CANNABIS CULTURE – Grafting is a procedure in which two plants are fused together, to make a single, two-part plant. The part below the graft union, including the roots and lower stem, is known as the rootstock. The top part of the plant, above the graft union, which bears leaves and fruit, is known as the scion.
History and Applications
Grafting has been used in woody crops like pears and grapes since at least 500 B.C.E. References to grafting appear in Chinese and Greek texts from the fifth and fourth centuries B.C.E., respectively, and indicate that the practice became prevalent centuries earlier. In the past few decades, vegetable growers also began grafting for various benefits. Cannabis too, can be grafted quite easily, and there are many possible applications.
In the year 2000, growers in Japan and Korea grew 700 million grafted vegetables. Vegetables, and other plants such as fruit trees and ornamentals, can benefit from rootstocks that are resistant to pests and pathogens. Breeders develop rootstocks resistant to diseases like root rot, infestations of root-knot nematodes, or even to attack from insects such as Phylloxera (which are often incorrectly referred to as ‘root aphids’, although they’re closely related). In fact, the grape industry in Europe was devastated by an infestation of Phylloxera, brought over from North America around the year 1860. European vinifera grapes had no native exposure to the pest, and therefore have little resistance to it. To mitigate the problem, growers began using rootstocks from North American grape species such as Vitis lubrusca, with native resistance to Phylloxera. The American rootstocks were a successful solution. Cannabis could also benefit from the breeding and development of rootstocks resistant to pests and diseases. For example, as laws loosen, and cannabis growers move to outdoor field production, root-knot nematodes will become a management challenge. Techniques like soil solarization can reduce pest nematode populations, but eradication may be very difficult to achieve. Resistant or tolerant cannabis rootstocks could therefore be very helpful.
Grafting can be used to combine the favorable root system of one plant, which may otherwise have poor attributes, with the favorable foliage, flowering, or fruiting of another plant, which might normally have a weak root system. Using this strategy, grafting cannabis could increase vigor and yield, due to combining favorable traits, while circumventing unfavorable ones. Tomato growers have reported increased yields, when growing heirloom varieties grafted onto superior rootstocks.
Like cloning, grafting is an asexual propagation technique; therefore, grafting can be used to produce the same level of crop uniformity as cloning. Rootstocks themselves can be clones, or they can be seedlings. Combining a seedling rootstock, with a clone scion, is a technique used in species that don’t have strong rooting as clones, such as some conifer species. Conifer growers germinate seeds, and once the seedlings have large enough stems, they graft scions of the desired clone onto the seedlings. From then on, the seedlings act only as rootstocks. Any shoots that might sprout from a rootstock are considered suckers and are pruned away. Suckers can be deterred by shading the stem with a corrugated wrap (which is black on inside to block light, and white on the outside to reflect it), until the plant develops a shady canopy and woody stem. At that point, suckers will be less likely to sprout.
Citrus trees are sometimes grafted onto clonal rootstocks, and sometimes onto seedling rootstocks. Lemons for example, are often grafted onto clonal rootstocks, and may blow over in strong winds, due to lacking a taproot, and being grown in sandy soil. Clones and clonal rootstocks have a sprawling root system, without a strong, central taproot, and therefore are less stable than seedlings. Grapefruit, in contrast to lemon, is often grafted onto seedling rootstocks, which have very deep taproots. The taproots penetrate into deep clay layers that provide stability, and a consistent supply of moisture. Grafting Cannabis onto seedling rootstocks might create more stable plants, for growth in deep organic soils, or sandy soils, especially where high winds are likely. For example, the windy conditions, and deep organic soil of the Great Plains of North America, could be problematic for plant stability. Not only do seedlings have a deep, strong, central taproot, they also put out what are known as anchor roots (Figure X). Anchor roots grow laterally from the stem near the soil line, and have high tensile strength, for stabilizing plants in high winds.
Another benefit, which cannabis plants could gain from the deeper root system of a seedling rootstock, is increased water uptake, when grown in drought or potentially water-stressed conditions. The less water stress a plant experiences, the more it photosynthesizes and grows. Deep layers of soil stay moist much more consistently than the surface, so access to this moisture via a taproot is beneficial.
How to Graft Cannabis
There are several different techniques that can be used to graft cannabis, such as splice grafting, saddle grafting, approach grafting, and cleft grafting. I’ve found that cleft grafting is simple and very successful (nearly 100% success rate) when grafting cannabis, so that’s the technique I’ll explain in this article. Conveniently, grafting can be done simultaneously along with rooting of a clonal rootstock; combining these two processes can save you time. Alternatively, if you have established seedlings or rooted clones, they can be used as rootstocks, but will need to fit in a humidity dome, or be kept in a larger high-humidity environment, until the graft union is established. Note that the grafting procedure should also take place in a shady, cool, humid location, to prevent water loss from the cuttings.
- To start, you’ll need vegetable grafting clips, which are available online, for various stem sizes. You can also use parafilm grafting tape to wrap the graft union, but I suggest using the clips. The ease and speed of using the clips will decrease labor, and increase the rate of successful grafts, by allowing you to move the grafted plants into a humidity dome sooner, and thereby avoid water loss.
