So, with a proliferation of “store” pre-made garden boxes with some assembly required maybe ist time to see how easily and cheap making one the old fashioned way is. I assume you have access to a drill and screws or hammer and nails. That really is all you need.
Decide on your box size. Maximum size is four feet wide (length can be anything). Keep in mind that lumber comes in pre-determined lengths of 8 feet, 10 feet and 12 feet and you will want to make use of the whole board with as few cuts as possible. For example a 4×2 box would need an eight foot cut in half, and another eight foot cut into two 2 foot lengths with 4 feet left over. If you used a 12 foot board, you would cut 4 foot + 4 foot + 2 foot + 2 foot with no waste.
Cutting is easy – any lumber store will cut your wood to size for you – usually for free if its a small amount of cuts.
What lumber do you use?
Well the size of the lumber varies – they can be made of recycled wood, leftover wood, scraps, whatever you want. For this box lets just go with a nice sturdy 2×6 (2×8 or 2×10 are great too but more expensive). The actual type of wood is essential as most untreated wood will rot very quickly and will need replacement after only a year or two. Cedar is a natural rot resistant choice but is very expensive. My cedar boxes I built 5 years ago will likely need replacing next year.
Pressure treated boards are commonly available, rot resistant and recent long term studies have shown/proven that the typical pressure treatment does not have ANY affect on plant growth or food safety. (More info here)
So lets go with a pressure treated 2×6 in a 12 foot length. The lumber yard cuts your lengths for you – two 2 foot lengths and two 4 foot lengths. Now you got your box at a “some assembly required” stage. At my local home depot, a 2x6x12′ pressure treated board is $9. Maybe a dollar to cut it so $10 for the lumber. Probably should get some nails or screws – nails are easiest. 3″ screws or nails – nails should go for around $5 for enough nails for a dozen boxes. Spiral or ardox nails, galvanized will last forever, but they only need to outlast you box – which most nails will. Screws tend to be about $10/100.
You have four corners, each corner will need two nails = 8 nails total (a third per corner wont hurt.
So your ready to assemble kit is now ready for $20 and you supply the hammer, compare that to the cheapest pre-made box at $43 at my local Home depot.
Two two foot sections go inside, hold them square and nail together (easier to start nails with the 4 foot section laying flat. Nails go through the four foot sections into the 2 foot sections, do all four corners – then you have a box.
Lay paper, cardboard on ground (you could also dig it up a bit ) under where the box will go. Wet the cardboard. The paper/cardboard will slowly disintegrate, killing grass underneath it.
Fill the box with compost/soil mixture. Soil should be mixed at a 1/4 peat moss, 1/2 compost and 1/4 vermiculite or soil of any kind. So for every bag of compost, you would add 1/2 of a similar sized bag of peat moss and 1/2 a similar sized bag of soil/vermiculite. THIS DOES NOT NEED BE EXACT! The key here is rich in compost. Bags of composted cow manure sell for around $3/bag, Peat moss around $10 for a large bag (multiple boxes) and soil for $3 per bag.
The 2×4 by 6″ deep bed here should take approximately 6 bags of compost, 3 bags of soil and the equivalent of three bags of peat moss (much larger bags). so with soil in your box you are up to about $45 all in per 2×4 box. Additional boxes will be slightly cheaper as you will have nails, peatmoss….