Starting Plants from Seed

• Rock Garden Plant Reference Table
• Containers
• Growing Medium
• Seed Germination Delays
• Temperature
• Light
• Moisture
• Fertilizing
• Potting On & Hardening Off
• Another Process
• References

Rock Garden Plant Reference Table

This spreadsheet contains a great deal of information about germinating seeds.

Containers

You can start your seeds in almost any type of container as long as it is at least 2 to 3 inches deep and has some drainage holes.

Growing Medium

To provide the optimum growing conditions, and to avoid disease and insect problems, seeds should be started in a soilless growing mix, not in garden soil. A good soilless mix is a moist and spongy blend of sphagnum moss, vermiculite and perlite. The finer the texture the mix is the better.
For a lot of rock garden plants, add sharp sand or fine granite chips (#1 turkey grit) both in the mix and a fine layer on top to keep the seed in contact with the soil. After the seeds have germinated and the seedlings have grown to the two-leaf stage, they can be pricked out and transplanted into a coarser medium that contains some compost and some more sand or granite chips. The growing medium should be thoroughly moistened before it is placed in your seed starting containers (warm water works best), or alternatively, water it in the containers and top it up with more soil as needed prior to seeding. Gently moisten the growing medium after seeding using a mister to ensure good contact between the seeds and the soil.

Seed Germination Delays

Most plants have mechanisms to delay seed germination until the seeds have been dispersed. The treatment required to "break dormancy" (or to overcome the delay mechanism) varies for different species.

The publications on seed germination by Dr. Norman C. Deno contain detailed accounts of germination requirements for thousands of species. Here are a few generalizations from his studies:

1. Swamp species usually require light for germination; this is also a common requirement for woodland species.
2. Germination at 40 degrees F is generally observed in cold desert species.
3. Many species require light for germination.
4. Seeds from common garden vegetables and annuals are conditioned by simply drying the seeds.
5. Some plants from harsh, dry environments need gibberellins for seed germination.
6. Some seeds need exposure to fluctuating temperatures to cause germination. The process of exposing seeds in moist medium to fluctuating temperatures, such as they would experience outdoors in nature, is referred to as "stratification".
7. Some seeds that have impervious outer seed coats will germinate more quickly after scarification. This refers to abrading the outer coat, such as with sandpaper or a file, or, alternatively, by piercing or clipping it, to allow water to penetrate the coat more quickly.
8. Some seeds need darkness to germinate and this is accomplished by covering the seed with soil.

Temperature

The temperatures for optimum germination listed on seed packets refer to soil temperature, not air temperature. Though some seeds germinate best at a soil temperature of 60 degrees F, and some at 85 degrees, most prefer a temperature of about 70. Some seeds even have to be stratified to cause germination to occur. Stratification is a process where the seeds are sown in a moist medium and exposed to varying temperatures in order to stimulate germination (or, rather, to overcome germination delay mechanisms). The initial temperature may be warm or cold, and repeated cycles of alternating warm-cold or cold-warm may be needed. Dr. Norm Deno's work on seed germination showed that effective stratification can be achieved at around 70 degrees F for the "warm" period, and around 40 degrees F for the "cold" period. Deno also found 3 months to be an effective time period for exposure to each temperature, however, it should be noted that Deno's work relied on establishing standards by which to compare germination statistics for different species. The home gardener may be able to achieve sufficient germination while "shortcutting" Deno's standards, particularly the lengths of the cold/warm periods.

If the above discussion sounds overly complicated, one can take comfort in the fact that many of the better seed vendors provide clear and practical instructions for seed germination right on the seed packets, based on their own experiences with germinating the species.

If the soil is too cold, seeds may take much longer to germinate, or they may not germinate at all. After germination, most seedlings grow best if the air temperature is below 70 degrees F. Most seedlings grow fine in air temperatures as low as 50 degrees, as long as soil temperature is maintained at about 65 to 70. You can keep the soil warm by using a heat mat, if you are starting seeds in some area where the temperature is considerably less than standard room temperature (70 degrees F).

Light

Most seeds don't require light to germinate but some do. If light is required, do not cover the seeds with soil. Those that don’t require light to germinate may be covered with soil, though it is not necessary to do so. Upon germination, seedlings require light. Most seedlings require 12 to 14 hours of direct light to manufacture enough food for healthy stems and leaves. The characteristic legginess indicates that the plants are not receiving enough light intensity, or enough hours of light.

When growing seedlings under lights, some growers use a combination of cool and warm fluorescents, or full-spectrum fluorescent bulbs, but unless you plan to continue to grow them under lights to the flowering stage, a regular fluorescent will suffice until you move them outside. The light bulbs should be placed very close to the plants - no more than three inches away from the foliage. Fluorescent bulbs do not generate enough heat to burn foliage, so even if the foliage touches the bulbs, it is not a concern.

