Managing Cannabis Growth

Controlling the Stretch in Cannabis Strains

Managing cannabis growth rates and final size before bud set can have a big influence on crop yield, harvest quality and the health of the cannabis plants in the garden. As a practical illustration, let’s say there are nine plants growing in. an area under lights.  If bud set occurs before the individual plants gain sufficient stature, yields will be low because there is lots of empty space between buds. If the same nine plants produce too much growth before bud set, the canopy will be jungle like and the crop may even run out of space, creating problems with artificial lighting and hurting quality and yield.

How Much Control?

Growth rates, especially in early flowering when strains make the most gains per day, are mostly determined by the genetics of your strain. A pure Indica may only grow 25% more than it’s size when flowering was started while an exotic Sativa can double or even triple in size. So the first lesson is knowing the genetic make up of your plant. At OldSchoolBA we typically list “Height” as a percentage, for example +65%–this means that you can expect the plant to stretch about 65% more from the size it was when flowering first started.  However, as we will discuss, this isn’t the only thing that will determine how much stature is gained before bud set, dictating how long you should veg your plants for to reach the ideal size before day length and dark hours start to tell the plant to bud.

Soil VS Hydro

Experienced hydro growers know that more often than not, hydroponic systems can accelerate plant development.  Sometimes by a lot versus soil, especially in vegetative growth and early flowering when cannabis plants gain their stature before putting all their energy into budding. Water culture systems like NFT, DWC, RDWC, etc can save a LOT of time in veg.  For example to grow a three foot round bush might only take a few weeks in water culture while it may take six to eight weeks in soil. Once flowering starts our “+65%” example could potentially turn into 100%, ie plants double in size.

In hydro/soilless systems like coco coir or Rockwool the grower can use moisture management to control growth rates to an extent because the growing medium still holds good oxygen when wet (although there are limits).  Maintaining wetter conditions in coco and rockwool in veg and early flowering will give more growth.  Some growers begin to allow the growing medium to dry out more in early flowering between watering cycles to stack flowering sites closer together for bigger buds if their plants are already a good size.

Environmental Factors for Managing Cannabis Growth

It’s always wise to maintain healthy conditions for your cannabis crop.  While outdoors the grower has little to no control, the greenhouse or indoor cannabis grower can achieve a high level of environmental control with good equipment and favourable conditions.

Higher humidity promotes stretching or elongation in cannabis them growth; the leaves try to make space for themselves to release excess humidity.  In drier conditions, the leaves and growth sites will stack closer together to help keep a healthy microclimate in order to prevent excessive moisture loss leading stress in drier conditions.  Typically, 65 to 70% is a good humidity level to maintain when you want faster growth rates while 50-60% relative humidity will promote slower growth rates and tight spacing between flowering sites.

Carbon Dioxide can most certainly elevate growth rates and even reduce flowering times.  Expect a 15% reduction in flowering time and reduce vegetative growth times by as much as 30% with supplemental CO2.


As a rule of thumb, higher intensities will promote faster development AND tighter growth spacing; both highly advantageous traits. Lower light intensities, while important for young delicate plants like seedlings, will promote stretching with lots of space between nodes in established plants.  Blue light wavelengths like CMH, MH, and higher Kelvin rated lamps will promote squat sturdy development and thick stems (perfect for veg and early flowering).  Lighting sources with more Red in the bandwidth (lower Kelvin ratings) promote more stretching and space between internodes and typically stimulate a strong flowering response.


There are many factors that can influence cannabis growth rates, including fertility (typically with higher values there is more potential for growth) and even dissolved oxygen levels at the roots.  However, the genetics of your strain will largely determine the growth rates you will see in veg and early flowering to manage what size your plants should be before bud set. Once you understand the tendencies of the strain you are working with (we list the % height wherever we can here at OldSchoolBA) you can start to manage other factors like lighting, irrigation and fertility to drive or slow growth traits like stem growth.  After flowers have formed you can use pruning to remove excess or unwanted stems or lower buds from the plant canopy, giving the perfect amount of space with the right environment for your seed genetics to reach their peak potential for yield and potency.