- A tall humidity dome (the kind that are close to 30 cm tall are best).
- A heat mat for propagation, preferably with a temperature controller, set at 25.5 degrees C (78 degrees F).
- Rooted or unrooted clones, or 13-18 cm tall seedlings with 4-5 nodes, to use as rootstocks
- Small rockwool cubes or other rooting medium, if using unrooted clones as rootstocks
- Fresh cuttings of the desired strain, to use as scions. The stem diameter of these scions must be nearly the same diameter as the rootstock stems. Each cutting should include the tip of a shoot, as well as leaves and axillary buds or shoots.
- Sharp new razor blades
- A mild nutrient solution with an EC around 0.4 mS/cm, and a pH around 5.6 (if using rockwool). If using other media, keep the pH closer to 6.0.
- If using an unrooted clone as the rootstock: Cut a shoot from a mother plant, about 15 centimeters long, like you would for a normal clone. The base of the shoot should have a semi-woody stem. Lightly wound the sides of the stem near the base, using scissors in a gentle scraping action. Only wound the section that will be below the surface of the rooting medium. Stick the clone in a nutrient-soaked rockwool cube, as you normally would for rooting. I prefer to avoid using rooting hormones, as it’s unnecessary for cannabis, and could have possible health implications. Ignore this step if you are starting with a rooted clone or seedling.
- Prepare your unrooted clone, rooted clone, or seedling, to be a rootstock, by cutting the stem parallel to the ground (a horizontal cut), thereby removing the top of the plant, and leaving only a stem. Preferably, the remaining stem should be at least 5 centimeters tall, to keep the graft union away from the surface of the soil or other medium. This will help prevent diseases, by keeping the graft union clean. If you’re using a seedling as a rootstock, cut the stem just below the cotyledons.
- Use the razor, to carefully split the stem of the rootstock, down the very center of the surface of the first cut. The split must remain centered as it goes down, or the stem might not be strong enough. Cut down about 1.5 centimeters.
- Prepare the scion, starting with a fresh cutting of the desired strain. Point the base of the cutting towards you, and carefully cut the base into a wedge shape, using two clean draws of the razor. The angle of the wedge should be as narrow as possible, and it should be about 1.5 cm long, as to fit into the cleft rootstock. Cut the very tip of the wedge to make it flat, so that it settles into the rootstock with good contact.
- Gently push the wedge of the scion into the cleft rootstock, then use the grafting clip to secure the connection. The clip should be positioned as to squeeze the cleft stem closed around the wedge of the scion. The two connected stems may not have the exact same diameter; to compensate, push the wedge of the scion to one side of the rootstock so that it aligns evenly with at least one side. This will allow a vascular connection to form between the two plants, in the cambial layer.
- Trim the tips of the leaves of the scion, to reduce transpirational water loss. Don’t remove more than half of each leaflet (finger). Some leaf material is needed for stored carbohydrates and hormones used in rooting, and for photosynthesis once the new plant is established.
- Place the newly grafted plant in the humidity dome, on the heat mat, and keep it in a warm, shady or moderately illuminated area, for about 7 days, or until you detect new growth. During this time, water carefully, to avoid overwatering and ‘drowning’ the scion.
- Begin acclimating the plant to normal growing conditions, by gradually increasing the lighting intensity, and gradually lowering the humidity level. If you started with an unrooted clone as your rootstock, wait at least until you find roots emerging from the bottom of the medium, to begin the acclimation process.
Following these steps, you’ll find that grafting cannabis is an easy process. You might even want to try other techniques, such as approach grafting, once you’re familiar with cleft grafting.
In 1898 Martinus Beijerinck coined the term ‘virus,’ when he described Tobacco Mosaic Virus, the first virus ever discovered. Plant viruses cause symptoms including contorted foliage, necrotic spots, and mosaic yellowing. One novel use for grafting is testing for plant viruses— a practice known as virus indexing. If you suspect that a plant is infected with a virus, but have no other way of testing it to make a determination, you can graft a scion from a healthy plant onto it. If the scion grows out displaying the virus-like symptoms observed in the rootstock, it indicates the presence of a virus. When virus indexing, remove one branch of the plant suspected to have the virus, and graft on a healthy scion. Protect the scion from bright light and dry air, until the graft union is established.
Multi-Strain Plants for Legal Compliance
One final, interesting application of grafting cannabis, is creating plants that produce more than one strain. In locations where the number of plants you can legally grow is restricted, scions of multiple strains can be grafted onto a single plant, to provide variety, without increasing the plant count. Each branch of a plant could have a different strain grafted onto it. A grafted, multi-strain plant could be flowered to finish its life-cycle, and produce a multitude of different buds, or could be used as a mother plant, to produce clones of several different strains. Check with your local authorities to verify legal compliance.
Grafting has many compelling uses, and could play a major role in the future of the cannabis industry. Imparting pest and disease resistance, increasing yields, providing stability in high winds, improving drought tolerance, virus indexing, and growing multiple strains per plant, are all ways that grafting can be applied to cannabis production.
Joe Bender is an experienced grower and the driving force behind Cannabis Crop Solutions LLC