I have grown lettuce in store bought, plastic trough containers and it works just fine.

At the same time, if you are going to go through the trouble of making your own from wood, why not just build yourself a 4' x4' 6-8" deep box and plant Square Foot style?

The problem with the trough containers is that you can't grow that much lettuce in one. If you figure 6" width and 30" length then giving each plant 6" of room you can only grow 5 plants to maturity. It will take 30-45 days to get to harvesting stage and only a couple/few to eat it all.

Plywood is fine, but as I am sure you know it doesn't last a long time when exposed to constant moisture and lettuce certainly grows best in moist soils. Cedar or redwood 1 by 6 or 1 by 8 lumber would be a more economical approach over the long term. If you need a solid bottom plywood covered with sheet plastic would help preserve the bottom a little longer, but I would make it 1/2" thick at least if you go with a 4'x4' design.

Long story short container grown lettuce, kept moist and cool, will grow quickly and taste great (variety dependent, of course), but give some thought to how much you can grow in a particular sized area versus how much you will eat.

I've used plywood for big long containers without problem and some of those containers will be on their 5th summer this year without any sign of warping. I paint every wood surface inside and out with that Glidden Porch and Floor paint with Polyurethane. So far Glidden is the only company that I have found that has Polyurethane in their Porch and Floor paint and that stuff is great for protecting wood from moisture and holds up well. Home Depot sells this stuff and you can get it in all kinds of different colors. It's oil based and kind of toxic so you have to paint everything outside or away from living spaces or you'll start getting dizzy. I've already gotten high off of it once. :-)

That said I've switched from using plywood for my big boxes and now use 1x10 rough pine wood which costs out almost equally to 3/4" plywood. It provides better structure to the box and makes constructing them much simpler.

mea2214 how do you built them. Could you give us details or pictures?

The plywood boxes I built are a little complicated to describe in words. I framed those boxes with 2x2s and screwed in 1/4" plywood as a skin. The frame actually keeps the structure square over the years. You can see my oldest plywood box at:

http://www.brandylion.com/images/front_box.jpg

Using rough pine 1xwhatever is much easier because you don't have to build a frame. Right now, I make all my containers 18.5" deep which provides more flexibility in future years. 1x10 pine boards are actually 9.25" wide so I strap two of them together. To strap them I cut up a bunch of 2x2s into 18" lengths. Make the long sides first by putting the 1x10s side by side and strapping them at each end with the 18" 2x2. I screw through the pine into the corner straps using 2 1 5/8" screws. You'll have to drill pilot holes first and only use two screws per board and alternate them because the short side will also screw into that same corner strap and you don't want the screws hitting each other. Two screws is plenty since there isn't much force from the soil against the walls.

Once you got the two long sides with their corner straps secured at each end connecting the 1x10s (or different depending on how deep you want your planter), you need to put an 18" 2x2 strap every two feet. So if your planter is 4' long you put one strap in the middle, if it's 8' long you'll be putting 3 straps spaced every two feet.

When your long sides are finished cut pieces of 1x10s to how wide you want your box. Set the long sides up on the ground and screw the pieces into the corner straps and you'll have a square box with no bottom. It helps to have a framing square for this step to make sure all the corners are 90 degrees.

For the bottom I use whatever scrap wood I have laying around -- even 3/4" plywood. You can use the same rough pine used for the sides too. Turn the bottomless box upside down and screw the boards into the sides. You don't need that many screws, perhaps 1 every 2 feet. If you use 1x10s or whatever you'll have to strap them together and the straps can be inside or out depending on how you plan to use the box. My boxes either go into a frame or sit on cut 4x4s or 6x6s. If you use 4x4s, space those appropriately. If you have a 4' box you can get away with 2, a 6-8' box will need at least three, 10' will need 4, etc. The screws holding the bottom into the sides cannot hold the weight of the dirt, the planter must be supported by some frame or structure from below and that depends on where you're going to put the box.

After you have the box screwed together, paint it inside and out and then drill holes in the bottom. I use 1/2" holes and I drill a lot of them. Then screw the box into the platform and fill with dirt.

This March/April I'll be building 5 more of these of all different sizes so maybe I'll put together a picture log if there's interest.

Here's a pic of 3 of my boxes early Spring last year. The white one on the right sits on 4x4s and the two on the left have their own frame that supports the bottom and the weight of the soil.

http://www.brandylion.com/images/rear-boxes2.jpg

The boxes on the edge of the roof line were the first ones I made out of that treated wood which I don't use anymore.

Here's a view later in the summer.

http://www.brandylion.com/images/summer-rear.jpg

Thank you! I am taking notes here...definitely want to do this for the summer!

Take care,

Pam

Is that polyurethane paint safe for use on containers used for vegetables? That might be worth checking into.

That's a good question. When I made the first boxes out of that treated wood I was told that arsenic can leech out of the wood into the soil so I never planted anything edible in those. As far as I know, they stopped using arsenic in treated wood sold at the big box stores but I still wouldn't take a chance. The nice thing about using treated wood is that you don't have to paint anything but treated wood is much more expensive than naked rough pine. BTW: In my area, the only big box store that sells the cheaper naked rough pine is Menards. Home Depot only sells finished pine, pine used for inside shelving etc. which is more expensive and really not necessary for outside projects -- especially since everything is painted.

As for the polyurethane, that's a good question. From my limited knowledge of chemistry, the poly kind of indicates it creates a polymer coating which should resemble a plastic seal so I doubt anything leeches into the soil. But I don't know. This year I'm going to use these boxes for lots of edible hot peppers and herbs and one box will have cucumbers in it so I'd be interested in knowing more about this too.

Arsenic based preservatives are no longer used for residential purposes but the alternatives are still toxic. I don't think that there is any preserved wood product that can be used to make containers for veggies, darn it all.

If you want to be really safe, line your boxes with stainless steel. It will cost a fortune, but last (almost) forever. I made containers from 2" thick styrofoam, painted only the outside, and plan to surround them with something wooden this spring. This way, the wood won't be soaked by the wet soil. My thinking was: if take-out food is offered in styrofoam, maybe it is inert enough to expose to soil with edible plants. (I have, till now, grown only flowers in containers).
Plywood, in general, is toxic. It contains formaldehyde and out-gasses for many years. Polyurethane finishes are tough, but over time with continual exposure to water, the wood will check and allow water in. Then you have the ingredients in the plywood migrating into the soil anyway. I made window boxes using marine grade fir plywood many years ago. I sealed them with a really nasty, clear floor-finishing product which is now banned in NYC. I followed that with several coats of polyurethane varnish, dried them out every fall after collecting seeds, lightly refinished the outsides only, and they looked great for 11 years. They were unlined, so eventually rotted from within, and, almost poetically, started to give out when I was forced to move. I think the combination of marine (or most any exterior glue) plywood, and a liner, is a good way to go.
If you're determined to use solid wood, and not line it, I would treat the inside with (lots of) mineral oil, as we do for cutting boards.

Amy

Has anyone ever used aluminum or plastic rain gutters for smaller lettuce types?
I know you wouldn't leave much room for roots, but for certain small lettuces and herbs, it might work fine. You can go to Home Depot or Lowes and get it in long sections. Holes for drainage, some sort of caps for the ends, a brick every foot or so underneath for support and you would have a very long container.

Dunno about the aluminum as I don't know how much leeches out (aluminum is toxic to plants above a trace amount).

I have not used a plastic gutter, but I have seen them used for hydroponic growing. Seems like a workable idea.