I NEED SOME PLANTER BOXES! – HOW DO I BUILD THEM?

Planter boxes come in all shapes and sizes. However, certain design elements are useful in all planter box projects!

This short article will provide a detailed description of how to build sturdy, long-lasting planter boxes, with a focus on the construction techniques and material selections involved!

TOOL LIST:

Chalk Box

Miter Saw (optional)

Circular Saw

Measuring Tape

Drill/Driver or Framing Gun

24″ Level

BEST PICKS:

A Compressor/Brad Nailer Combo can speed things up tremendously in this project!

When you use tube adhesive, a good Drip-Free Caulking Gun is a must.

MATERIAL LIST:

6 Mil Plastic greatly helps to preserve your lumber from moisture

Liquid Nails

Galvanized Framing Nails

Exterior Screws

5/4 Deck Board

1/2″ Treated Plywood

Treated 2×4

Treated 4×4

PLANTER BOXES OVERVIEW:

Custom planter boxes with vertical trim and top rails.

The key to all successful planter box projects to use materials that can stand up to exterior conditions. Boxes will inevitably come in contact with the ground, standing water, and a variety of damp soils and aggregates.

All fasteners in planter box projects should therefore be galvanized, or ceramic-coated. The lumber should be treated, preferably with a GC (“Ground Contact”) rating.

I use a lot of adhesives in my outdoor projects. However, I don’t trust all of them to bond with treated lumber. One of the few adhesives I’ve found that is specifically formulated for these projects is Liquid Nails (see link above).

MOISTURE BARRIER:

I believe strongly in using a moisture barrier to protect the inner walls of your planter boxes. External conditions are bad enough, but interior conditions of your planter boxes will always be wet and dark.

Therefore, I wrap all interiors with heavy 6-mil plastic. (I’ll explain later how to maintain decent drainage.)

STEP #1: CREATE A DECK PLATFORM

The planter boxes I created for this client were “elevated” beds. Top rails of the boxes were at about counter height. The floors were lifted off the ground by a deck platform system.

4×4 post legs and 2×4 rails create the planter box platform.

This is a great way to get the box up to a better working height so you don’t have to bend over or get on your knees to do some gardening. Of course, you also won’t have to fill your box with three feet of soil!

To create these floor platforms, I simply cut 4×4 posts to a predetermined height (about 14″).  I then created a rail system across their tops with treated 2×4 pieces, mitered at their corners.

Cut treated 2x4s with a miter saw.

Typically, having the 2x4s lying down like this would not be the strongest design. But, in this case, I installed the plywood walls later to provide them with plenty of extra support.

I then attached pieces of 5/4 deck board across the width of these frames, creating a platform. Next, I installed the deck boards with a lot space between them (about 1″). This way, I created natural drainage fields at the bottom of the box.

Standard deck board is used to create planter box floors.

Finally, I shot these components together with galvanized framing nails and brad nails.

(NOTE: These planter boxes were positioned on a concrete pad, which provided a perfectly flat foundation for them to sit on. However, in a grassy yard, the 4×4 posts could have easily been lengthened to sit on small footers, or even masonry blocks,  below ground level. For some notes on below-grade foundations, check out this article I wrote on footings and the frost line.)

STEP #2: INSTALL PLYWOOD WALLS

Treated plywood is used to create the planter box walls.

I used panels of treated 1/2″ plywood to create solid barrier walls around the perimeters of the deck platforms. I made sure to keep the walls lifted about 3/4″ off the concrete pad. That way, the drainage area beneath the planter boxes could occasionally be sprayed out with a hose. (You can see the long spacer block lying on the ground in the picture above.)

To cut plywood panels to size, I used a tape measure and pencil to lay out the dimensions of the cuts. Then, I used a chalk box to snap long “plot lines” between these measured marks.

Use a chalk box to snap cut guidelines on treated plywood.

I cut my plywood components to size. Next, I attached them to the deck platforms by shooting galvanized framing nails through the plywood walls. After that, I shot nails into the edge of the horizontal 2x4s, and into the 4×4 legs. I built the planter boxes this way to prevent the 2x4s from bowing under weight over time. This way, I didn’t need more bulky structural members.

