When seedlings produce their first set of true leaves, it's time to transplant them into larger containers. Transplanting gives plants room to grow and improves air circulation. It also stimulates the growth of bushier feeder roots.
Fill the transplanting flat (or other larger container) with moistened growing medium. I recommend you use the same soilless mix used for germinating the seeds. Firm the mix to within 1/2 inch of the top of the container. I use six 2- by 2-inch plastic six-pack pots in each waterproof tray.
I make a planting hole for each seedling with my finger, then use a small pointed plant marker stake or wooden popsicle stick to lift seedlings. I hold each individual plant by its leaves rather than by the stem, using the stake to lift the roots as I gently pull the plant up.
Keep the rootball intact when setting the plant in its new spot. Most plants can be set slightly deeper than they were growing in the germination flat.
Firm the soil over and around the newly set plant so that the soil and roots are in close contact and there are no air pockets.
Set the transplants back under the lights unless they wilt or droop. Droopers go in a shady, cool spot for a day until they perk up.
Check your transplants every day by feeling the soil surface. It should be moist but not soggy. Watering from the bottom directs the water to the root zone and prevents puddling on the soil surface, which could lead to damping-off. It's also less messy. I give the plants a drink of room temperature water every three or four days; more often as they get bigger.
Plants without enough light will be weak and leggy. As with the seedlings, I use four fluorescent lights above each flat and tape reflective mylar to the front and back sides of the fixtures to maximize the light directed to the plants. As the transplants grow, I increase the distance between the top of the plants and the bulbs to about 2 inches so that the light can spread more evenly over the flats.
I grow transplants at the same temperatures that the seedlings were grown: 60 to 70°F during the day, with temperatures 10 degrees lower at night. You can slow down fast-growing plants by lowering the temperature while keeping light levels high.
Transplants do best when the relative humidity is between 50 and 70 percent. Maintain humidity levels by misting plants with a sprayer, setting flats on top of trays filled with pebbles and water or covering your light setup with a plastic tent. Don't keep them too cooped up, though. I give transplants a little exercise to toughen them up by having a fan lightly blow on them. A fan can also help prevent green algae or mold from forming on top of the soil surface.
I give transplants a constant, lean diet every time I water them. Use one-quarter the recommended amount of fertilizer per gallon of water.
I set planted flats in a waterproof tray filled with 1/2- to 3/4-inch of water. Because the watering is done from the bottom of the flat, seeds don't get washed out or disturbed. The trays also provide a water reservoir that can help keep your growing medium moist but not soggy. Tepid (65° to 70°F) water is best. Never allow the soil to dry out completely.
To maintain humidity during germination, I place the flat in a plastic bag, cover it with plastic food wrap or use a plastic humidity cover. I don't seal the flat completely, as mold may form if air can't circulate.
Seed flats need bottom heat of 75° to 85°F. I place my flats on top of the fluorescent light fixtures, but you could also put them on top of a refrigerator or near another heat source, such as a wood stove or radiator. Or use one of the bottom heat mats made for this purpose.
Once seeds germinate, they need light. I use fluorescent bulbs that are specifically designed to provide plants with as much of the solar spectrum as possible. Each of my fixtures holds four fluorescent bulbs. I also tape pieces of reflective Mylar ("space blanket") to the fixtures' sides to concentrate light on the plants.
I adjust the height of the fluorescent lights so they are nearly touching the sprouted seedlings, and I use a timer to turn the lights on for 16 hours and off for eight hours each day.
Seedlings need less heat than germinating seeds. Most seedlings will grow best at temperatures of 60° to 70°F during the day and night temperatures that are 10 degrees cooler.
The Director of Research here at the National Gardening Association, Bruce Butterfield is widely known for his understanding of gardeners and the gardening world. But few outside this building know of his passion for starting seeds. For 10 weeks every spring, at a cost of $50 for seed, soil and pots, plus $70 for electricity, he starts more than 500 plants. Why? As Bruce notes, "It's pretty magical to have hundreds of seedlings pop up while it's still lousy outside. Besides," he adds, "it makes me a better gardener because I'm in touch with the entire process."
I use 2-deep plastic flats with multiple water holes in the bottom and fill them with a dry soilless growing mix, firming it down until it is 1/2 from the top of the flat.
Sow seeds in furrows three times as deep as the seed's diameter. If seeds need light to germinate, cover them very lightly if at all.
Space small seeds 1/8, medium seeds 1/2 and large seeds an inch apart. Space rows 1-1/2 to 2 inches apart to allow seedlings room to grow their first true leaves before transplanting. Label each row with the name of the plant and variety, and the date of sowing.
Before making the transition to the real world of the garden, transplants need to toughen up. One week before you begin hardening your plants outdoors, cut back the amount of water you give them, stop their fertilizer habit and keep temperatures slightly cooler.
Harden transplants gradually. Start by putting them outside on a day when temperatures are above 50°F. Leave them out for just half a day (or less) in a sheltered spot protected from strong winds. Be sure to keep the plants watered, as they dry out more quickly outside.
Over the course of a week, extend the amount of time your transplants spend outside and increase their exposure to sunlight, wind and a range of temperatures. At the end of the week, your plants should be ready to go into the ground. A cloudy, warm, windless day is best for easing the plants' transition into their new homes.
Photography by National Gardening Association (top) and John Goodman
Article published on June 23, 2008.