Native Plant Sale
|Seed Dormancy in California Native Plants
|by Susan Ashley
Flannelbush (Fremontodendron californicum) seedpods. Photo by Jason Hollinger
Growing California native plants from seed, although generally rewarding, has its challenges. The seeds of some species, given the ideal environment of water, light, and temperature, simply refuse to send out the tiny root tip known as the radicle, which signals germination. A secondary requirement must be overcome before germination can take place. This requirement is termed dormancy. The challenge for propagators is to overcome this naturally occurring dormancy in an artificial environment.
Dormancy enhances survival of a species by ensuring that seeds will not germinate until environmental conditions are optimal for survival. A short warm spell in February, for example, could spell disaster to seedlings if followed by a snowstorm.
There are two major types of dormancy: external and internal. External, or physical, dormancy, is caused by either a hard seed coat or chemical barriers in the seed coat that prevent water uptake. Internal, or physiological, dormancy is caused by mechanisms within the seed itself that delay germination until certain requirements are met. External dormancy is fairly straightforward: The seed coat must be scratched or softened until water can penetrate.
Internal dormancy can be caused by a variety of factors. The seed might be shed with an immature embryo that requires weeks or months of after ripening to mature. Chemical inhibitors might have to be broken down by cold or warm moist storage, a process known as stratification, or by leaching with water. Other seeds will remain dormant until their requirement for light or darkness is met. This is called photodormancy. In many instances more than one requirement must be overcome. Compound dormancy is common in the seeds of California native flora. The seed coat might be hard, and internal chemical barriers might be present (e.g., flannelbush, Fremontodendron californicum); or the seed might require leaching and cold stratification (many desert species); or the embryo might be immature, and the seed could require several cycles of warm then cold moist stratification (e.g., wild ginger, Asarum caudatum).
Seedpod of matilija poppy (Romneya coulteri). Photo by Robin
The seeds of matilija poppy (Romneya coulteri) are shed with an immature embryo, but in addition to the time needed for the embryo to mature, they require exposure to smoke or ash from a fire to release dormancy. The discovery of smoke sensitivity in the seeds of fire-follower plants is a fairly recent discovery. It was once thought that the heat of the fire was the factor at hand in breaking dormancy, and many seeds with external dormancy do indeed need heat shock to alter the seed coat. Heat alone does not break dormancy, however, for a great many seeds that germinate after a wildfire. Smoke is also required. The compounds in smoke residue that precipitate onto the soil, as well as the compounds in ash and charate, dissolve in the first rains following a fire and bathe seeds that have lain dormant for years, thus releasing dormancy.
Swanton Valley, California hillside after the 2009 Lockheed Fire, Santa Cruz County. Photo by Peter Ashley
The first year after a fire, resources are greatest for seedling establishment. The soil has been enriched by the ash, light is abundant, there is little competition, and the removal of surface duff allows water to penetrate the ground. Dozens of chaparral species have become adapted to dormancy release by smoke, or a combination of smoke and heat shock (Arctostaphylos sp.), or smoke and cold moist stratification (Romneya coulteri). The beautiful tree or bush poppy, Dendromecon rigida, appeared on a hillside in Swanton Valley, California, after an intensely hot fire (the Lockheed Fire of 2009). The fire burned off a dense stand of Schreiber’s manzanita (Arctostaphylos glutinosa) and knobcone pine (Pinus attenuata). Tree poppies had not grown on that hillside for more than four decades, but the following spring the entire hillside was covered with a thick stand of them. Today the tree poppies are slowly giving way to a new stand of Schreiber’s manzanita and knobcone pine, also fire followers, but their seeds will remain in the soil until the next conflagration.
The same Swanton Valley, California hillside the following Spring. Note the Dendromecon rigida. Photo by Peter Ashley
California bush poppy (Dendromecon rigida). Photo by Pete Veilleux
So how do you, as a propagator, replicate what works in nature? Foremost, observe where the plant grows. Seeds of high-altitude plants require cold stratification to simulate natural winter conditions. For cold stratification you can mix seed with damp perlite or clean sand and refrigerate for at least 6 weeks at 40⁰ F. Gibberellic acid, a harmless plant growth regulator available as a white powder, will shorten the time needed for cold stratification. As an added benefit it will release internal dormancy in some seed.
Seeds such as those of wild ginger (Asarum caudatum) and leopard lily (Lilium pardalinum) that can take up to a year to germinate in the wild have immature embryos and thus require 1 to 3 months of warm stratification. Place the seed mixed with dampened perlite in a 70⁰ F location, such as on the top of the fridge, and leave it there for 4 to 6 weeks. To avoid moldy seed, first dip the seed in a solution of 1 part bleach to 9 parts water, rinse in plain water, and then stratify.
Wild ginger (Asarum caudatum). Photo by Peter Stevens
Seeds of plants from temperate or tropical high rainfall areas may require leaching with water to remove chemical inhibitors. To leach seeds, tie them in an old sock or stocking and hang it in the toilet tank (not bowl) so that water repeatedly flushes over it for up to 7 days.
Seeds of desert plants and plants that pass through the gut of foraging animals must be scarified or abraded to loosen the seed coat. You can use hydrogen peroxide 3% to penetrate the impermeable seed coats of Salvia species. Simply soak the seeds for up to 1 hour, then rinse and sow. Hydrogen peroxide also delivers oxygen to the embryo, kick starting germination. You can also use sandpaper, boiling water, or sulfuric acid (careful handling required!) to soften an impermeable seed coat.
Seeds of fire followers require smoke treatment. To replicate fire exposure, you can soak seeds in liquid smoke from the grocery store, in a solution of 1 part liquid smoke to 9 parts water. Soaking time is variable, and it is best to experiment.
One of the most unusual dormancy-breaking treatments involves burying the seed in a net bag in the ground for a year. After that, smoke treat the seed, cold-stratify it, and then sow, watch, and wait. This method has been recommended for the seeds of Pacific bleeding heart (Dicentra formosa), woolly blue curls (Trichostema lanatum), and tree poppy (Dendromecon rigida), three notoriously difficult seeds to germinate.
Woolly blue curls (Trichostema lanatum). Photo courtesy Wikimedia Commons
To learn more about the rewards and challenges of propagating California native plants, check out the internet, take propagation classes such as those offered by the Friends of the Regional Parks Botanic Garden, and best of all, talk to people who are propagating California natives from seed.
Susan Ashley has taught plant propagation at Merritt College, Diablo Valley College, and the Regional Parks Botanic Garden. She has a small backyard nursery specializing in California native plants and unusual shrubs.
WAYNE RODERICK LECTURES WINTER 2016-2017
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|December 11; 10 am - 12:30 pm
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