All seeds may be sown outside in late spring, or started earlier indoors. Hardy trees and shrubs, perennials and bulbs may also be sown in fall; cold will assist in germinating these. Houseplants can be sown any time of year, though ideally in spring or early summer for most rapid growth.
Note that many seeds have specific germination requirements: special tricks are described below, and the needs of many species are described in the alphabetic listing. Make sure you have ample seed if using trial and error.
The following table suggests general germination temperatures for various kinds of seed. Note that the requirements of individual species may vary considerably: do not commit all your seed to any trial of germination. More specific information is given under individual plant listings.
Ideal temps are given, though I've easily germinated plants expecting 80F/ 27C in 65F/18C conditions. However, expect slower germination, a lower percentage of germination, or both at less-than-optimal temperatures. Also, seedlings are more likely to do poorly with temperatures that are too high than too low.
Use a sterilized growth medium of some sorts, the finer, the better. For example, mix sterilized soil with equal amounts of peat and fine vermiculite or perlite, for a good starter medium, or use some other mix as described under Growth Media.
Potting soil itself may be too rich in nutrients and can actually damage seedlings. Still, many people use such soil, often without obvious penalty. If a soilless mix is used, then feeding with fertilizer, fish emulsion, or similar is required once seedlings reach the two-leaf stage.
Level soil without compacting it. Dustlike seeds are left uncovered: mix these well with a larger quantity of sand to spread them easily and evenly. Fine seeds are covered with a fine of layer of soil. Otherwise, bury once to twice the depth of the diameter of the seed.
Conventional wisdom held that most seeds prefer to germinate in the dark, and seeds are often covered for that reason, though most will germinate in light or dark. Ideal is subdued indirect light, unless otherwise specified. If germinated in the dark, uncover with first germination, and expose to air.
Initially, plant into already somewhat moist (not wet) soil, and where possible, water from below, by standing in tray of water for 20 minutes. Use plastic bags tented over the pots, or specially-designed propagaters, to prevent moisture loss by evaporation, watching that the sun does not bake seedlings in this enclosed environment. Use wicking systems as a good alternative method of providing water.
Humidity is also needed for germination; about 50% works well. Too much moisture, however, encourages damping off, an often lethal fungal infection. Be careful, and uncover seedlings immediately with the first germination. From this point on, use a small fan to provide gentle air circulation directly to the seedlings, to help prevent fungal problems, and also build sturdier plants.
Soil warmth often greatly helps germination: set seeds above the fridge (if light is acceptable), on a heating pad, or use temperature-adjustable soil heating cables. Note that moist soil is cooler than the ambient temperature.
Good light is required, i.e. a south-facing window, greenhouse, or proper artificial lighting. Provide at least 14 hours of good light a day, and if used, keep fluorescent tubes as close to the plants as possible.
Prick out seedlings into individual pots when the first true leaves have appeared and they seem robust enough. Handle gently by their leaves, not stems. Free roots without traction, make a hole in new, moist compost with the butt end of pencil and lower in place, then firm around. Young seedlings may benefit from a mixture of soil, compost, optional vermiculite and bone meal.
Keep cool at 65F/18C, in good indirect light. Perennials especially prefer cooler temperatures, down to about 40F/4C at night. Water lightly, from below or with a fine mist. Give seedlings plenty of room to minimize the number of transplantations; many plants should not be transplanted after this first move.
Feed seedlings after the second set of leaves has developed, with diluted 10-52-10 or 20-20-20 solutions, compost tea (if not too rich in nitrogen) or fish emulsion, or simply transplant into rich organic growth medium.
Plants destined for outdoors should be gradually introduced when the weather warms up: place in a protected spot for two hours one day, three the next, etc., and decrease nighttime temperatures gradually. This hardening off should take place over 1-2 weeks.
Fall-sown seeds can be started outdoors; for better protection in harsher winters, sow into a cold frame, in the shade. Always mark where sown.
For cacti, water from below after germination, especially if seedlings are very small, and allow to dry out between waterings. Bottom heat and warm ambient temperatures, 75F/24C daytime, 70F/21C at night, ideal. Seed and young seedlings should be protected from the midday sun.
Do not prick these out until the second year after sowing, when bodies are some 1/4" thick, and distinct roots and spines have formed. Succulents grow much quicker and may be transplanted within weeks or months.
Transplant into light, porous medium, being careful to protect the tender roots. Long roots may be trimmed but roots should never be bent; the neck of the roots must be at the soil surface. Water for several days after transplanting; transplant only during the growing season.
Bonsai-destined seedlings are sown into gritty well-drained compost, and usually grow faster, and are more vigorous, if these are started outside. Water several hours prior to transplanting.
Finally, many seeds go dormant if not sown immediately upon ripening. If unsure how to break dormancy, try several methods with each type of seed, using a small amount only. Often shiny hard coats need chipping or soaking, soft seed coats only stratification.
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