender seedlings might perish during a late cold snap, or too late in summer when they won’t live long enough to produce new seeds.
For seeds sown in late summer or fall, stratification occurs naturally during winter’s freeze/thaw cycles. Seeds may also be stratified indoors for planting outdoors later. The key is to simulate the conditions seeds experience when breaking dormancy in nature—to trick them into thinking they have experienced winter.
Methods for Stratifying Seeds
The chill time needed to break dormancy varies among plant species and can range from just a couple of weeks to many months. Look online for commercial nursery websites that offer step-by-step stratification instructions and chill times for the seeds they carry.
Dry Stratification—This pre-treatment subjects seeds to a period of cold temperatures without any moisture. It works well for plants such as cleome, foxglove, some poppy species, and hardy perennial grasses that thrive in dry soils or dry winters. Simply seal dry seeds in a plastic baggie or glass jar, label and date the contents, and store the container in the refrigerator for one to four months, depending on the plant species. After the chilling period, remove the seeds and plant in spring after the last frost date (typically April 15–25 in our hardiness zone).
Moist Stratification—This keeps seeds in contact with a moisture-retaining medium during the chilling process and is used for wildflower species that evolved under moist, cold winter conditions. These stratification methods mimic those conditions.
Chill seeds in the refrigerator using a moist paper towel. This works well with small seeds. Place seeds on a dampened (but not soggy) paper towel or coffee filter. Make sure the seeds have good contact with the moist paper. Fold the paper over the seeds to form a packet. Place the packet in a plastic baggie or other sealable container and refrigerate for about one to three months depending on the plant species. Monitor the seeds periodically to make sure the paper towel is still damp but not too wet. Otherwise, the seeds may become moldy. At the end of the chilling period, remove the seeds and either sow them indoors under grow lights or outside on raked soil once the danger of frost is past.
Chill seeds in the refrigerator in a moist medium such as sand, vermiculite, or soil-less seed starting mix. This works well with larger seeds, such as beans. Mix the seeds into the moist (but not soggy) medium, making sure they have good contact with the medium. Place the mixture in a plastic baggie or other sealable container, label and date the contents, and refrigerate. During the chilling period, check periodically to make sure no mold has formed. Once the chilling period is complete, remove the seeds from the moist medium and plant outdoors as you normally would.
Chill seeds outdoors on the ground. Scatter seeds on raked soil outdoors in the fall after the soil has cooled. Leave the seeds uncovered or very lightly covered with soil to prevent birds and other animals from consuming them. Over the winter months, the natural freezing and thawing cycles plus moisture from rain and snow will break down the seed coats. This method requires very little effort but may take more time than controlled, indoor refrigeration because winter weather can be variable. You may also scatter seeds on soil outdoors in late winter or very early spring before the soil starts to warm up, but keep in mind how long the seeds need to be exposed to cold temperatures so that they break dormancy before warm weather arrives.
Chill seeds outdoors in pots or planting trays. Make sure the containers have holes for good drainage. Fill with sterile soil or a soil-less seed starting mix. Sow the seeds and lightly cover them with grit, vermiculite, or coarse sand to limit the amount of light they receive and to prevent birds from eating the seeds. Store the containers outdoors over the winter months in a protected area away from the wind and where they will receive precipitation, light, and cold conditions. If squirrels or birds are a problem, lay a flat piece of chicken wire or other similar barrier over the pots and weigh it down to discourage digging. Monitor periodically to keep the soil moist and mold free until the seeds germinate in spring. Transplant into the garden once growing conditions are favorable and the seedlings have developed at least two or three sets of leaves.
Examples of Seeds that Benefit from Stratification
The seeds of most ornamental annual species don’t require stratification. However, a few that do include larkspur (Consolida), love-in-a-mist (Nigella), annual poppies (Papaver spp.), annual pincushion flower (Scabiosa atropurpurea) and bells of Ireland (Moluccella laevis). Examples of perennial species that benefit from stratification include:
- Black-eyed Susan (Rudbeckia species)
- Columbine (Aquilegia)
- False Aster (Boltonia)
- False Indigo (Baptisia)
- False Sunflower (Heliopsis)
- Ironweed (Vernonia)
- Joe Pye Weed (Eupatorium maculatum)
- Lavender (Lavendula)
- Milkweed (Asclepias syriaca)
- New England Aster (Symphyotrichum novae-angliae)
- Perennial Sunflower (Helianthus species)
- Rattlesnake Master (Eryngium yuccifolium)
- Turtlehead (Chelonia)
- In particular, wildflower species that produce their seeds in fall benefit from stratification. For more information on these species, search online for the Missouri Botanical Garden’s guidance on Native Seed Propagation Methods.
Some seeds may not sprout if moisture cannot penetrate their hard, impermeable seed coats. These seeds should be scarified before they are stratified. Scarification is the process of breaking, scratching or softening the seed coat so that moisture can penetrate and start the germination process. This typically occurs on its own in nature; for example, when seeds are swallowed by birds or other animals and pass through their digestive tracts, acids weaken the seed coats. We can simulate the process with these mechanical, chemical or thermal interventions.
Thinning—Lightly rub seed coats with fine grit sandpaper or file them with a metal file to thin and weaken them.
Breaking—Gently crack seed coats slightly with a hammer or nick seed coats with a knife or nail clippers. For large seeds such as beans (legumes), avoid cracking or nicking the seed at the small scar (called a hilum) where the seed was previously attached to the mother plant.
Chemical bath—Soak seeds in bleach or vinegar for 10 minutes to several hours to weaken seed coats.
Decay—Place seeds in a nonsterile, warm, damp container for several months where microbes can cause the seed coats to decay and break down.
Heat—Pour warm water over the seeds and soak them for 12 to 24 hours. Certain plant species require heat rather than cold to jumpstart the germination process. Some even require smoke and heat from fire to weaken their seed coats.
Preparing Seeds from Trees and Shrubs
Seeds of most tree and shrub species that evolved in the mid-Atlantic area benefit from stratification, and many also require scarification due to their tough skins. Iowa State University Extension offers an online resource on when and how to collect and germinate seeds for a variety of species, including redbuds, maples, oaks, hickories and fruit trees. To find it, search for Germination of Tree Seed. Also search online for Virginia Cooperative Extension’s Publication 426-001, “Plant Propagation From Seed,” which outlines basic seed stratification procedures for trees, shrubs and other plants.
Take a Free Class on Seed Saving and Sowing
At 2 p.m. Saturday, October 21, the Piedmont Master Gardeners will offer “Native Plant Propagation—Seed Saving and Winter Sowing,” a Garden Basics class on how to collect or purchase native plant seeds and prepare them for planting. To be held at Trinity Episcopal Church, 1118 Preston Avenue in Charlottesville, the free program will feature a hands-on seed preparation activity. Register at piedmontmastergardeners.org/events/ to reserve your place in the class.