Earlier this summer, a generous neighbor gave me some columbine (Aquilegia) seeds from her beautiful garden. Naturally, I was thrilled to receive the seeds but wasn’t sure when to plant them. Some gardening sources advised planting them in early spring. Other sources recommended planting them in fall. So, which was correct – spring or fall? While the sources differed on when to plant the seeds, they were consistent on one point: Columbine seeds need to be exposed to a period of cold temperatures before they can break dormancy. But why?
First — What does it mean to break dormancy?
Seeds may appear lifeless, but that is not the case at all. Fundamentally, a seed is a plant in embryonic form enclosed in a protective covering (seed coat). The thickness and hardness of the seed coat combined with chemical germination inhibitors (hormones) within the seed coat protect the embryonic plant until conditions are optimal for germination. 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 a process called stratification.
Stratification
Stratification uses temperature to break dormancy. It occurs when dormant seeds are exposed to a period of cold temperatures followed by warm temperatures and moisture. This combination of alternating temperatures 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. This mechanism applies to plants that evolved in temperate climates such as those found in North America. Plants species that evolved in hot climates, such as the tropics, don’t experience cold temperatures. Therefore, the seeds of those species don’t need to be stratified.
For seeds that fall to the ground from plants or are sown in late summer or fall, stratification occurs naturally during winter’s freeze/thaw cycles. This method has worked for Mother Nature for hundreds of thousands of years. But seeds may also be stratified indoors for planting outdoors later. The key to achieving successful stratification indoors is to simulate the conditions seeds experience when breaking dormancy in nature. In other words, the goal is to trick seeds into thinking they have experienced winter.
Methods for Stratifying Seeds
Before stratifying seeds, research the amount of chill time they need to break dormancy. It can range from just a couple of weeks to many months, depending on the plant species. Then, decide which stratification method will work best. While there are many methods, a few of the easier ones are described below. In addition, check out some of the commercial nursery websites for step-by-step stratification instructions and chill times.
Dry Stratification– This is a pre-treatment that 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 as you normally would once the danger of frost is past.
Moist Stratification– This method requires seeds to be in contact with a moisture-retaining medium during the chilling process. It works well for plants, such as wildflower species, that evolved under moist, cold winter conditions. Those conditions can be mimicked using any of the moist stratification methods below:
- Scatter seeds on raked soil in the fall after the soil has cooled. Leave the seeds uncovered or very lightly cover them with soil to prevent birds and animals from consuming them. The normal freezing/thawing cycles plus moisture from rain and snow over the winter months will break down the seed coats. The benefit of this method is that it requires very little effort. The downside is that winter weather can be variable with some warm days interspersed with cold ones. As a result, natural moist stratification may take longer than refrigeration, which is controlled and consistent.
- Scatter seeds on soil in late winter or very early spring before the soil starts to warm up. This is a good solution if you didn’t remember to plant the seeds in fall. Keep in mind the number of weeks or months the seeds need to be exposed to cold temperatures so that you scatter them in plenty of time before warm weather arrives.
- Chill seeds in the refrigerator using a moist paper towel. This method 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 sow them outside on raked soil after the last frost date (typically April 15 – 25 in USDA Zone 7).
- Chill seeds in the refrigerator mixed in with a moist medium such as sand, vermiculite, or soil-less seed starting mix. This method 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.
Baptisia seeds prepared for cold, moist stratification. Photo: Pat Chadwick - 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 with grit, vermiculite, or coarse sand to limit the amount of light and to prevent birds from eating the seeds. Store the containers outdoors over the winter months in a protected area away from the wind where they will receive precipitation, light, and cold conditions over the winter months. 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
Seeds of most tree and shrub species that evolved in the mid-Atlantic area of the U.S. benefit from stratification. The amount of chilling time needed depends on the species. Iowa State University Extension’s publication on Germination of Tree Seed provides advice on when and how to collect tree seeds for germination of a variety of species including redbuds, maples, oaks, hickories and fruit trees.
The seeds of most ornamental annual species don’t require stratification. However, a few that do require stratification include larkspur (Consolida), love-in-a-mist (Nigella), annual poppies (Papaver spp.), annual pincushion flower (Scabiosa atropurpurea), and bells of Ireland (Moluccella laevis).
A few 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 those species, check out the Missouri Botanical Garden’s guidance on Native Seed Propagation Methods listed under Sources at the end of this article.
Scarification
Some seeds may not sprout if moisture cannot penetrate their hard, impermeable seed coats. Those 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. While this process often can be accomplished through natural means, it can also be accomplished by mechanical, chemical, or thermal intervention such as:
- 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.
- Acid 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.
- Breakdown by digestive tract – As seeds that are swallowed by birds or other animals pass through their digestive tracts, acids weaken the seed coats.
- Heat – Some plant species require heat rather than cold to jump start the germination process. Pour warm water over the seeds and soak them for 12 to 24 hours. In nature, smoke and fire are two additional heat-related methods for weakening the seed coats of certain species.
The seeds of many tree species benefit from a combination of scarification and stratification because of their particularly thick, hard seed coats. Virginia Cooperative Extension’s Publication 426-001 outlines basic seed stratification procedures for trees and shrubs.
In conclusion
Seeds of many plant species that evolved in temperate climates require a period of freeze/thaw temperatures to break dormancy and germinate. By understanding how seeds work in terms of stratification and scarification, it’s possible to improve their germination rates.
As for the columbine seeds mentioned at the beginning of this article, sources varied on the amount of time they should be stratified with the average time being 3 to 4 weeks. This means they may be planted outdoors either in early spring before the soil warms up or in fall so that they are exposed to the winter cold. As long as the seeds receive sufficient chill time, either approach will work.
FEATURE PHOTO: Examples of seeds that benefit from stratification. Starting at top of photo and moving clockwise: Baptisia, milkweed, coneflower, blanket flower, coreopsis, and columbine. Photo: Pat Chadwick
SOURCES:
Plant Propagation, The American Horticultural Society (Toogood, Alan, 1999)
Botany for Gardeners, Third Edition (Capon, Brian, 2010)
The Ever Curious Gardener – Using a Little Natural Science for a Much Better Garden (Reich, Lee, 2018)
Germination of Tree Seed, Iowa State University Extension
How to Prepare Seeds for Sowing, Lady Bird Johnson Wildflower Center
Native Plant Seed Propagation, Rutgers Cooperative Extension Fact Sheet FS1329
Native Seed Propagation Methods, Missouri Botanical Gardens
Plant Propagation From Seed, Virginia Cooperative Extension publication 426-001
Seed Stratification: What Seeds Require Cold Treatment?, University of Illinois Urbana-Champaign Extension
