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Organic Gardening

Get dirty, have fun and grow more food with great gardening tips from real-life gardeners.

Using Open Flats (Seed Trays) to Grow Sturdy Seedlings Easily

We do use some plastic plug flats, flower pots and trays of cell packs, but we also like to use our home-made wood plant trays, as we can grow big sturdy plants in them, and reuse the flats year after year.

Make Your own Seed Flats (Plant Starter Trays)

I recommend choosing a standard size for your flats, to make life simpler when fitting all the flats into your warm sunny growing space, as well as when calculating how much to plant. We have a large garden, and we use flats 12" by 24". We make two depths: 3" flats for sowing seeds in, and 4" flats for spotting out the seedlings to grow them on. I don't recommend bigger than 12 x 24 x 4" as the filled flats get very heavy, and none of us needs to lift extra weight, when that can be avoided by a bit of planning.

Lettuce seedlings. Photo by Kathryn Simmons

We gather small scrap boards and make up a batch of flats at a time. We usually end up making a couple of half-size 12 x 12" flats to use up the wood scraps too. I like Eastern Red cedar or pine. Avoid oak. Not only is it heavy, but it splinters painfully, and is not so easy to work with as soft woods. Avoid ply, and all other manufactured boards, as the glues and fillers can be toxic to plants. Likewise avoid pressure-treated wood.

We cut 12" wide end boards of the thickest pieces, about ½ to ¾" thick, and 3" or 4" wide. Because I'm working with scraps, I generally cut the collected boards into the biggest possible parts. If you are buying or milling your own lumber, you can plan out your cutting list exactly. The side and bottom boards are thinner, maybe ¼ to ½" thick. The side boards need to be 24" by 3" or 4". The bottom boards are 24" long, of random widths. In fact you can use obliquely cut or waney-edged pieces for the bottoms of the flats, if you are creative.

First assemble the "side walls" of your flats, drilling through the thinner sides and into the thicker ends. This is a nice basic woodworking task for beginners of all ages. Use exterior grade screws, because they will be wet a lot of the time in use. Once you have the four sides together, turn the frame over and fasten bottom boards, leaving small gaps (up to ½") between them. This helps make it possible to combine various widths of board. Turning the frame over gives you a flat surface to fasten the bottom boards to, in case your sides and ends were slightly different widths. The gaps will help with drainage and stop the wet boards buckling. I keep a supply of ready cut and drilled boards to make running repairs during the season.

Greenhouse, early spring. Photo by Twin Oaks Community

Sowing Seeds in Open Flats

Take a 3" deep seed starting tray and line it with a double layer of newspaper (to stop the compost falling out the gaps). Have the paper come part way up the sides, but never poking out above the compost, as this wicks the water out of the compost. Fill the flat with compost - we use a plastic dustpan which happens to be just the right width, and works much better than a trowel or a shovel. Scrape the dustpan across the frame of the flat to ensure it is evenly and completely filled.

Next make tiny furrows for the seeds. We use a plastic ruler pressed into the surface of the damp compost and pushed back and forth. Sow the seeds, aiming for 3-5 per inch for most crops. Cover the seeds over shallowly (except for celery and some flowers which need light). Water and grow the seeds indoors.

Spotting Out Seedlings into Transplant Flats

Once the seedlings have emerged and the seed leaves opened fully, it's time to spot the seedlings out into the deeper flats to grow on until you transplant them outdoors. Fill the bigger flats with compost in the same way. We have a dibble board with 40 wood pegs glued into holes in a 12 x 24" piece of plywood in 8 rows down the 24" direction, 5 offset rows in the 12" direction. The pegs are about 2 ½" apart from all their nearest neighbors. This gives the 40 plants about 7 square inches each, which is a nice lot of space. We press the dibble board down into the surface of the compost, making 40 holes at once.

Cabbage seedlings, showing the pattern of holes made by the dibble board. Photo by Wren Vile

We use a butter knife to loosen the seedlings in the seed flat. Then, handling them only by the seed leaves (which are tough and disposable), we shake the seedlings apart. We use the knife to deepen the hole in the transplant flat if needed, then we jiggle the seedling to get the roots pointing downward in the hole. With the knife and the other hand, we press the compost firmly around the plants. For small plants such as lettuce, we spot into 3" deep flats, but bigger brassicas and tomatoes need 4" flats. Water and wait.

This is a good way to avoid contributing to the problem of agricultural plastic trash and be self-reliant in gardening equipment. You can also grow stronger plants by giving them a larger compost volume than plastic plug flats or cell packs provide. 

Pam Dawling works in the vegetable gardens at Twin Oaks Community in central Virginia. She often presents workshops at MEN Fairs. Pam also writes for Growing for Market magazine. Her book, Sustainable Market Farming: Intensive Vegetable Production on a Few Acres, is available at, Pam's blog is on her website and also on

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