Building Your SFG BoxesBy Mark Roach
Square Foot Garden Boxes can be made from almost any untreated wood, stone, cinder blocks, bricks or even ‘man made’ wood and recycled plastic. We recommend using wood for your first SFG box as it is more portable, cheaper and easier to work with, although not as long lasting as brick or stone. Wooden boxes also have the advantage of being able to put them on stilts or trestles so that you don’t have to bend down to tend your garden.
It’s important to use untreated wood as you don’t want any chemicals to be leached into your growing soil and be taken up by the plants. In South Africa, most timber yards and places like Timber City or Builders Warehouse will also cut the wood to size for you for free. If you are under a strict budget you can also pick up free (or very cheap) wood from building demolition sites and construction dump sites although you will have a lot more work to do cutting, joining odd bits together and sometimes cleaning the wood yourself.
Measuring the wood
I find it easier to work in metric measurements and ‘squares’ when building my Square Foot Garden boxes. Remember that 1ft is approx 30cm so a 1×1 meter box is close enough to 3×3 foot (or 3 squares by 3 squares). A 1.2×1.2 meter box would be 4×4 squares, a 1.5×1.2 meter box would be 5×4 squares and a 2×1.2 meter box would be 8×4 squares. You will notice that I do not go above 1.2m (4 squares) wide with any of these measurements. This is because you do not want the boxes to be too wide or you will not be able to reach and tend to the middle squares without tramping on your growing soil (a big no-no).
I have also now learnt from experience to have all four box sides cut to the same length and then to rotate the corners instead of having to measure and then add/subtract the width of the planks to the ‘inside/outside’ sides to try and get a square box. This is not rocket science and the measurements don’t have to be perfect but having every ‘square‘ square does make things a little easier than having 6 – 8 odd centimeters left over at the end of your box when you’re trying to put on your grid
Constructing the box
Build your box on a flat surface using course thread deck screws and drilling holes for the screws in the first piece of wood that you’re joining and letting the screws embed themselves in the second piece (this prevents the planks from splitting when tightening the screws). The length of the screws should ideally be 2-3 times the width of the planks that you are using.
By the way, I would also recommend using some wood glue on the joints and not using planks less than 20mm thick as you are going to get some warping of the wood once it has been exposed to the elements. My first boxes were not glued and they warped pretty badly (after only two days in the garden) and almost pulled the screws out completely. In the future I will also be using a few steel L-shaped corner brackets on the outsides of each corner (especially at the base of the box) for additional strength.
Whether you put a bottom on your boxes or not is up to you. If you plan on simply placing your SFG boxes on the ground as a semi-permanent fixture then you can save some money by simply placing some flattened cardboard boxes and newspaper in the bottom of your box before adding your soil to prevent weeds and grass in your existing garden soil from growing up inside your box – You can also purchase and use proper (expensive) weed cloth to do the same thing.
SFG boxes that are going to be placed on trestles, tables, decks & balcony’s, or anywhere where they are to be moved around regularly, can be built with a plywood sheeting base with holes drilled through for drainage (at least one 6mm hole per square plus one in each corner). The thickness of the plywood sheeting depends on the size of the box. A 1×1 meter box (3×3 squares) would need about a 12mm thick sheeting. For bigger boxes or boxes that are going to be moved often, we would recommend going with 18mm or even a 22mm plywood sheeting.
Preserving the wood
Once your boxes are completed you can give them a couple of coats of linseed oil to preserve the wood if you want. You can also paint or stain the outside of your boxes but leave the inside unpainted so that there is no possible contamination of your soil.
Special structure boxes, pyramid boxes, railing boxes, L-shape boxes and more will be covered in other articles but for now, you should have enough info to go out and get started building your first Square Foot Garden box.