Encouraging Root Growth in Cannabis

Light and CO₂

Plants growing in high light tend to root better. Higher light means more CO₂ absorption for photosynthesis. This is important, as plants tend to designate about 40 percent of their absorbed CO₂ to the root system. In outdoor production under cloudy conditions, which can happen in the northwest, root growth can be limited at times.

Soil Temperature

Plants tend to grow the best roots at soil temperatures between 68°F and 86°F. This is of course not a problem for indoor production, but soil temps can at times be outside this range in outdoor production. In Florida and the Caribbean media temperatures can be above 120°F. In the tropics, roots tend to grow best on the north side of the container.

Moisture and Salt Levels

Roots like to grow in dark spaces where there is consistent moisture, but also decent aeration. It is a myth that drought stressing plants makes them grow more roots. At the same time, drying out overly wet, poorly aerated media back to proper moisture can increase root growth. Good irrigation timing and quantity are essential. Always monitor salt levels as well. When salts are high, it is very hard for roots to absorb moisture and nutrients. Root growth suffers.

Potting Soil Media

Physical media factors are critically important. I like to grow in media with somewhat lower moisture holding capacity. Such media are harder to overwater, and easier to leach if soluble salts become a problem. When retaining less moisture, a mix can be irrigated more aggressively without risking root disease. Particle size, aeration and drainage all matter.

Coir, or coconut fiber is a popular media component, as it contains abundant potassium but low calcium. Coir in mixes tends to help root growth, though watering practices will be different with coir as opposed to peat-based media. Some growers like to combine the two with some perlite. Compost in substrates can add valuable nutrients, carbon and microbes, though good physical structure must be maintained. Bark can supply aeration and pore space without holding excessive moisture.

pH Balance

At low pH, elements such as aluminum and manganese can become toxic to plants, causing stunting and limiting root growth. Some root pathogens such as Fusarium and Cylindrocladium are more aggressive at low pH. Aluminum is especially toxic to growing root tips. High pH can cause nutrient imbalances such as phosphorus and trace element deficiencies, which reduce photosynthesis and therefore root growth