Cannabis Propagation: 8 Tips

Cannabis plants have basically three growth stages in their life cycle: propagation (whether from tissue culture/clones/seeds), vegetation, and flowering. Each of these stages has different requirements to maximize the crop’s potential. Environmental conditions, production methodologies, and horticultural techniques all vary depending on where the plants are in their development. 

In the first part of this three-part series, I present my top tips for successful cannabis propagation. Parts 2 and 3 will share tips on vegetating plants and flowering crops, respectively. 

Given that there are different requirements for outdoor growing, indoor growing, and mixed-lighting settings, the tips presented here may favor one environment over another, but all have relevance to multiple environments. 

1. Start with more tissue cultures/clones/seedlings than you need. Having too many plants might be difficult to manage at first, but the plethora of options can allow you to cherry pick only the best specimens to transfer to vegetation (stage 2). The more options you have, the easier it is to eliminate any undesirable plants (such as any male plants or any plants that do not exhibit quality traits) in favor of only the ones best suited for your production needs. 

2. Remove any unnecessary plant material. Whether propagating from clones or seed, it is best to remove the lower leaves early so that the energy and resources the plant otherwise would have allocated to keeping that unproductive lower foliage alive can be employed to make the existing vegetation that much healthier. On a 6-inch clone, anything below the top two leaf sets and the top shoot can be trimmed or eliminated. 

Trimming lower foliage on clones can help strengthen existing leaf structures and improve rooting as energy is redirected away from new growth.Ahha Epemeeba | Adobe Stock

“Lollipopping,” as it’s called, is a common practice in many forms of agriculture. Tree farmers, for example, regularly remove lower branches on a tree trunk or sapling at the base of a tree (commonly referred to as “suckers” because they are useless and “suck” energy from the tree). 

3. Cover rockwool or media to prevent algae growth. Algae can grow on the surface of rockwool, perlite, and coco fiber-based growing media. Light, combined with moisture and a food source, encourages algae spores to grow and bloom on the media’s surface and can sometimes cover the entire top surface. On rockwool, this algae growth can prevent the media from absorbing maximum amounts of moisture. The algae also will compete for nutrients which can cause nutrient deficiencies. Algae also encourages fungus gnat proliferation. 

Therefore, the media should always be covered or shaded to prevent light transmission. Multiple products made to prevent light from contacting the media are available, including simple black plastic strips (2-4 mm thick). 

4. Let your roots breathe. Many growers are aware that plants “breathe in” CO2 through leaf surface pores in the photosynthesis process. What often gets overlooked is that the plant’s root system needs oxygen. If you don’t believe it, you can test it by placing a plastic bag around a plant’s root system and only pump CO2 into the plastic bag; the plant ultimately would die. 

A rooting plant is not trying to photosynthesize to replicate more vegetation. A rooting plant requires elevated oxygen levels to aid in root production. This is not to say the plant only requires oxygen in stage 1, but to clarify that a rooting plant needs more than just CO2. When rooting clones, it is beneficial to periodically exchange the air in the cloning environment, whether it is a whole room or simply a small cloning tray. I prefer to exchange air twice a day. 

5. Never place an unrooted clone under intense direct light. A clone does not require intense light until it has formed roots. An unrooted clone exposed to excessive light levels will attempt to photosynthesize, but it cannot do so without roots to uptake moisture and nutrients, so it will sacrifice and cannibalize its existing leaves in an attempt to produce new vegetation. 

Unrooted clones require approximately 800 lumens per square foot of grow space, and a rooted clone requires no more than 1,500 lumens per square foot. That said, all cultivars are unique and some may tolerate different light levels during the rooted clone stage. 

Tissue cultures have a similar lighting requirement as a clone, except that radiant heat is an additional concern in tissue culture, so minimal lighting is preferred (under 800 lumens per square foot). Delicate seedlings require little lighting when initially germinating, but as roots form and leaves multiply, the lighting requirements ramp up in intensity. 

darren415 | Adobe Stock

6. Keep temperatures cooler compared to stages 2 and 3. Ambient temperature is the temperature of the air that surrounds you. Radiant heat is the hotter temperature that radiates from an object (e.g., the hood of your car when left in the sun). Neither clones nor tissue cultures nor seedlings prefer elevated temperatures. Always monitor the air temperature surrounding these delicate plants, as well as their media temperatures and, if possible, the air layer at the tops of the plants to ensure there is no elevated radiant heat near plant tips. I prefer to keep propagation rooms between 70 and 75 degrees Fahrenheit. 

7. Adapt your watering schedule to your propagation method. Seedlings and clones require periodical watering, while tissue cultures require no watering until they are moved to a media. 

After a watering, clones and seedlings should be allowed to slowly transition from wet to damp—but they must never fully dry out. The key is to keep the media moist enough to encourage root development and growth, but at the same time not kill them with kindness by overwatering them. Overwatering can both stress out a clone or seedling as well as encourage mold, mildew, and disease. Slowly drying out the media encourages oxygen to be drawn into it, and the oxygen, in turn, aids in root development and growth. Never overwater or underwater seedlings or clones. Oxygen-rich water should be utilized for all stages of plant growth. 

8. Acclimate your propagated plants prior to moving them to vegetation. Regardless of the method, all propagation plants require hardening off (or acclimatization). The light levels and temperatures required in stage 2 of plant environments are different than stage 1; therefore, after the plants are rooted and begin to photosynthesize, they need to be acclimated to the new environment slowly to be able to tolerate the increased light and temperature levels. 

A good way to do this is to slowly increase light levels in the clone area over the course of the final propagation week. Another option is to have a specific hardening off area in the stage 2 room where light levels are minimized to mimic the stage 1 area at first before slowly increasing them over the week. 

Be mindful of the process, as the increased light levels will also bring higher temperatures and humidity as plant transpiration increases. Do not try to harden off your plants too rapidly—like the tortoise, slow and steady wins the race. 

Good clones make good plants, which in turn make good finished product. That said, the vegetative plants you take clones from are just as important to your propagation success as following these tips.

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