Sowing perennial seed is one of my favorite tasks of winter. There's so much anticipation wrapped up in this simple practice, even more so than sowing say lettuce or tomato seeds with their quick germination. Perennial seed, both woody and herbaceous, may take weeks or months before the first hint of green is seen, so even in this waiting there is an air of mystery. My eyes hover over the trays as I search for life, and I find myself wondering if and when.
Many perennials have evolved to use the exposure to wet cold as an indicator for when they should emerge from dormancy and resume growth. When I teach the concept of this physiological dormancy to students, I liken this period to an alarm clock. Seeds have to get the appropriate amount of wet chilling time for the maximum number of seeds to germinate (or “wake up”). Without this exposure, none or only a few may start growing (as far as the few goes, variation does exist within a population!). This moist chilling results in physiological changes to the embryo that then allow the seed to overcome dormancy. You can hypothesize how this could be selected. Those that emerge early die in the cold. Those that bide their time awake to a glorious spring and can pass on their traits.
The staggered germination of Echinacea sanguinea. Note that there are a few larger ones with two or three true leaves that evidently didn’t need any chilling to enhance germination, but many seeds did and have only their pair of cotyledons. Don’t mind the wavy-leaf Streptanthus on the bottom or the couple of Penstemon tenuis that creeped into this block.
While this method of overcoming of dormancy occurs in nature, we horticulturists call the practice where we expose seeds to cold and moist conditions cold stratification (there is also warm stratification for some seeds, hence the addition the modifier). Usually the seeds are placed in moistened sand, perlite, vermiculite, peat moss, or some blend of those; I've even used paper towels and coffee filters if they go into the fridge. Now, mind you, all seeds don’t need this exposure. Again, I recommend Norman Deno's wonderful online Seed Germination Theory and Practice to investigate the requirements.
I've always wondered about the etymology of the word stratification. Strata means layers, not seed. Then, I read in Hartmann and Kester’s Plant Propagation: Principles and Practice how nurseries used to make layers of seed that were separated by moist substrate in outdoor pits. These “strata” were allowed to overwinter there. One of the earliest historical writings about stratification is found in Sylva by John Evelyn in 1664. I've shared his writing below of how he recommended stratifying, and to save myself from writing "[sic]" every other word, just note that spellings back then were a bit different.
“Chuse your seed of that which is perfectly mature, ponderous and sound; commonly that which is easily shaken from the boughs, or gathered about November, immediately upon its spontaneous fall, or taken from the tops and summities of the fairest and soundest trees, is best, and does (for the most part) direct to the proper season of interring, according to institution.
Nature herself who all created first,
Invented sowing, and the wild plants nurs’t:
When mast and berries from the trees did drop,
Succeeded under by a numerous crop.
Yet this is to be consider’d, that if the place you sow in be too cold for an autumnal semination, your acorns, mast, and other seeds may be prepared for the vernal by being barrel’d, or potted up in moist sand, or earth stratum super stratum during the winter; ... Chuse a fit piece of ground, and with boards (if it have not that position of it self) design it three foot high; lay the first foot in fine earth, another of seeds, acorns, mast, keys, nuts, haws, holly-berries, &c. promiscuously, or separate, with (now and then) a little mould sprinkled amongst them: The third foot wholly earth: Of these preparatory magazines make as many, and as much larger ones as will serve your turn, continuing it from time to time as your store is brought in. The same for ruder handlings, may you also do by burying your seeds in dry sand, or pulveriz’d earth, barrelling them (as I said) in tubs, or laid in heaps in some deep cellar where the rigour of the winter may least prejudice them; and I have fill’d old hampers, bee-hives, and boxes with them, and found the like advantage, which is to have them ready for your seminary, as before hath been shew’d, and exceedingly prevent the season. ”
Don't you just love the poem in the middle there?
Of course our technique has improved since the 1600's. In the past I have put the seeds in the fridge, and I have colleagues who also use this cooler method. But, I like to take advantage of natural winter chilling to germinate seeds. It is so much easier than bagging seeds up, putting them in the fridge, and then remembering to sow them later on. Why artificially create what nature already offers? Also, there is some evidence that the diurnal changes in temperature may positively promote germination.
Last winter, I filled the cold frame with 4 inches of growing substrate and sowed several different species directly into that media. We were traveling for the holidays, and I was worried about the seeds drying out too much in individual cells while we were gone. This method worked well. I got good germination on most species, but sowing en masse made it difficult to separate them when planting up later on.
The cold frame last winter. Old, used labels that were headed to the trash became block edges for different species.
This year, I sowed the seeds into trays to make it easier on me. We didn't travel over the holidays, and I was able to more closely monitor their moisture levels. Because I knew I would run out of room in my cold frame this year, I built a small low tunnel frame where I can cover a 4 × 8 ft area with floating row cover supported by the same hoops I used to cover my vegetable beds. The floating row cover provides just enough protection to keep the frost off.
In my rush to get the seeds sown and because a few of them of them still have fluff or chaff on them, I don't fret much about trying to get one seed per hole. I can always transplant or move seedlings around later in their cells. I will note that I flame some seed like Liatris, Andropogon, and Asclepias in a bucket to get the fluff off the seed.
I am fascinated by seeing the differences in germination time, even for some species in the same family. For example, Penstemon tenuis germinated fairly quickly within a week while Penstemon digitalis took a month from sowing to start showing cotyledons.
Penstemon digitalis takes longer to germinate than Penstemon tenuis (not pictured).
Winter is a good time to clean out the fridge for seeds and go through old boxes. I had hoped to be done by now with my winter perennial sowing, but I keep finding more species that I saved earlier this year to sow now. For most of the country you should still have a little time to get seed that need stratifying sown outdoors or just use the fridge method I mentioned earlier. Either way is sure to result in more plants this year for your garden.
Ipomopsis rubra was another fast germinating species. I transplanted them out shortly after this photo.
Amsonia hubrichtii emerge from their cigar-shaped seed.
The thread-like cotyledons of Lilium formosanum
