If you live in the city like we do and your growing space is limited, the growing tower is your ultimate solution. I do not know who was the first to think about it, but I find it ingenious. All you'll need for making your own tower is a plastic sewage pipe and a drill with a hole saw.
We decided to make holes of 4 cm in diameter, leaving horizontal space of about 10 cm between any two holes and vertical space of about 17 cm.
The main question was whether we should drill first and then fill the pipe or fill it first and then drill. Obviously, it is much easier to drill first but then the earth spills out of the pipe when you fill it. To prevent it, just cover the holes with duct tape before you start filling the pipe.
The outcome - 18 plants in just one planter.