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
Published on February 24, 2017
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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.

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