Propagation, Germination Requirements of Seeds
Germination Requirements of Seeds
by Bill Cullina
New England Wild Flower Society
What is interesting and maybe a little intimidating about seed sowing are the different requirements that have to be met before a particular species will germinate. Most temperate plants have some mechanism that prevents their seeds from germinating during the wrong season, when they’d be killed by harsh weather. These mechanisms involve some combination of temperature fluctuation, the presence of moisture, light, or inhibitors in the fruit or husk and/or the need for mechanical scaring of the seed coat.
To make things simpler, we indicate on every seed packet what treatment that species will need for optimal germination. Again, this is meant as a guide, and following these recommendations will usually lead to the fastest and most consistent results. But if all the different treatments seem intimidating, simply sow the seeds outdoors in the spring or fall and let the seeds germinate when they are ready These Codes are as follows:
NOTE: The Germination codes for species grown by The New England Wild Flower Society Native Plant Nurseries handout is a listing that references these codes.
Germination Code A: seed will germinate within 4 weeks if sown at 70 degrees F
This is a common pattern with tropical and annual plants, and a few hardy perennials fall in to this category as well. Addition of water is the mechanism allows the seed to germinate, so this pattern is more common with dry land and desert plants that must be ready to grow when the rains come (technically this is referred to as non-deep physiological dormancy). In order for many A type seeds to germinate, they need a period of 1-3 months of dry storage (after ripening) prior to planting. Thus, seed collected in fall and sown immediately will germinate poorly, while seed that has been cleaned and stored dry (preferably at 40 degrees) for 3 months will come up well. Still others, like many in the Aster family, will germinate with an A treatment, but emergence is erratic and slow. Cold stratification (B) will lead to more uniform germination in this case, and for that reason, I treat most of the A type seeds I sow as B. Partly this is because the length of dry storage or after-ripening necessary to overcome dormancy can be much longer to complete than cold stratification (Asclepias seed may require a year of dry storage to germinate well, but only 60 days of moist, cold stratification will yield the same percentages) A period of moist cold will generally have no ill effects, and it makes the process less complicated.
Germination Code B: seed will germinate upon shifting to 70 degrees after 90 days moist, cold stratification at 40 degrees
This is by far the most common germination pattern among the temperate wildflowers. It is vital that the seed be sown and well moistened before exposing it to cold. Water “activates” hormone and enzyme systems in the seed that block germination and allow the embryo to measure the length of cold that it is receiving. Dry, cold storage will not usually substitute for moist stratification if the seeds are truly type B germinators. The easiest way to germinate seeds of this type is to sow them outdoors in fall – either directly in the garden or in prepared bed or pots kept in a cold frame or crawl space that does not get too cold. Natural temperature changes are often much more effective than those you might achieve by placing the flats in a refrigerator. However, if you receive your seeds late in the winter, live in an area where winter temperatures do not stay regularly below 50 degrees, or do not have a place to keep them outdoors, you can sow them in pots and place them inside a plastic bag in the refrigerator (not freezer) section of your cooler. A method that takes even less space is to mix the seed with some damp vermiculite in a self-seal baggie and put them in the cooler that way. After cold treatment, sow the lot in a container just as you would any seed. Liatris spicata, Spigelia marilandica
Germination Code C: seeds germinate only after multiple cycles of warm and cold, typically 40-70-40-70
Multicycle germination is common in the Lily family and woodland forbs. It is a slow process that discourages many beginning propagators, but it is really not too difficult. Long germination times are necessary for several reasons. First, many plants, like Trillium, Polygonatum, and Dirca aretwo stage germinators – that is after their first cold period they germinate underground to form a rudimentary rhizome or bulb and root system, but must be exposed to another cycle of cold and warmth to send up the cotyledon, or in the case of Jeffersonia, the first true leaves. Secondly, some (Jeffersonia, Caulophyllum, some Quercus) are hypogeal germinators, with cotyledons that remain umder ground to feed a developing root system the first year or two. Thirdly, plants like Goldenseal have embryos that are not fully developed when the seed is ripe, and need a full growing season to mature the embryo and prepare for germination. Others, like Blue Cohosh have extremely hard seed coats that need time to weather away so that water can permeate. In all cases where the seed is both hydrophyllic (*) and a C type germinator, germination may be enhanced by collecting the seed when it is immature and sowing it immediately outdoors. Roughly this means collecting the seed 3-4 weeks early, but you may have to experiment with a particular genus to get the timing right. Seed should be plump and full size, but still green or white without a hardened/darkened seed coat. I have had success with Actaea, Trillium, Clintonia, Hydrastis, and some Disporum using this method but it does not always work. Furthermore, most C germinators need exposure to the cycling or fluctuating temperatures and long warm and cold seasons (4-6 months) that you find outdoors, and germination under artificial conditions of refrigeration/greenhouse is hit or miss. Seed that is sown in beds or cold frames and kept watered and allowed to go through the natural seasons will give the best results. Trillium cuneatum
Germination Code D: seed needs a period of warm moist stratification followed by cold stratification and will germinate after shifting back to warm (70-40-70)
