Inspect the containers regularly for signs of germination. As soon as seeds sprout, remove the plastic and transfer seedlings to an area with bright light and a cooler temperature: grow lights left on for 16 hours a day are ideal, or place plants in a sunny southern window.
Keep the soil damp but not soggy, using room-temperature water. For soilless mixes, feed with half-strength fertilizer (with low, roughly equal nitrogen, phosphorus and potassium numbers) when the first true leaves appear, then twice a week thereafter. For potting soils, start fertilizing two weeks after the leaves appear, fertilizing every 10 to 14 days thereafter.
Once the seedlings have their first true leaves, thin to the most vigorous seedling per container. When the leaves begin to touch, transplant to a larger container.
Transplanting to the garden
Before planting out, move seedlings to a sheltered location outside. Gradually introduce them to cooler temperatures and direct sunlight, starting with one hour of morning sun and increasing it by an hour or so daily. Bring seedlings inside if frost threatens. Don't plant heat-loving varieties outdoors too soon, as they will never recover from the shock or, worse, die.
Dig a hole several times larger than the root ball and mix in lots of compost and some organic fertilizer. Place the seedling in the hole; fill in with extra soil, firming it gently, then water. Keep it moist until a root system is established. If frost threatens, cover vulnerable plants.
Named hybrid varieties of perennials, such as daylilies, irises and peonies, will not come true from seed. Although the offspring may not be the same as their parents, it can be exciting to see what you get.
While some perennials are easy to grow, others require more effort and patience than vegetable seeds. Some take weeks to germinate and others take a long time to grow to flowering size (peonies, for example, can take five or more years). Many perennial seeds need a period of cold that mimics the winter they'd experience germinating outside, so you need to condition those ones in the refrigerator.
Veggies are easy to start from seed. Some, however, such as cucumbers, melons, okra and squash, don't like to be transplanted; these should be started in peat pots (biodegradable pots are fine, as long as they break
down fairly quickly) or individual containers instead of cell packs, removing the plants carefully during transplanting so their roots are not disturbed.