Attach ply walls to all four sides of planter box.

Finished plywood walls on two planter boxes.

Because the ply panels met at the corners, they did not have to be meticulously fastened to one another. They would be further supported and concealed by framing members on the inside, and trim pieces on the outside.

STEP #4: INSTALL UPPER FRAMING

With the dimensions of the planter boxes roughly formed, I now installed more interior framing.

I began by creating a 2×4 “rim” along the upper edge of the plywood walls. Then, I used clamps to temporarily hold these pieces in place while I attached them with 1-1/2″ screws through the plywood walls. (I also attached the long framing members to the short ones. Lastly, I drove 3″ through their ends, locking the top frame together.)

Screw plywood walls treated top rails.

With this rim frame in place, I cut and assembled L-shaped verticals to sit between the rim and the deck platform. Once again, I screwed through the plywood walls into these vertical framing members to trap things in place more effectively.

Install vertical supports at inner corners of planter boxes.

Once this framing was in place, the boxes were extremely rigid! I dragged them around without worrying about damaging anything. I spaced them a little ways apart in order to more easily trim them.

STEP #5: LINE PLANTER BOXES WITH HEAVY PLASTIC

Before I began trimming, though, I took the opportunity to line the interior of the planter boxes with heavy 6-mil plastic. This I did by cutting two pieces that would cover the floor and turn 6″ up the walls. I then cut long pieces that would cover the walls and turn 6″ onto the floor.

Line planter box interiors with heavy 6-mil plastic for moisture protection.

To hold the plastic in place until the soil was deposited, I attached the plastic with galvanized 1/4″ staples . Then I also cut numerous small gashes through the bottom layers, in the spaces between deck boards, to allow for moisture “weeping.”

(You can use washed stone to fill the bottom 5″ of planter boxes to help create an adequate drain field below soil.)

STEP #6: ATTACH VERTICAL TRIM

Attach exterior trim to planter box walls with adhesive and brad nails.

Trim on these boxes can be done in an endless variety of ways. Horizontal decking, galvanized metal, and even shake cedar look good. In this case though, my client opted for tightly spaced vertical trim. I like the way it came out–it has sort of a cabin look.

To install this trim profile, I simply attached 1×6 treated trim pieces to the outer walls of the boxes, butting them together side-to-side.

Like the plywood walls themselves, I kept the trim pieces lifted 3/4″ off the ground. To attach them, I dotted the backs of the pieces with blobs of Liquid Nails Extreme (linked in the “Overview” section). Then, I fastened them with 1-3/16″ galvanized brad nails, which are not quite long enough to punch through the inner wall.

THIS PART WAS TOUGH:

Corners presented the main difficulty. I had to be sure that pieces were staying plumb to the corner they were approaching. I did this by measuring from the top and bottom of each new piece. Also, every side of the box required one piece to be ripped to width so corners would meet evenly.

Rip trim pieces to custom widths to create square corners.

I know that some separation between trim pieces will occur when the drying process begins. But, this glue+brads method actually helps mitigate shrinkage parting because of widespread attachment, and I think the long-term effect will be negligible.

STEP #7: ATTACH TOP RAILS

Cut and install mitered 5/4 cap pieces on top edge of planter boxes.

I wanted to hide the framing and end grain visible at the top of the box. Also, I wanted to create a convenient ledge for setting tools on. To do this, I installed a flat rail on the top edge of the box to act as a “cap”.

I ripped down 5/4 deck board so it would be just wide enough to cover the lumber below, plus have a 1″ overhang to the outside: about 5″ in all. I then cut mitered corners on the pieces, and installed them with adhesive and brads, just like the trim boards.

To touch-up, I sanded the rim of the new cap, since this is the area most likely to come in contact with skin and clothing.

PLANTER BOXES WRAP-UP:

With the planter boxes assembled, I just gave the adhesive a day to dry, then moved the boxes into their final locations. They felt incredibly strong and rigid, and seemed entirely up to the task of holding heavy soil and water weight. They also looked great.

A couple weeks later, I got the perfect follow-up to any custom project…a text from my client, confirming a job well done:

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