Perennial Seeds May Need a Winter - Dormancy
Seeds of many perennials do not germinate as easily as annuals. Perennial seeds often contain germination inhibitors, abscisic acid, dormin, waxes and other oils. This is nature's way of ensuring seeds germinate when conditions are optimal. Annual plant seeds generally have a shallow dormancy and do not need a winter to germinate, they only live one season. The techniques below are for perennial seeds only, do not use these techniques on annual or bi-annual plant seeds.
In the wild, dormancy is broken by spending time in the ground through the winter so that its hard seed coat is softened by frost and weathering. This cold moist period triggers the seed's embryo to grow and eventually break though the softened seed coat in its search for the sun and nutrients.
In all cases below use well-drained seed starting or potting soils. See our wiki Potting Soils: Customizing to a Plant's Needs.
Some seeds require scratching or nicking the hard seed coat to allow moisture to enter the seed to begin germination. This mimics natures weathering, freezing and thawing, or the gnawing of rodents. Medium to larger seeds can be nicked with a knife, filed or rubbed with sandpaper. Test whether a seed needs scarification or not by soaking in water overnight. If swells up, it is imbibing water and probably does not need scarification.
Rub smaller seeds between sandpaper or emery paper. Hobby rock tumblers can be used to scarify larger seed volumes. Abrade only the outer coating, embryos should not be cracked or damaged to remain viable. Commercial nurseries scarify using solutions of sulfuric acid. Do not scarify on the hilum attachment scar.
We do not recommend hot water scarification. This method can greatly damage your seeds by burning them and rendering them non-viable.
Some seeds have a double dormancy requiring both scarification first followed by cold stratification. Others such as Black Cohosh require both warm and cold stratification. See a varieties' growing instructions for its scarification and stratification requirements.
Cold stratification is the exposure of seed to conditions of cold temperature for germination to begin. The range of cold temperature or vernalization required varies from one crop to another. In general, an optimum temperature for most crops is between 33˚F to 40˚F for at least 8 weeks. Each of our seed packets and online listings should state the desired cold stratification period in weeks. Do not freeze seeds unless you are certain that they contain less than 10 percent moisture.
Here are six methods of stratification. Try different methods. Buy multiple packets of difficult to germinate seeds. Split your seeds between different methods, start dates or cold stratification lengths. Label name and start date with a lead pencil.
Seed requires the recommended treatment only once. For example, do not cold stratify and then fall sow. This is a double cold stratification. Once the initial stratification is complete the seed is ready to germinate. Should there be a fall warm up, the seed will begin the process of germination and may be killed by the freezing temperatures of winter. In addition, once the seed has completed stratification, it is ready to start growing. In this state the seed will use up its energy reserves at a rapid rate and will have a much reduced shelf life at warmer or cooler temperatures.
Seed should be stored cold prior to sowing or stratifying. Allow the seed to warm to room temperature, then soak in water 24 hours, for most species. Seed must be moist to stratify. Most seeds are dried to below 10 percent moisture for best storage. Seed below 10 percent moisture does not stratify well and may just sit dormant. The moisture level must be increased to 60 percent or more to allow the seed to begin to stratify. Cold stratify in damp vermiculite in a refrigerator during December and January. Experienced growers like vermiculite because it washes off the seeds easier. For small seeds like Arnica, put them in a bag with a damp, but not wet, paper towel that does not contain bleach. Avoid fine sand because it is harder to work with. Sow in plug flats or undivided trays in the greenhouse when seeds comes out of the refrigerator. Read packet instructions to see if bottom heat is required, for most it is not. Reminder to keep the seeds damp, not wet.
2. Outdoor Treatment
This method mimics natural freeze-thaw stratification. Mother Nature will naturally cold stratify seed over the winter for seeds planted outside in colder areas. Sow seeds in well-drained starting soil mix. Place the flats on the North side of your house or in full shade. Be sure seeds are protected from rodents, birds and cats. We cover our flats with weighted 1/4" or 1/2" hardware cloth. One advantage of this method is that young seedlings do not have to compete with weeds as they get started and can be planted out when more mature. If you purchase your seeds after mid-March, do not use this method because you may need more cold winter evening temperature days than are left in the season. If you have a dry winter, place pieces of 1/4" ice or 1/2" snow on the flats to melt every few days, not a lot, just enough to keep some moisture on the flats.
3. Fall Sowing
Direct sowing in the garden or pots in the Fall is the traditional method in Europe. This method naturally exposes perennial seeds to winter conditions. Be sure to stake where you sow and date. Wood popsicle sticks fade and rot over the winter. Use larger stakes, plastic stakes written with a black Sharpie last the longest. Plant at a depth appropriate for the variety. Seeds requiring light to germinate, just press on the surface.
4. Winter Solstice Sowing
A variation of Fall sowing is planting later in the winter starting with the Winter Solstice on up to February. This method takes full advantage of winter cold and spring heaving and the growing energy of the earth as days lengthen. This works well for hard to start seeds like Good King Henry, we direct sow once in each month from November through March to find the right stratification window. In our zone 7b, mid-December was the proper sowing time to germinate Good King Henry.
Direct sow in place or sow in well drained flats. Some cover their flats with plastic with holes or screens to let snow. Make this a part of your annual Winter Solstice celebration. You can soak medium to larger seeds overnight to aid germination. If seeds do not require light to germinate, sprinkle with dry soil to cover per sowing depth on the seed packet.
