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

Here's how to make reusable wood flats (seed trays) for seedlings, and use them to grow sturdy vegetable starts to transplant into your garden. This is a way to avoid contributing to the problem of agricultural plastic trash, and to be self-reliant in gardening equipment. You can also grow stronger plants by giving them a larger compost volume than plug flats or cell packs provide.

Reader Contribution by Pam Dawling
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by Wren Vile
Cabbage seedlings, showing the pattern of holes made by the dibble board.

We do use some plastic plug flats, pots, and trays of cell packs, but we also like to use our homemade wooden trays, because 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 that are 12 by 24 inches. We make two depths: 3-inch flats for sowing seeds, and 4-inch flats for growing the seedlings. I don’t recommend flats bigger than that, 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.

We gather small scrap boards and make up a batch of flats at a time. We usually end up making a couple of half-sized 12-by-12-inch flats to use up the wood scraps. I like Eastern red cedar or pine. Avoid oak. Not only is it heavy, but it also splinters painfully, and it’s not as easy to work with as soft woods. Avoid plywood and other manufactured boards, as the glues and fillers can be toxic to plants. Likewise, avoid pressure-treated wood.

We cut 12-inch-wide end boards of the thickest pieces, about 1/2 to 3/4 inch thick, and 3 to 4 inches wide. Because I’m working with scraps, I generally cut the collected boards into the biggest possible parts. If you’re buying or milling your own lumber, you can plan out your cutting list exactly. The side and bottom boards are thinner, maybe 1/4 to 1/2 inch thick. The side boards need to be 24 inches long by 3 to 4 inches wide. The bottom boards are 24 inches long, of random widths. In fact, you can use obliquely cut or waney-edged pieces for the bottoms of the flats, if you’re 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’ll be wet a lot of the time they’re in use. Once you have the four sides together, turn the frame over and fasten the bottom boards, leaving small gaps (up to 1/2 inch) between them. This will help make it possible to combine various widths of board. Turning the frame over will give 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 from buckling. I keep a supply of ready-cut and drilled boards to make running repairs during the season.

lettuce seedlings

Sowing Seeds in Open Flats

Take a 3-inch-deep seed starting tray and line it with a double layer of newspaper (to stop the compost from falling out the gaps). Have the paper come partway up the sides, but never poking out above the compost, as this will wick 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’s 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 push it back and forth. Sow the seeds, aiming for 3 to 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 have 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-by-24-inch piece of plywood in eight rows down the 24-inch direction, and five offset rows in the 12-inch direction. The pegs are about 2-1/2 inches apart from all their nearest neighbors. This gives the 40 plants about 7 square inches each, which is a nice amount of space. We press the dibble board down into the surface of the compost, making 40 holes at once.

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, and 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-inch-deep flats, but bigger brassicas and tomatoes need 4-inch 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 MOTHER EARTH NEWS FAIRS. Pam also writes for Growing for Market magazine. Her book Sustainable Market Farming: Intensive Vegetable Production on a Few Acres is available on Pam’s website.

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