You've set up light shelves and heat mats. You've sterilized your seed starting mix and considered other tips to prevent damping-off. You've ordered seeds for more plants than your yard and your neighbors' yards together can possibly hold. You are more than ready to start turning your seeds into rows and pots of healthy seedlings!
Although it's possible to sow seeds one by one into little pots on your light shelf, many plants seem to do better when transplanted once between sowing and being planted out in the garden. I usually start seeds in rows in shallow, domed seed starting trays. After the seedlings have their first true leaves (the first pair of leaves are cotyledons, or "nurse leaves"), they are potted up into individual pots or cell packs. For plants with a naturally branching or clumping habit such as basil or lobelia, I use a method that herb guru Tom DeBaggio calls "clump transplanting."
You can purchase official "seed starting trays," often with vented domes to hold a little extra humidity around the seedlings. Or you can create your own, using any container that will hold a good inch or so of potting mix. Take-out containers work great! Poke a bunch of holes in the bottom for drainage and in the top for ventilation. A hot metal skewer works well to make holes in a dozen nested containers at once. Separate containers after each jab so they don't "weld" together. Be careful, and work in a well ventilated area.
|Basil seedlings in closed vented container. Note little puff of condensation in lower left corner.|
Use a good soil-less potting mix. If it's not a "seed starting mix" as such, you may want to lighten it with a little extra perlite. I also add a pinch of polymer moisture crystals to my mix, so I don't have to water as often. Fill your seed starting container with moist, preferably sterile mix. 
|Petunia seedling tray, aka salad container. Droplets of condensation inside lid indicate too much moisture in potting mix.|
Sow seeds in rows at least an inch apart, so that a row of well rooted little seedlings will be easy to break away. I sow seed quite thickly for clump transplanting. There's generally no need to cover seeds; just press them gently into the surface of the potting mix. Don't forget to label the container using a permanent marker or paint pen. Jot down the date also, to keep track of germination times.
When the container is closed, there should be enough humidity to make a little puff of mist on the inside of the lid. If large droplets of condensation form, then your mix is too wet -- prop the lid open for a few hours to let it dry out a bit. The container can be easily watered from the bottom as needed. Place it in a tray of water for a few minutes, until it no longer feels lightweight.
At the first sign of germination, I make sure my seedling tray is as close as possible to the fluorescent light tube. When seedlings have their first true leaves, and for sure by the time they have their second set of true leaves, they are ready to be transplanted. Looking at the seedling tray from the bottom, you can see vigorous roots searching for more growing room.
Tom DeBaggio's clump transplanting method  makes for healthy, sturdy seedlings. It's very simple. You don't tease away a solitary seedling from the massed rootlets in the seedling tray. Instead, you gently separate a clump of seedlings and plant them together.
Having a larger clump of roots in the pot means it's harder to drown the little seedlings. Damping off issues of root and stem rot are less likely when there are more roots to take up more water. With plants like basil, seedlings clumps give you a nice, full appearance before you even start pinching back the stems.
Tom recommends 3 or more plants in a clump. I've had young friends who enthusiastically potted up huge clumps of basil seedlings, and others who carefully selected a perfect pair. It's all good. In Tom's words, "I don't count them, but take what comes apart most easily with the least root damage. It would defeat the beneficial effects... to prick out individual seedlings and gather them in clumps... the larger the leaves, the fewer seedlings [should be] in the clump."
I like transplanting to pots no smaller than 48 cell inserts for standard nursery flats. Most of my seeds are started 6 to 8 weeks before planting out. Seedlings that will be started sooner (like wave petunias) or grow larger (like tomatoes) get a 2 inch pot. Fill pots or cell packs ("sheet pots") with moist potting mix, and poke a hole in the mix with a stick or with your finger.
Handle seedlings by their leaves, and gently massage their roots to separate them into clumps. The tender stems should be touched as little as possible. Here's Tom's other piece of invaluable advice: "Set the seedlings into the pot lower than they were growing in the seedling flat," preferably with the true leaves level with the growing medium and the cotyledons (nurse leaves) covered.
Gently pat the potting mix around the seedling clump. Water carefully around the seedlings to settle the potting mix around the roots. Your newly transplanted seedlings are ready to go back on the shelf, very close to the lights for best growth. After several more weeks, they will be ready for hardening off and planting out in your garden.
Sowing and transplanting is a straightforward, two-step process. There's no "one true way" to go about it, but using covered seed starting trays and transplanting with Tom DeBaggio's clump method definitely works for me. Before you know it, you will have turned a few seed packets into flats of beautiful seedlings for your garden!
Photos by Jill M Nicolaus
Move your mouse over the images for additional information.
 See my article "The Dreaded Damping Off (and How to Prevent It)" for tips on sterilizing seed starting mix.
 See my article "Seed Starting 101: Setting up Lights" for more information on the importance of light to growing sturdy seedlings.
 I owe a great debt to Tom DeBaggio and his "little book" that made me believe I could succeed in starting basil and other plants from seed. See my review in Garden Bookworm.
DeBaggio, Tom. Growing Herbs from Seed, Cutting & Root: An adventure in small miracles. Interweave Press, 2000. ISBN #: 1883010780