The colder your winter the later you can plant. If you have cold winters and it is very cold out, 30˚F (0˚F), put your seeds in the fridge for two weeks before sowing outside to reduce the shock.
5. Snow Planting
This one is for the kids. After a heavy snow, go outside and broadcast your seeds on the snow where you want them to grown preferably over a prepared garden bed. Have a snowball fight or toss snow over the seeds so birds do not eat them all. This method works best for varieties that can handle cold but do not require cold stratification such as hardy annuals, biennials or short lived perennials.
6. Cold Water Soaking
This method works best for medium and larger seeds a few weeks before last frost. You are trying to imitate snowmelt. Place seeds in a small jar and fill with cold water. Viable seeds should sink, although this is not true for all seeds. Many flat seeds or seeds with edges float. Change water daily, your are trying to wash germination inhibitors in the seed. Sow after two weeks. You can also try putting seeds in a small muslin bag and suspending them in the toilet tank. What could be easier, automatic rinsing. If the seeds do not swell when soaked in water, they may need scarification to allow water to imbibe through their water tight covering.
When a seed requires both a warm and cold stratification, the warm stratification is done first, followed by the cold stratification. A warm stratification is done to soften the seed coat or allow the seed embryo to mature. Warm stratification is at the temperature of 68-86 degrees F˚.
Light Dependent Germination - Surface Sowing
Light dependency is controlled by a protein pigment called 'phytochrome' that resides in the seed coat. The survival advantage allows seeds to bide their time until they emerge into light and have the right conditions to develop into a mature plant. Usually required for very tiny seeds such as mullein and St. John's wort.
Mix tiny seed with tablespoon of fine sand to help prevent germinating in clumps. Water small seeds from below or light misting. Violent watering may dislodge the seed at the critical time and result in germination failure. Seedlings grown in flats should be potted up and grown out for several months until they reach sufficient size to transplant.
If direct sown seeds dry out in your climate from sun or wind and do not require light to germinate, keep moist by covering with a single layer of burlap, light colored cotton sheet or half an inch of loose grass clippings. Remove burlap or sheet after germination. Shading with a window screen or white row cover above them the first season will help prevent drying in hot climates. You can also cover with clear plastic until germination to retain moisture but be very careful not to overheat. As soon as sprouts appear, remove covering.
Although not dead, some seed resists imbibing water and remain dormant even when sown in optimal conditions. As much as 70 percent of seeds may not germinate. Hard seed may be waiting for a variety of conditions to germinate, heat dependence, light dependence, temperature variation, expansion and contraction, time after an initial flush of germination, passage of several seasons or the presence of gibberellic acid. Gibberellic GA3 acid is a germination stimulanting hormone produced in the seed when it imbibes water but not all plants can produce their own and require it in the soil. Gibberellic acid is released when fungi break down and is not present in sterile potting mixes. See our wiki Potting Soils: Customizing to a Plant's Needs about how to make your own live potting soil mixes.
Early Bottom Heat Germination
Starting tomatoes and peppers in January or February in the greenhouse sometimes requires a little more effort than in mid-April. These crops must be subject to continuous warm soil temperatures, 70-85˚F is optimal for tomatoes, peppers and yerba mansa. However, nighttime temperatures early on can be so cold that despite using a heat mat your surface soil temperature may be below 60˚F. Try doubling up on the heat mats to keep soil temperatures warm at night. Cover flats with a clear acrylic domes in the evenings. If you are having very cold temperatures add a row cover or second dome at night. Keep an eye on them so you do not solarize your starts during the day. Never cover your flats with clear plastic.
Invest in a soil thermometer to see exactly what your actual surface soil temperature is vs. what the heat mat controller may be set at. Some controllers can control up to four heat mats. If your tomato germination is slow, this may be the problem. We also place our tomato plug flats in a leak-proof tray filled with 1/2-3/4" of water to keep the environment moist.
Deep Potting Soil for Perennials
Perennial seeds requiring several months or even years to develop normally before transplanting out require deeper soil beds than provided by plug trays designed for annuals. Annuals develop quickly and are ready to plant out in a matter of weeks. Fill a gallon pot(s) with good potting soil, sprinkle the seeds on top, barely cover, tamp securely and keep evenly moist and in the light until germination occurs. Plant the whole packet because sometimes only a small portion of the seed will germinate on the first flush. Thin by transplanting to dedicated pots after the second real leaves develop. Hold on to the original pots and subject to outdoor winter conditions if you can after transplanting and see if more seeds germinate the following season.
Sow perennials seeds in sterile potting soil. Keep seeds moist but do not over water. Do not sow in peat moss or Perlite. See our blog, Making Your Own Potting Soil.
Tamping and Misting
Tamping gives the seeds a sense of place. If they shift after they have sent out their rootlet, a more delicate seed may not be able to reorient and be exhausted. Mist seed flats instead of spraying to prevent moving the seed around until starts are well established even if required two or three times per day. Do not allow water to pool over the seeds, it should drain quickly in good potting soil.
Good growing from Restoration Seeds.
Seed Germination Theory and Practice by Norman C. Deno, 1993-1994.
The Organic Seed Grower by John Navazio.
Seed to Seed by Suzanne Ashworth, 2002.
Starting From Seed: The Natural Gardener's Guide to Propagating Plants, Brooklyn Botanical Garden, 2000.
Plant Propagation, The American Horticultural Society, 1999.
The Medicinal Herb Grower, Volume I by Richo Cech, 2009.
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