This is an incredibly simple garden made out an old tire, and can often be built at extremely minimal to zero cost.
The tire garden also has the advantage of being entirely portable, meaning that it can be put anywhere there is space for it (it by no means has to go on the roof, although that is probably a great place for it). Due to this feature, the tire garden can be placed where foraging animals cannot munch on your vegetables.
The tire garden is also slightly deeper than the shallow-bed garden, making it better for deeper root crops.
A tripod trellis can be added to stake up plants like tomatoes.
Lastly, tire gardens can be placed on blocks lifting them off the surface of the roof, minimizing or negating potential damage to the roof.
Materials for a Tire Garden:
The Volume (V) V=h*(r^2) of the same growing medium as the shallow-bed garden (r=radius, h=height).
An old tire.
A sharp knife or machete to cut the tires.
Circular plastic sheets the width of the inside of the tire, or wire mesh of the same dimensions covered with a thick layer of newspaper.
Possibly small sticks, branches, or similar items to make trellises for plants like tomatoes that need them (three per tire).
Instructions to Make a Tire Garden:
(1) Take an old tire and cut around the top rim with a machete, saw, or other cutting device (Take note that the piece you cut out is a mirror image of the bottom of the tire).
(2) Remove the circular piece that was cut off and line the bottom of the inside of the tire with the plastic sheet or the wire mesh/newspaper combination.
(3) Take the piece that was cut off, flip it upside down and place it back into the tire, firmly locking down the plastic sheet or the wire mesh and newspaper.
(4) Fill the tire with growing medium and plant!
Next week: guidelines for how to layer the soil of your urban gardens