iGrow Gardening: Seed propagation and seed dormancy

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By David Graper

SDSU Extension Horticulture Specialist

Several terms can be used to describe a seed when discussing seed propagation. First, to be successful in seed propagation, the seed must be alive. If a seed is dead, it of course is not capable of germinating. When buying seed, always check the seed label to be sure that it was packaged for this year and that the germination rate is good, hopefully over 80 percent. Seed death can be caused from a wide array of circumstances. The seed may simply be too old or had improper storage conditions, just to name a few. Seed should be stored in a cool, dry place. It should not be exposed to fluctuating temperatures or ever exposed to moisture during storage. Storing seed in a freezer in a water-tight container like a zip-lock bag or plastic box with a tightly fitting cover would work well. We have had some tomato seed stored in a regular freezer at SDSU that is about 30 years old and it is still has a fairly high rate of germination.

Secondly, the seed must be viable, which means that the seed is alive and will germinate given the proper environmental conditions. Finally, the seed must not be dormant. A dormant seed is alive but will not germinate even if it is given ideal germination conditions. Seed dormancy is a common attribute of many horticultural plants, especially amongst temperate zone perennial herbaceous and woody plants.

There are two main types of seed dormancy, physical and physiological. Physical dormancy relates to some component of the seed, usually the seed coat, which can inhibit seed germination, usually by preventing or delaying the uptake of water which is called imbibition. Since imbibition is the necessary, first step in seed germination, if it cannot occur, the seed will not be able to germinate. Seeds with this type of dormancy are also said to be “hard seed” which usually refers to the hard or impervious seed coat, the outer layer of the seed. Physical dormancy can usually be overcome by physical damage or weakening of the seed coat to allow imbibition to occur. This can occur in nature by a variety of means. As seeds fall from a plant, they may be exposed to normal weathering as imposed by freezing/thawing cycles, abrasion by wind or water, being chewed on by an animal, or by passing through the digestive tract of an animal where digestive acids work to break down the seed coat. In some cases, it can take many years for the seed coat to break down sufficiently to allow for seed germination to occur. Or, in the case of an animal eating the seed, that animal may spread the seed for some distance before depositing the seed in its feces. Some animals may carry the seed away and simply bury or cache it, as we see in squirrels and blue jays respectively. Here seed dormancy also helps with seed dispersal and in some cases, the animal feces provides a nutrient source for the young plant after it begins to grow.

Home and commercial propagators can overcome physical dormancy by abrading the seed coat of seeds with sandpaper or by tumbling seed with sand or fine gravel. We call these methods of scarification. Think of “scarring” the seed coat top help you recall that term. Kentucky coffee tree has very large, hard seed with a seed coat that is about 1/8” thick. It can take many years of weathering before tiny cracks or fissures develop in the seed coat to finally allow for imbibition and the seed to germinate. Hard seed may also be soaked in hot water or acids to dissolve the seed coat. However, whichever method is used, care must be taken so as not to damage the tender seed parts beneath the seed coat, especially the embryo. Sensitive plant (Mimosa pudica) is a fun plant to grow from seed but it is often difficult to get to germinate without soaking the seed in hot water for a day or so. By then the seed will often have begun to sprout and can carefully be transferred to a pot of growing medium or flat to continue growing.

Physiological dormancy comes from within the seed itself. Plant hormones like abscisic acid may be produced by the seed during seed maturation or later during storage. The presence of that abscisic acid prevents the normal processes of germination. Under moist conditions with cool temperatures, around 40°F, these germination-inhibiting compounds may be leached out of the seed or simply broken down by the seed so that the seed can then germinate. It usually will take 4 to 8 weeks for this to occur. This can be of benefit to the plant in the case of temperate zone plants that shed their seeds in late summer or fall. If those seeds were to germinate quickly, the young seedlings could be killed by freezing temperatures before becoming sufficiently established and cold hardy to survive the winter. Or they might just be killed outright in the case of annual plants. In nature that cool moist period is provided by the cool moist conditions that most seed would experience while lying on the ground, getting covered with leaves perhaps, and getting moisture from rains or melting snow. We call this process stratification. Think of a seed lying on the ground covered with layers or “strata” of leaves and snow to help you recall this term. Once the weather warms in the spring, the seed’s dormancy would have been broken, allowing the seed to germinate when favorable temperature and moisture conditions occur.

