Sea of Green (SOG, not to be confused with ScrOG) is the theory and method of harvesting lots of small plants, matured early to get the fastest production of buds available. Instead of growing a few plants for a longer period of time, in the same space, many smaller plants are grown that mature faster and in less time. Thus, less time is required between crops. This is important to you when the electricity bill comes each month. One crop can be started while another is maturing, and a continuous harvest, year round can be maintained. 4 plants per square foot (1 foot = 30 cm) will be a good start for seedlings. 1 plant per square foot will allow plenty of room for each plant to grow a large top cola, but will not allow for much bottom branching. This is OK since indoors, these bottom branches are always shaded anyway, and will not grow very well unless given additional light and space.
The indoor grower quickly realizes that plants that are too tall do not produce enough at the bottom to make the extra growing time used worth while. An exception to this rule would be if it is intended the plants are to go outside at some point, and it is expected that the light/shading issue will not be a factor at that point. The plants, if started at the same time, should create what is called a "green canopy" that traps most of the light at the top level of the plants. Little light will penetrate below this level since the plants are so close together.
The gardener is attempting to concentrate on the top of the plant, and use the light and space to the best advantage, in as little time as possible. Use of nylon poultry fence or similar trellising laid out over the green canopy will support the plants as they start to droop under the weight of heavy fruiting tops. Stakes can be used too, but are not as easy to install for plants in the middle and back of the room, where reach is more difficult. It's easy to want big plants since they will produce more yield per plant, but it's usually better with limited space to grow smaller plants that mature faster and pack into smaller spaces.
Sea of Green was developed in Holland. Instead of fitting 4 large plants in that small room, fit 12 small ones on a shelf above 12 other small plants. These plants take only 3-4 months to mature from germination to ripe buds, and harvesting takes place constantly since there is both a vegetative and flowering area devoted to each, with harvests every 45-60 days. It's not the size of the plant, but the maturity and quality of the product that counts. Twice as many plants grown half as big will fill the grow space twice as fast, so harvests take place almost twice as often. Get good at picking early flowering plants, and propagate only those that are of the best quality.
It's better to just grow 2 or 4 times more plants, since they will produce more, faster, in the same space. Also, "training" plants with twist-ties is a great way to get them to bush out a bit. Just take any type of plastic or paper twist tie and wrap it around the top of the plant, then pull it over until the top is bent over 90-180 degrees and then attach this to the main stem lower on the plant. Do this for one week and then release the plant from its bond. The plant can be trained in this fashion to take less vertical space and to grow bushier, to fill the grow space and force lower limbs to grow upward and join the green canopy.
This technique takes advantage of the fact that if the top is pulled over, it creates a hormonal condition in the plant that makes it bush out at all lower internodes. Sea of Green entails growing to harvest the main cola (top) of the plant. Bottom branches are trimmed to increase air flow under the "blanket" of growing tops. Use these cuttings for clones, as they are the easiest part of the plant to root.