How I Built My Seedling Flats

B43mXNVCIAEU9fQAn important part of Grow Biointensive is starting your seeds in flats (wooden boxes), and then transplanting the seedlings when they are hardy enough to reliably survive. If you don’t want to build them yourself, you can always buy them from Bountiful Gardens. But since I needed 16 flats, the prospect of spending ~$300 + shipping on flats wasn’t palatable. Besides, where possible I really wanted to learn how to build basic growing infrastructure, such as flats.

Here’s the thing, if you’re a carpenter or woodworker or general handyman, you’ll say “OK so you’re making small wooden boxes. Very easy.” But if you haven’t done this kind of thing before, like I hadn’t, it can be daunting. Nonetheless I decided to give it a try, and it went fine! My intent here is to share with you my learnings and opinions, to try and take away some of the mystery of it, and also to help you get a better end product on your first try.

The High Level


  • IMG_20141214_194854lg (NXPowerLite Copy)For the sides: untreated Redwood fence board, ripped in half length-wise to make 3 ½” high sides. I used this wood: FSC ConCom Redwood Flat Top Fence Picket (11/16 in. x 7-1/2 in. x 6 ft).
  • For the bottom: Redwood bender board. I used this wood: Benderboard Redwood (1/8 in. x 3-3/8 in. x 8 ft.
  • Cedar would also work fine.


  • 14” x 11”
  • This gives you five x 14” pieces per half fence board, or seven x 11” pieces.
  • On the 11” width, it means you can nicely space the 3 bottom bender boards, get just enough of a gap between them for drainage, and not have to rip (cut in half length-wise) any of the bender boards. If your bender boards are a slightly different size, adjust the width accordingly.
  • The resulting flat is about 65% the size of a full flat, and so not too heavy to carry.


  • “Drive Straight” self-drilling lath screws, 1 ¼”.
  • Pre-drill 3/32” for attaching sides; no pre-drill for attaching bottom bender boards.
  • One screw per side of board works fine, for both the sides and the bottom. The bottom benders will warp less with 2 fasteners per side, but in general 1 is working fine for me.


  • Table saw to rip fence boards length-wise.
  • Miter saw to cut fence and bender boards to length.
  • Safety equipment for using the table and miter saws.
  • Drill to pre-drill side board holes and screw in fasteners. If you have two tools, you can avoid needing to switch bits.
  • Tape measure, pencil, carpenter’s square.

Fair questions

Why not use bender board on the sides, like the HTGMV book says?
I did this and broke several in the first year. When it’s full of soil and you tug it on a bender board side, it’ll snap. If it happens at the wrong instant, you can lose a full flat of seedlings. Not worth the hassle.

 Why not make full sized 11″ x 23″ flats, as commonly used on Ecology Action farms?
In my experience full sized flats are extremely heavy, especially when the soil is wet. It’s easy to drop heavy flats, destroying the seedling, or even worse, to get a back injury from lifting the flat.

I also think the full flats are more brittle because of the relationship of weight to length, but I haven’t used them much. My 6″ deep half flats are the same weight as a full 3″ flat, and I have to be super careful when I move those around.

These 65% flats are a nice balance of size and weight. If you wanted an even lighter box, consider a 11” square box, which would roughly be a half-flat.

Do you really need to pre-drill the sides?
When I made my first batch of boxes, I was cracking about 1/3rd of the boards I was screwing into. It’ll depend on how fresh your wood is, too. It’s disappointing to have wood crack, and so I decided to take a moment to pre-drill using my old screw-gun, which resulted in zero cracking. The dryer your wood is, the more important it is to pre-drill.

Do the self-drilling screws really hold?
Yes, holding power hasn’t been a problem at all. The bigger issue is if a board cracks, and the self drilling screws are less likely to do that.

Do you need to treat the wood?
I haven’t treated the wood and so far it seems to be holding up well. If you know how to do it, you could. There is some skill to doing it safely since the treatments can be highly flammable before drying, and any rags you use in the process need to be taken care of properly. I haven’t done it, but looked it and decided to go without for now.

