COLD STRATIFY ALL SEEDS FOR AT LEAST 30 DAYS UNLESS THE PLANT SPECIFIC INSTRUCTIONS MENTION A LONGER PERIOD OF STRATIFICATION. SEEDS will not GERMINATE IF YOU DO NOT STRATIFY.
Seeds provide a simple and economical way of raising large numbers of perennials, although it has limitations. Many cultivars do not come true from seeds, and even commonly grown species display some natural, albeit acceptable, variation in the seedlings. However, there is always a chance of producing a seedling that is superior to its parents.
Some cultivars do, however, come reasonably true to type, including Delphiniums, Lupines, and Oriental Poppies (Papaver orientale). Seedlings with colored, marbled, or variegated leaves, such as Heuchera cultivars, vary in color, so poor forms need to be rogued out at an early stage.
Seeds also offer the only way of raising monocarpic species, such as Meconopsis, that die after the first flowering. Perennials that are very slow to increase vegetatively, such as Hepatica and Pulsatilla, may be raised in large numbers commercially from seed.
WHEN TO SOW PERENNIAL SEEDS
Euphorbia, Gentians, and several others are best stored in a cool place until autumn and sown then. Seeds of later-flowering perennials, if sown in autumn, will not germinate until early spring. In most cases, such as for most Chrysanthemums and Asters, these seeds may be stored over winter and sown in spring.
TREATING DORMANT SEEDS
Some perennial seeds have built-in dormancy to delay germination in the wild until conditions occur that are beneficial for seedling development. There are several ways to break this dormancy before sowing to obtain a good rate of germination. Hard protective seed coats in perennials are most common in the Pea family (Fabaceae). The seed coats must be scarified so that moisture can enter. Gardeners are often advised to file seed coats, but anyone who has tried this with dozens of Lupine seeds knows it is painful and time consuming. A better way of scarifying a large amount of seeds is to rub a batch with fine-grade sandpaper. Place the seeds on top of the sandpaper and get another sandpaper to rub the seeds with.
If the seeds are large or from plants grown in hot, dry conditions, pour boiling water over them and allow to stand in the cooled water for 24 hours. Sow soaked seeds immediately; otherwise, they will die.
Many perennials, particularly those from mountainous or harsh climates, have seeds that do not germinate until after a cold period. The seeds must be chilled (stratified) before sowing in spring by placing them in a refrigerator, or sown in autumn in regions with cold winters.
A few perennials, such as peonies, are doubly dormant and require a period of cold, then warmth, followed by a second spell of cold. If the seeds are not sown fresh, they take two years to germinate naturally. This can be overcome by subjecting the seeds to artificial temperature changes.
To override chemical inhibitors in the seeds of some perennials, the seeds are sown as soon as they are fully formed before the inhibitor is activated, sown after storing when it has broken down, or soaked in water for 48 hours to leach out the chemical, as with rhizomatous irises.
PREPARING CONTAINERS FOR SOWING
Perennial seeds are often sown in pots or half pots of 3 ½ inches (9cm) to 5 inches (13cm). Seeds that germinate quickly and easily, such as of Delphiniums or Lupines, or those of plants that dislike root disturbance, are best sown singly in cells or plug trays; use one with cells large enough for seedlings to reach a good size before potting.
Soil-based seed soil mixes are best for most perennials unless the seedlings will be transplanted soon after germination. A good homemade seed soil mix can be made of two parts sterilized soil, two parts peat or leaf mold, and one part sharp sand. For autumn sowings, equal parts coarse sand and peat, bark fiber, or soil works equally well.
To prepare a container for sowing, fill it generously with soil mix, tap to settle it, scrape off the excess, and firm with a presser or base of an empty pot.
SOWING SEEDS IN CONTAINERS
Take care not to sow too thickly, which could lead to spindly seedlings and damping off. Cover with screened soil mix or, for seeds that need light to germinate or germinate quickly, top dress with vermiculite. Large seeds may be space-sown, pushed into the soil mix with a presser and covered with ¼ inch (5mm) of soil mix. Seeds that must not dry out fare better when sown on moss.
After sowing, water containers using a fine hose or by standing the container for 30 minutes in a tray of water; this avoids disturbing the soil mix surface and seeds. Cover the container or place it in a closed case to prevent moisture loss, and shade it from the sun if necessary. Remove the cover after germination.
For most seed germination, an ideal temperature is 60F (15.5C). Keep seeds of very hardy plants at 50F (10C); they will germinate at lower temperatures, but it takes longer. Tender species need a minimum of 68F (20C).
If containers are sown in autumn for stratification by winter cold, cover the seeds with a shallow layer of fine gravel or coarse sand to discourage weeds and protect seeds from rain. Pack the containers into an open cold frame or sink in a plunge bed. The bed keeps the soil mix moist and protects clay pots and plant roots from cold damage. Cover the containers with fine mesh to protect the seeds from birds and rodents.
Seeds of perennials can be fickle. Seeds that normally germinate quickly may not do so, and supposedly dormant seeds may germinate rapidly. It is wise to keep pots or trays of seeds for a year after the expected germination date.
HANDLING THE SEEDLINGS
Seedlings need bright light and regular watering. If using rockwool plugs or another inert medium, feed the seedlings once they have two true leaves with a liquid fertilizer according to the manufacturer’s instructions.
Transplant seedlings 30 or 40 to a tray or individually into plugs, cells or pots as soon as they are large enough to handle. If the seedlings germinated under cover at a frost free temperature, it is better to pot them when they are slightly larger. Always handle seedlings by the leaves. Use soil-based potting mixes or a mix of three parts sterilized soil, two parts peat or leaf mold, and one part sharp sand.
Grow on the seedlings in a sheltered place until well established. Plant out fast growers into their final positions in the same year, but delay planting out slow developers until the next spring. These are better potted or grown on in a nursery bed for a year.
SOWING SEEDS OUTDOORS
Easy perennials may be raised in a seedbed; the seeds are best spring sown in drills similar to annuals or biennials. If needed, thin the seedlings as they grow; when they are about 3 inches (8cm) tall, lift and plant them out.
Seeds that germinate slowly may rot if the soil mix decomposes, so these are better sown directly into a seedbed in a cold frame. Sow them in rows, label, and top dress with fine gravel. Keep the bed moist and weed free; be aware that organisms working through the bed may displace the seeds.
Seedlings may need potting or transplanting after only a few weeks; if left too long, they become crowded and drawn as they compete for light and air.
Please be advised that this is a GENERAL guide to germinating Perennial seeds. There are multiple resources on the web that are available and specifically tailored to the genus of the plant.
From personal experience, we had the best success by using a plastic seed tray, dome lid, heating mat and plant lights to get the seeds started. This way we were able to control the temperature and light requirements of the plants.