So, from a practical standpoint, if a gardener wants to plant seed from a temperate zone perennial plant that requires stratification, an easy approach would be to plant the seed in the garden or a desired location in the fall. This will allow for natural stratification to occur. Gardeners can even try snow-seeding where the seed is sprinkled onto prepared soil right before a snowfall or even on top of the snow. Ideally it is better to get the seed right on the soil, but even if it is applied on top of snow, when the sun comes out, it will often warm the darkly colored seed, allowing it to melt its way down through the snow. However, since we often see our snow melting by the end of March, the seed needs to be “planted” before about the first of March for this method to be successful. Otherwise there might not be a long enough period of temperatures in the 40°s to allow for stratification to occur and the breaking of dormancy.

However, if a gardener or propagator wants to germinate the seed during the winter or early spring, artificial stratification can also be accomplished quite easily. One method would be to plant the seed in a flat or pot of moistened germination media. Place the pot in a plastic bag with a few holes punched in it to allow for air exchange and place it in a refrigerator or cooler. Commercially, entire flats may be sown and placed in a cooler where humidity is maintained at about 95% RH. After about 6 weeks, the flats can be moved to a germination chamber at the optimal germination temperature and 95% RH to allow the seed to germinate. Of course, different plant species may have slightly different stratification requirements.

In some plant species, physiological dormancy may be much more complex, requiring periods of cool and moist, followed by a period of warm and moist and then another period of cool and moist before dormancy will be broken. Other stratification scenarios may be found with other plant species. And, in some cases, some means of scarification may also be needed to overcome physical dormancy. Thankfully, a home gardener or commercial propagator does not have to germinate seed by trial and error. Often germination treatments are listed right on the seed packet or in the seed catalog. There are also many good reference manuals and other resources available on the Internet that can provide germination protocols to help assure success. Even when the requirements are known for a particular species, it can be difficult to germinate some seed.

Spring Fever 2018 Gardening Event

Spring Fever 2018 Gardening Event will be held in the Rushmore Room at Ramkota Hotel in Rapid City on March 3, from 8 a.m. – 4 p.m. This year’s conference features two presentations by Linda Chalker-Scott of Washington State University, speaking on “Evidence Based Gardening Information” and “The Root of the Problem, When Plants Don’t Thrive”. John Ball, SDSU professor and Extension forestry specialist will present “Selecting Trees for Western South Dakota.” Short “table talks” topics include: “Growing Vegetables in Containers on Your Deck or Patio”, “Caring for House Plants”, “Conserving Water in Your Garden”, and “Using Compost and Mulch in Your Garden”. Lunch catered by Minerva’s is included in the $35 registration fee. The event includes a Silent Auction, Door Prizes and a Free Table. Registration is limited to 200, please register before Feb. 27. Visit www.blackhillsgarden.com under the “Welcome” tab for registration blanks and more information.

South Dakota State Horticulture Society 2018 Spring Workshop

South Dakota State Horticulture Society 2018 Spring Workshop will be April 21 at the Plains Dining and Recreation Center, located at 960 4th St. NE in Huron, S.D. Registration begins at 8:45 a.m. with the program starting at 9:30 a.m. We are extremely fortunate to have Mr. Brent Heath, of Brent and Becky’s bulbs as our speaker for the workshop. Brent is a noted authority on bulbs who has spoken all over the country and world, educating and promoting the use of flowering bulbs of all sorts. He will speak on four different topics: “Bulbs as Companion Plants”; “Pest Resistant Bulbs”; “Tantalizing Tulips”; and “Lovely Long Lasting Lilies and Awesome Alliums”. Cost for the day-long workshop, including lunch is $30 for members; $35 for non-members. Send payment to Glenda Oakley, treasurer, by April 4. For mailing address, email her at goakleyhuron@yahoo.com. Mr. Brent Heath is co-owner with his wife Becky of Brent and Becky’s Bulbs. Brent and Becky, along with their family has been growing bulbs since 1900 when Brent’s grandfather moved to Virginia after falling in love with the Daffodil. Today, they still love the Daffodil, but offer so much more! Bulbs, seeds, perennials, plants, supplements, books, gifts…they’ve really evolved! They live and work on their 28-acre farm and gardens in Gloucester, Va. They are well-known flower bulb suppliers, garden writers, photographers, lecturers, consultants and educators as they developed 8-plus acres of educational demonstration gardens. You can view their website, catalog, learn more about Brent and their company and read more about the topics on which he is speaking at www.brentandbeckysbulbs.com.

Scarified seed showing exposed cotyledons. iGrow photo
Sensitive plant seed soaking in hot water. iGrow photo
Small seedling beginning to emerge after stratification in a cooler. iGrow photo
Kentucky coffee tree seeds 5 days after scarifying. iGrow photo
Narcissus bulbs in flower. iGrow photo
Sensitive plant seed 3 days after scarifying in hot water. iGrow photo