In More Detail

Sourcing Wood

If you can get access to salvaged wood, and it’s in decent condition, then use that. But don’t use it if it’s been exposed to chemicals, or painted. I used a number of hard old fence boards from a neighbor, and found that as long as I predrilled my side holes to avoid cracking, it worked well.

If you’re buying your wood, I recommend finding one reliable vendor to buy from. In particular for your bender board, you’ll want to consistently use the same width. I’d personally rather not  try to rip a piece of bender board to be narrower and so the width of the bender board becomes a driver for the dimensions of the box. I haven’t tried to rip bender board, but it strikes me as the kind of thing where it’s easy to have a piece break off and come flying at you off of the table saw.

Type of Wood

You’ll want to use a wood that is naturally resilient to weather, such as Redwood or Cedar. I advise against using chemically treated wood because it’s easy to see those chemicals getting into your soil, into your plants, and then into you. It’s just not worth the risk.

For the sides of the box, I’ve been buying 11/16 in. x 7 1/2 in. x 6 ft. Redwood picket fence like this one and then cutting it down to the size I need. I’d say you want the wood for the sides to be at least ½ inch thick. Since this is a “less expensive wood” from a “less expensive store” I always have to carefully pick through the pile to find pieces without cracks or other major flaws.

For the bottom of the box, I’ve been buying Redwood bender board like this one although you have to be very careful to get a piece that doesn’t have knots or cracks. Wear gloves to the store and don’t be afraid to search through the pile for the good pieces.


One fastener or two? Up to you. Note drainage gap between boards.

Initially I was using a different fastener for the bottom bender boards and the side fence boards. However after a year of heavy usage, I found that both fasteners were doing just fine, and so there’s no point is using two different kinds, and also no point in spending the money for the more expensive “deck screw” kind.

So I’ve settled on using self-drilling lath screws, 1 ¼” “Drive Straight” which Home Depot sells. On the upside it saves me a lot of time because when I’m attaching the bottom bender boards, I don’t need to predrill and the bender boards don’t crack. I was initially concerned about holding strength, but this hasn’t at all been an issue.

I do predrill the holes for the side fence boards, to minimize the chance of cracking. I used a 3/32” bit and had zero cracking. A 1/8″ bit would probably work fine, but I like to think I’m getting better holding power by pre-drilling with a slightly smaller bit.

I did some boxes with 1 fastener on each side of the side boards, and some with 2. I didn’t see any performance difference, so I’m just going with 1.

I also did some boxes with 1 fastener on each side of the bottom bender board, and some with 2 on each side. I did see warping on some of the bottom benders with just one fastener, but not widespread. Frankly the bottom bender boards are going to fail first anyhow, and I’m not sure it’s worth the extra fasteners. On the other hand, the wood should last longer with 2 fasteners because of less warping. So you’ll have to decide what you want to do.

Assembly tips

IMG_20141214_194705 (NXPowerLite Copy)I found that using clamps made it much easier to put the boxes together. I used a pair of 24” Harbor Freight Aluminum bar clamps that are performing well enough for me with occasional use.

When screwing in the bottom bender boards in particular, set your drill for minimal torque. The bender boards are thin and so it’s easy to drive the screw too far and split them.

Last word: safety

There’s one thing you must keep in mind: cutting wood with power tools is dangerous! Seriously! If you aren’t an expert in this, find someone who is. Keep in mind, though, that many  “handyman” types have terrible safety habits. A good bet is to find places like Woodcraft or Tech Shop that have classes taught by people who are safety conscious, and tend to know what they are doing. Otherwise, there are some decent safety videos to be found on YouTube. Here’s one of someone purposely doing the wrong things, so you can see how scary it can get.

Anyhow, if I haven’t scared you off yet, I hope your flat building is fun and fruitful. If you have any questions, let me know.