When it comes to producing very high yields per square foot of growing space there are very few who can come close to the yields produced by commercial cannabis growers. The wikipedia page on cannabis cultivation is an epic 11,000 word overview that provides a huge amount of detail on different techniques and solutions available for growers and this barely even scratches the surface of what is available online.
Cannabis farms are highly illegal in most of the western world and as a result are often concealed indoors away from the prying eyes of the authorities. As a result of this, farms are frequently kept in windowless rooms such as basements, loft spaces or warehouses and sheds. One of the results of this, other than being able to evade detection, is that the growers have 100% control over the farm’s environment with levels of temperature, water and light all fully managed by the farmer. This tight level of control gives organised growers laboratory conditions for maximizing yields per square foot.
Farmers use a wide variety of techniques to squeeze every last drop of growth productivity out of the plants with techniques such as reducing the number of hours of sunlight a plant gets after a fixed growth period to invoke a hormone response that increases the harvest weight, controlling and changing the food inputs depending on the plant growth cycle stage and so on. If the research and rigour applied to cannabis farming could be applied to legal urban farming of food plants, urban farming could become very profitable for farmers in small spaces.