Making seedling flats

My new greenhouse is getting warmer by the day (in fact I will soon have to finish the venting) and it’s time to start sowing the first spring crops inside it. I decided that a new greenhouse should have new flats to go with it (a fresh start), so I made a whole stack of them and took some pictures of the process. If you don’t already know, flats are shallow open wooden planting boxes, used for starting seeds of the easily transplanted crops, such as Brassicas, tomatoes, lettuce, onion, peppers and more.

The parts

Flats are pretty simple and there isn’t a great deal of design involved in making them. They can be any size you want, though you should make sure they are of a size that fits the wood you have without too much cutting (cutting wastes both wood and time). I found that 24 x 14 worked well with the wood I had (the four bottom boards are just under 3 ½” wide). The depth of a flat is more critical, as you need enough depth for the roots, but don’t want them to be too deep, as this makes them heavy and requires more soil mix to fill them. I find that 3 ½” is a good average depth, as well as a standard lumber dimension. Some gardeners like to have two different depths, a shallow one (3”) for starting seeds and a deeper (4”) one for pricking out. There only other important design criteria is that there should be a narrow gap between the bottom boards for good drainage.

The sides

I didn’t have to buy any material for my flats because I salvaged a lot of 1X redwood when a neighbor tore down an old fence (if ever you see a redwood or cedar fence being replaced, grab the old materials before they go to the dump). I also had some 4” X ¼” tongue and groove redwood paneling that another neighbor gave me when he was selling his house, and this was perfect for the bottoms. You can use any old wood for making flats (only buy it as a very last resort), though if it isn’t rot resistant they won’t last anywhere near as long (which is okay). I have redwood flats that have fallen apart from use and the wood is still as good as new.

Half finished

I ripped the fence boards down to a width of 3 ¼” on my table saw and cut them into 22 ½” pieces for the sides and 14” pieces for the ends. When nailed together this gave me a finished flat measuring 24” X 14”). The bottom boards were a full 24” in length. You can see from the photographs how they all go together.

Nailing the bottom

I used 1 ¼” staples (and an air stapler) to nail the flats together (because I already had them and didn’t have any nails). The gun makes the process go very quickly and with all the pieces cut I could assemble a flat in under 3 minutes (I’m guessing now, I never actually timed it). If I didn’t have the staples I would have nailed the sides together with 1 ½” galvanized common nails and attached the bottoms with ¾” galvanized common nails. I can’t think of anything else to say here, as I said before this is a pretty simple process and a picture is worth a thousand words anyway.

That was easy