Seed starting indoors is exciting and infectious, and you’ll soon find that your windowsills are crammed with little pots of seedlings and you have an overwhelming desire to study greenhouse designs. But a greenhouse is not necessary for a satisfactory experience in gardening, as long as you follow some practical considerations such as light and temperature requirements and space. A tray of seedlings is only a starting point, as these seedlings will have to be transplanted on to larger containers as they grow, taking up more and more room.
For many gardeners, providing suitable warmth can be as easy as placing the seed tray near the hot water heater or near the home heat source, on top of the refrigerator, or in a sunny windowsill. If the packet instructions call for very warm temperatures, or if conditions in your home are generally unsuitable for even the minimum germination temperature, commercial propagation mats are available. These mats provide constant, steady warmth at a thermostatically-controlled setting and are essential for proper germination of many tropical seeds.
After germination, the general recommendation is to drop the temperature by 10°F from the optimum germination temperature and grow seedlings on in a cool, well-ventilated area with a good light source or under grow lights. Keep out of cold windowsills at night where drafts could damage seeds and young seedlings.
Most seeds germinate best if the tray is covered with a sheet of glass or loosely covered with a sheet of plastic wrap to retain moisture. Light and dark requirements are noted on the seed packet and should be followed carefully. Seeds that are non-specific in their light requirement for germination will not have a notation. For seed that require darkness for germination, check periodically under the darkening cover for signs of growth. Once the majority of the seedlings are through, remove the darkening covering and bring into bright but indirect light. Remember that seedlings under glass or plastic wrap and placed in a hot, sunny windowsill will literally cook as temperatures can easily reach over 100°F.
When seeds germinate, the first leaves to appear are the cotyledons or seed leaves. These are usually a pair of oval, fleshy leaves that bear no resemblance to the mature leaves of the plant.
The conventional advice is that seedlings should not be pricked out or transplanted until the first true leaves appear. In the case of large seedlings such as cucumbers or squash however, plants are large enough to handle before the true leaves develop. It is sound advice to plant these large seeds in individual containers and eliminate the need for transplanting.
Remove tiny seedlings from the sowing container into individual pots of potting soil can be a delicate business. As seedling stems are easily bruised, always handle seedlings by their seed leaves. To facilitate removal of the seedlings, use a tapered stick, a narrow flat-ended screwdriver, or a metal device called a widger to separate and ease out the seedlings, taking care not to damage the delicate roots. Where several seedlings are growing in a very small space, it is best to transplant a clump of seedlings and then snip off all but one or two. If seedlings seem sturdy enough, you can gently tease 2 or 3 seedlings apart, but any damage to the root system will make survival risky.
Prior to transplanting, fill the clean new pots with pre-moistened potting soil. Using the end of a pencil, make a small hole in the center of each pot to accommodate the transplant. After easing the seedling out of the sowing tray, move directly into the new pot and firm potting soil around the delicate root system while still holding onto the seed leaf. Water immediately with a gentle spray of lukewarm water. Set the pots out of direct sun and protect from wind for several days. It is not advisable to use a fertilizer at the time of transplanting as feeder roots are invariably torn and more likely to be damaged by fertilizer salts. After about 2 weeks, commence fertilization with diluted (1/4 strength) liquid fertilizers.
Invariably, there will be more seedlings to transplant than pots to accommodate them. As a very rough guide, figure on 50 transplants produced from a full-size nursery flat.
Seedlings such as half-hardy annuals, half-hardy perennials and many vegetables that are started indoors with heat must be gradually acclimated to the stronger light, winds and generally cooler night temperatures of the outdoors prior to planting out in their final locations. This conditioning is known as “hardening off” and traditionally takes from 7 to 10 days. The correct timing of plants for both hardening off and final site planting depends on the plant’s genetic cold hardiness and climate factors for your particular area.
When seedlings have reached an appropriate size and the time is right for the individual plants to go outdoors in their final location, start the process of hardening off by placing pots or flats outdoors for several hours a day in a location with some morning sun and protection from winds. Return to the protection of an unheated porch, garage or greenhouse for the late afternoon and evening hours. Slowly increase the amount of time plants are left outdoors and increase the light they receive to the appropriate light levels over a period of 1 to 2 weeks, eventually leaving plants outdoors all night. At the end of this period, plants are fully ready to go into the garden.
Remember to protect plants from predicted hard frosts, freezing winds and heavy rains, which can dislodge seedlings. A useful aid in both growing cold-hardy seedlings and hardening off tender plants is a cold frame. A cold frame is an unheated 4-walled structure with a glass or plastic roof. Materials can be as inexpensive as discarded lumber and an old window sash. During the day, the glass or plastic top should be raised for air circulation, but at night it is lowered to protect seedlings from frost and freezing winds.