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Determining Plant Density

June 28, 2016

When designing a new indoor garden, some professional growers might be tempted to fill it up with as many plants as they can possibly fit in any given space. However, visions of massive yields can cloud their perception of reality and actually lead to smaller yields. For example, all of the plant’s leaves must be able to receive light before they can process it. If plants are overcrowded, leaves on lower branches won’t receive the maximum amount of light.

Calculating space requirements

How do growers decide just how many plants they can grow in a given space? That depends on the particular variety and strain, and there are size characteristics for each one:

  • Most indica and Afghanica hybrid strains originated in the Hindu Kush Mountains of India and Nepal, where they have a relatively short growing season. Plants with these genetics tend to be physically short with relatively small diameters.
  • Sativa strains originated in tropical and sub-tropical latitudes. They are used to a significantly longer growing season and tend to grow much taller and broader. Sativa-dominant hybrids tend to grow somewhat shorter.
  • Many auto-flowering strains produce short plants. They employ genetics from the cannabis species C. ruderalis to trigger the auto-flowering sequence, which is dependent on the age of the plant rather than the length of the light cycle. In fact, many auto-flowering strains are advertised as “stealth” plants due to their intentionally short height.

Based on these characteristics, it’s practical to assume that auto-flowering plants and small indica varieties need approximately 1 square foot per plant. Larger strains of indicas and sativas need 2 to 3 square feet per plant. However, some strains can require as much as 4 square feet per plant to prevent overcrowding. Consequently, you will need to experiment with the different strains you intend to grow to find the maximum plant density. Knowing this information will provide you with a good idea of how much room each plant will need.

Spacing for light

Conventional wisdom tells us that grow room lights positioned at the optimum height will penetrate about three feet deep into the canopy. But there are two major schools of thought that follow different spacing methods to ensure that all plants receive enough light.

  1. On one hand, some growers say that plants should be positioned with enough space to allow their lower leaves to receive and process all of the light that they can. The more leaves a plant has, the more light and minerals it can process and the more food it can produce for itself. This leads to both optimum yield and optimum quality, as long as sufficient space, light and CO2 are provided. With this method, one plant per 2 to 4 square feet, depending on the strain chosen, is often necessary to ensure optimum yield.
  2. In contrast, others say that plants can be crowded together and their lower branches removed; this forces each plant to concentrate its efforts on the upper branches. The lower branches of most plants often produce the lowest yields of the lowest quality bud, so removing lower branches will increase plant density. With this method, one plant per 1 square foot is often sufficient.


By Bill Bernhardt
Cannabis Cultivation Today articles are for informational purposes only and should not be considered legal guidance or advice on grow practices. You should contact an attorney or a qualified cultivation consultant for specific compliance and cultivation advice.

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