It seems to be a common problem among beginner seed-starters (in areas with long winters, at least) to start seeds too early, under less-than-adequate light sources, which can result in weak seedlings that are prone to disease and often to failure. This can be avoided by starting seeds later, or preferably, by increasing the intensity of the light supply. (An indoor light system that consists of 2 standard 4-foot fluorescent ballasts, with 2 lights in each ballast, over each shelf of the seed-starting stand, seems to provide sufficient light for an indoor growing period of about 13 weeks, in the experience of one contributer.) Fluorescent lighting is simple to use and widely-available, although it is not the only light source that may be used.

Moisture

Germination requires constant moisture. It is important that the soil be kept moist but not soggy to prevent the seeds from rotting. There are different ways to achieve this. Some gardeners cover their flats with clear plastic until the seeds germinate. Some inexpensive seed-starting trays come with clear plastic covers.

As soon as your seeds have sprouted, remove any plastic covering to reduce moisture and humidity levels. Check the soil every day to ensure that it is moist, not wet. Too much moisture will retard root growth and lead to disease problems. Letting the soil dry out a bit between waterings helps prevent molds and fungus from growing on the soil surface. Don't use water that has been through a water softener. Try to make sure that the moisture reaches the bottom of the growing container so your seedlings will stretch their roots out and create a nice, fat root-ball.

Most seedlings like a humidity level of 50 to 70 percent. Higher humidity levels and poor air circulation can lead to fungus growth on the soil surface and disease problems. If the air in your house is very dry, you can keep your seedlings happy by setting them on capillary matting, or in a waterproof tray filled with small stones and a little water. If your plants are in a small room, you may consider running a small fan to keep the air circulating.

Fertilizing

Once your seedlings develop their second set of true leaves, it is time to start feeding them. Young seedlings are very tender and can't tolerate a full dose of fertilizer. Baby them with a half-strength dose until they are three or four weeks old. Since your seedlings are growing in a sterile, soilless medium, fertilizing them is absolutely critical.

Potting On & Hardening Off

The next process is to thin, "pot on" and "harden off" the seedlings. "Potting on" refers to removing seedlings from the original pot (which may be quite overcrowded, particularly if very fine seeds were sown) and moving them on to individual pots for further growth. If seedlings were grown indoors, they will need to be hardened off prior to being planted outdoors, that is, accustomed to outdoor conditions and to sunlight, mainly. For many gardeners, hardening off involves setting pots or trays outdoors or in the sun for gradually-increasing lengths of time. (Note that this must be done with caution to prevent seedlings from being sunburnt, which results in the foliage turning limp and then whitish; seedlings may not be able to recover from sunburn.) (In the relatively cool, but very sunny climate in which this contributor gardens, setting seedling trays in the shade for a few days hardens them off with a minimum of effort.) It is most convenient to wait until after the last frost in your area to begin hardening off indoor-grown seedlings; otherwise, one may feel compelled to move trays indoors whenever overnight frosts threaten.

Another Process

A lot of rock garden plants can be germinated and grown to the transplanting stage by a very simple process that is sometimes referred to as "winter sowing." During the late fall to mid-winter, seeds which have been potted-up and watered in containers can then be set outside in the shade or in a cold frame - or in the sunny open, depending on your preference or experience in your area - where they will experience the natural temperature fluctuations that promote germination in many species. Stratification is thus achieved in a very simple manner. Note that this method need not be limited to species that actually require stratification. The containers may be covered to prevent "alien" seeds from sprouting in the pots, and to provide more constant moisture, but again, the necessity of a cover depends on your preference and local experience. Leaving the pots or trays uncovered may also be beneficial in allowing melting snow and rains to wash the seeds, thereby removing germination inhibitors (e.g. certain seeds with fleshy coatings, such as Arisaema).
The containers are left there until spring when the warm temperatures or temperature fluctuations will start the germination. When germinated, keep the containers in a strong light but no sun (if they have been set in shade). When large enough (a couple of sets of leaves), the seedlings can be transplanted to larger pots and hardened off. If the containers were set out in sun initially (assuming that your conditions allow it), hardening off will not be necessary.

References

Gardener's Supply Company
Seed Germination Theory and Practice Second Ed. - Dr. Norman C. Deno
First Supplement to the Second Edition of Seed Germination Theory and Practice - Dr. Norman C. Deno
Second Supplement to Seed Germination Theory and Practice - Dr. Norman C. Deno