We built a large glass covered deck, and we figured that since we now have an outdoor living room with lots of light, we already have the beginning of a greenhouse. The only thing missing was some soil to put the plants in. There were lots of existing planters available, but we didn't find a suitable one.

There were plenty of planters available in stores and online, but almost all of them were tiny. Unfortunately gardening seems to be the one thing that planter designers rarely understand. The more soil your plants have available, the easier it will be for them to get nutrients and water. A smaller soil space runs the risk of drying up on a hot day. We wanted our planter to have plenty of depth.

There were also plenty of instructions to build a planter, but most, if not all of them had one problem. They had holes all over their bottom for drainage. Since we wanted to put our planter on a wooden deck, we didn't want water to leak all over every time we water the plants. We wanted to have a single drainage hole. Drainage is a must in a planter, if the roots sit in too still water for too long, they will suffocate.

Supplies

Pressure treated wood boards for the wood surface of the planter (not in contact with the soil)
Stainless Steel Screws
Plastic film between the wood and the soil, to prevent the wood from rotting.
Plastic drainage sheet to protect the plastic film, and to allow for better aeration.

Step 1: Painting the Boards

I decided to paint the boards before assembly, because getting the paint between the boards in an assembled planter would be so difficult. Since our wood was fresh from the factory, I had to dry it out first to get the paint to stick. I first cut the boards to length, because water evaporates faster from the end grain, then I used a fan, along with a dehumidifier in a warm, enclosed space (our garage). I tested the boards with a moisture meter. The boards dried out (enough for painting at least) surprisingly fast, in just two days.

Step 2: Base

I wanted to have a drainage hole in the middle of the planter, so I designed the bottom of the planter to slope toward the middle. That way it was quite easy to direct the excess water to flow to the middle, where I later built the drainage.

Step 3: Sides

I screwed on the side boards to the base, one side at a time. I added a board I had cut in half to the top of each side. I didn't want the screws to show, so I added them from the inside of the planter.

Step 4: Plastic Film

I added film to protect the boards from the wet soil. Even pressure treated lumber will rot eventually if it's in constant contact with water, moist soil included. I also didn't want any pressure treatment chemicals to leak from the boards to the soil. The film I used was food grade polyethylene film used to cover strawberry beds. In two of the bigger boxes my film wasn't wide enough. I used a clothes iron to weld two pieces of film together. I used a sheet of waxed paper on both side of the plastic, to prevent it from sticking to everything. I tried first without the paper, not a good idea.

Important note here, practically every instruction I've read about making your own planter box tells you to add staples everywhere. Won't the water get through those holes? To prevent this, I stapled the plastic film only to the top of the box where it's above the soil level.

Step 5: Drainage Hole

To allow the water to drain out we need to add at least one hole to the bottom. I drilled a hole through the base, then added this fitting, it was called a cable inlet. A simpler version would be to have some slack in the plastic film, and push that through the hole with a finger.

Step 6: Drainage Sheet

I added some drainage sheet meant to protect a house from damp earth. This will protect the fairly thin plastic film from damage. This will probably be most useful when we have to use bigger gardening tools like shovels, for example when improving the soil.

Step 7: Water Outlet

At first I tried using a plastic tube to lead the water out from the planter. The tube got kinked in the small space under the planter, so I decided to switch it for a funnel. I had to drill a hole for the funnel, but I figured that if I ever remove the planter, I can simply plug the hole.

Step 8: Edging

I added a mitered board to the top of the planter to hide the ugly plastic, and to allow sitting on the edge.

Step 9: Gravel

I added gravel to the bottom to server as a drainage layer. Then I covered the gravel with some garden fabric to prevent soil from getting mixed with the gravel. After that I added some sand to serve as an extra layer of drainage, and after that a fat layer of soil and compost.

Step 10: Finished!

I measured the soil depth to be about 45cm (+ sand & gravel underneath). Each of our four planters now has well over 500 liters of soil.

I can't report yet on how durable the planters are, but the excess water seems to be coming out of the drainage hole as intended. The water flows straight into the ground below, so I'm fairly confident the design is solid and will minimize the risk of moisture rotting the wood. I will try to report back in twenty years or so. Feel free to remind me. :)

Our tomatoes have been in the planters for a couple of weeks now, and seem to be growing spectacularly - so far at least. They are now starting to need some strings to hold them up. We tried to choose short varieties, so our view wouldn't be obstructed too much, but it seems like the plants didn't get the message. :)