This is a common cycle with early ripening, hydrophyllic species like Bloodroot that naturally experience this pattern when they fall to the ground in early summer. Since I sow most of the hydrophyllic species immediately outdoors, they get the proper treatment as a matter of course. However, when we sell seed of plants like Bloodroot that has been stored moist at 40 degrees since collection, it will not germinate until it receives 3-4 months of warm then 3-4 months cold temperatures, so typically will not emerge until the second spring if sown outdoors in February of March. Some hypogeal species, like many of the true lilies, form a bulb and roots at 70 then sent up a cotyledon after 3 months at 40. There are others, like Black Cohosh, that have immature embryos which require 3 months of moist warmth to develop before shifting to cold. Since both the Lilies and Cimicifugas ripen late in the year, I usually sow the seed in flats in a warm greenhouse (or on top of the refrigerator, near a radiator – any warm place) and leave them for at least three months before shifting them to a cold frame or the refrigerator for 3 months. In this way they come up well the following spring instead of waiting an additional year. Clematis albicoma, Lilium canadense
Germination Code G: Physical or Chemical inhibitors in the pulp and skin of fleshy fruits
Many if not most seeds that ripen in fleshy fruits must be well washed of all pulp to remove possible germination inhibiting chemicals it contains and also to remove physical barriers to water absorption . See seed cleaning. Hydrastis canadensis
Germination Code H: Seed requires light to germinate
Small seeds that lack the resources to germinate deep underground are sensitive to light, and will germinate only when a certain minimum intensity of light hits the seed coat. This is a cue that they are positioned at or near the soil surface. As a rule of thumb, I treat any seed smaller than a grain of salt as a type H, because they need to be sown shallowly anyway, whether light is important or not. There are also a few large-seeded species that require light, but these are rare. The easiest way to handle H seeds outdoors is to sow them over the surface of a tamped and dampened seed mix and cover them with an 1/8th inch layer of washed, coarse quartz sand. Quartz sand transmits some light just as glass does, yet provides some protection from drying and weather. I use #1 filter washed filter sand, available at swimming pool supply stores, as it is uniform and coarse, but you can make your own by screening regular builders sand though a window screen then washing what remains in the screen to remove dust. It will have a few large pebbles that can be picked out, but is otherwise a good size – really a fine gravel. Alternatively, you can sow the seed on the surface of a moistened flat of seed mix and enclose the whole thing in a large, self-sealing bag. If the seed is type A,H, put the bag under grow lights at 70 degrees (you can put it on a windowsill, but it is hard to find a spot with enough sunlight light for most plants that will not at the same time heat up the bag too much). Seedlings can be left in the bag for up to a year, but check them regularly for water and fertilize every month with 1/2 strength liquid fertilizer when they are actively growing. Wean them out of this saturated environment slowly when they are large enough to transplant by first opening up the bag part way for a week or two. B,H seed can be treated the same except put the flats in the refrigerator for 3 months prior to setting them out in the light. Lobelia siphilitica
A final procedure that is an excellent way to start small-seeded, acid loving species like Galax, Shortia, and Epigaea and Gautheria is moss-sowing. If you have a damp spot in the yard where moss grows well and stays fairly damp through the growing season, you can shake the seeds into the moss carpet and carefully water them in. You may have to supplement watering to keep the moss damp, but this method can work remarkably well. Alternatively, you can establish a moss carpet in a container by transplanting some tufts of moss. The tight growing, carpeting types like fern-leaf moss are easiest to get going (a mix of 1:1 peat and sand is good for acid-loving mosses, but others grow well on the surface of regular potting mixes as anyone who grows plants in containers can attest). Sphagnum works well, and certainly use it if you have some growing wild because it usually stays evenly moist. You can grow it by burying some of the long strands almost to their tips in a partially submerged container of 1:1 peat and sand, but it needs a season to become thick and established. All mosses are best watered with rain or distilled water. Dryopteris goldiana
Germination Code I: Seed requires scarification because of an impermeable seed coat
Many plants in the bean and mallow family have hard seed coats that will not let water in to the embryo. Mechanical abrasion of the seed coat is necessary for imbibation and germination. Many legumes will germinate almost immediately after scarification but can take a year or more otherwise. There are several methods I have used. Placing a pile of seeds between two pieces of fine sandpaper and grinding the top piece back and forth quickly with your palm is easy and effective. It is not necessary to remove much of the coast, and typically 15 – 30 seconds of grinding is sufficient. Some people swear by the hot water method, where near boiling water is poured over a pot of seeds and they are allowed to cool and soak overnight before planting. The soaking allows the seed to imbibe all the water they need. I have had mixed results with this treatment. It works well with Baptisias and Prairie Clovers, but not so well with Lupines. The milkshake blender method of cleaning seeds I describe on page (???) also helps scarify them as well, and is an easy way to treat things like Blue Cohosh. Sowing the seed outdoors in fall will also weather the seed coat by spring, and for Baptisias, a combination of sandpaper scarification and cold stratification in a cold frame gives me consistent results.Baptisia macrophylla, Callirhoe involucrata
*Hydrophyllic Seed: intolerant of dry storage
Includes all the ant-dispersed woodland species as well as a number of others that do not take desiccation very well. Freshly collected seed should be sown immediately or alternatively cleaned and stored in sealed plastic bags at 40 degrees. Seed that ripens in fleshy fruits or has an attached elaiosome is best stored with a handful of slightly damp vermiculite mixed in. This maintains a consistent saturated atmosphere for the seeds. Small seeds as well as seeds that ripen in dry capsules and lack elaiosomes can be stored without the vermiculite, which will discourage molds.
Native genera containing at least some hydrophyllic species:
(Portions excerpted from: The New England Wild Flower Society Guide to Growing and Propagating Wildflowers, and Native Trees, Shrubs, and Vines by William Cullina)