How We Made An 8-Foot Planter Box

October 03, 2017   //  Posted in: Backyard, DIY, Gardening   //  By: Emily   //  one response

Elaborate landscaping and flowering gardens exist in our dreams; not so much in reality. Container gardens are my domain for now, and between herbs and potatoes and flowering front porch planters, we kind of rocked it this season. I always knew I’d get around to adding great, large planter on the treehouse, and early in the summer, it came to life (and of course it was completely assembled and installed in, like, an hour… just a reminder to stop putting stuff off, E.)

We couldn’t be happier with how it blossomed this growing season.

A DIY 8-foot long large planter box mounted to our kid's treehouse.

Things I used:

  • Two 5/4×6″ 8-foot deck boards
  • Four pieces of 2×6, cut to 6″ in length
  • Steel mesh fencing
  • Drill with assorted bits
  • 20+ 3-1/2″ exterior screws (this thing will never fall apart, nor fall down)
  • air compressor and a nail gun fitted with 1/2″ staples
  • my favorite natural stain made of vinegar and steel wool
  • a very tool-savvy tween who enjoys herself some DIY

If you’re looking for a fast project, this is it. The full 8-foot deck boards extend as the front and back of the planter–selected because deck boards are lighter than 2x lumber–and the cut 2×6 pieces form the ends of the planter, and act a few cross-braces for added durability. Sandwiched in there, the 2x offers bigger target to hit when you’re drilling through a deck board and trying to hit something on-center. And they’re undeniably more solid. If you’re considering your own DIY window box, after 4 months hanging filled with dirt and plants on our treehouse, this is a construct that I would do again 1,000,000x over. It’s solid as can be.

Step 1: Make your big rectangle

Dry fit the 8-foot boards, with the 2x pieces sandwiched perpendicularly in between. Do a rough layout, clamp it in position, and predrill everywhere you need to attach. Don’t scrimp on screws, use enough to feel confident that it won’t warp (3 at each joint should do the trick). Use the long screws to assemble the outer rectangle that will become your planter.

Step 2: Install your base

Drainage is a big issue with planter boxes made completely of wood. If it doesn’t have good drainage, your plants might suffer and the wood will deteriorate faster, and maybe troubleshooting this aspect of the build is why it took me 3 years to get this made. I eliminated my concerns about rotting by installing a heavy-duty steel mesh across the bottom of the planter, like a little planter hammock, if you will. I draped it along the inside of the assembled rectangle, trimmed it to size, and then we used a staple gun to pin the metal in place around the inside bottom edge of the planter.

Building a DIY planter box with a steel mesh bottom, to allow for drainage and root health.

Step 3: Add cross braces for reinforcement

How many cross pieces you install depends on your confidence with the construction to this point. I added two–evenly spaced at 1/3 and 2/3 marks along the 8-foot length, knowing that it would really help to keep the planter square and distribute the weight of the soil against the front of the planter. You’ll get the visual in the next photo.

Step 4: Install that planter!

I guess it’s worth noting that I also stained it prior to installing, using what is typically my favorite natural stain. It usually takes a few months to fully transform the color of the wood, but I think the solution I applied hadn’t brewed long enough; even today, the color is much lighter than I expected it would be. Shoulda been more like the color of this garden box accessory. Another coat of stain to come in the future.

Every single time I’ve considered adding large planter boxes beneath a window, I’ve felt conflicted because of the commitment factor – either you’re putting them there for life and committing to maintaining them, or you’re putting holes into your siding or trim and opening up the chances of rainwater seeping through those openings. I’m really sorry I don’t have the right answers for those of you doing that type of planter box, because my planter is outdoors, in an unfinished space that already gets wet, attaching to a railing that is already a solid, sturdy natural hardwood. All I can say is that I used 12 giant exterior screws to mount this to the railing, which averages a screw every 8″, and some might say that’s overkill but there’s no such thing as overkill when you’re securing a heavy wooden box that’s going to be filled with heavy soil and growing plants directly above where your children hopscotch all day long.

Attaching an 8-foot window box to a treehouse railing for a raised planter area.

Step 5: Fill it up

Whoa there. First things first. Along the bottom, before adding dirt, we shaved pieces of moss from the yard and laid it soil-side up in the planter. With the moss facing down, we formed a living base that continue to thrive beneath the plants, and would also prevent muddy run-through without affecting proper drainage. I’m happy to say that it worked well and continued on with its green, mossy self.

Using moss as the bottom layer in a window box.

Our assortment of small market-bought herbs, flowers, and cascading greens filled in nicely, too. I trimmed the herbs like it was my job (photographed them too, for this piece on why/how to prune and grow herbs for DIY Network), and just this week began to transplant them indoors to see if I can get them through the winter to re-plant next spring.

Bonus Points:

I attached a row of Pon Pushpins (my fave of all the pushpins) on the railing, so that the kids could ID the plants and remember which was which as they grew. It was pretty brilliant, if I do say so myself, since the plant markers are covered by soil, or moved around and hidden within a few weeks of planting. This not only helped us remember which plant was which (apparently we aren’t people who can tell the difference between parsley and cilantro nor stevia and lemon basil, when under pressure <shrugs>) but it also taught the kids to be mindful of what the plant likes, what it looks like when blossoming, etc.

Pon Pins used in our treehouse as an alternative to sharp pushpins on a cork board.

If you’re not familiar with Pon Pins yet, welcome to your next happy thing. An alternative to push pins, they press in like you would expect, but rest flush against the surface and host a tight coil in which you can slide artwork and photos through without devastatingly having to puncture holes into your stuff. To get them secured in wood, I simply drilled a small hole, poked the end of the Pon into some strong adhesive, and then tapped it into the predrilled hole. Have not had an issue with them popping loose yet, though the labels for our plants have been the only things occupied this year.

View from inside the treehouse, with peeks of our large railing-mounted planter box and Pon Pins holding our plant ID cards.

Our gardens were time-sucks this summer, and I loved every minute of it. There are a few other things I want to get around to sharing this fall that might help you plan for next spring, but you can also see peeks of these things on Instagram if you scroll around the feed @merrypad.

  • Mark
    10 months ago - Reply

    Nice Article!

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