Mycorrhiza in Cannabis – Root Fungi and Inoculation

Mycorrhiza is a symbiosis between roots and soil fungi that most land plants form. Cannabis forms arbuscular mycorrhiza, named after the arbuscules, the tree-like branched structures the fungus forms inside root cells. Shops sell mycorrhizal inoculants with fungal spores that promise higher yields. Whether inoculation works depends on soil, feeding and cultivar. In established garden soil the fungi are usually present already, so an inoculant mainly pays off in sterile soil.

What does the fungus trade with the root?

The fungus supplies the plant mainly with phosphorus, plus nitrogen and zinc, and receives sugars from photosynthesis in return. The fungi live only with a living host plant. Brassicas and some other plant families form no arbuscular mycorrhiza. The fungus can supply a large share of the phosphorus even when the plant does not visibly grow better.

  • Outdoors, do not grow brassicas before cannabis if you want the soil to build up its fungal population.

When does mycorrhiza help, and when not?

Mycorrhiza helps most in phosphorus-poor soil, and its benefit falls as soil phosphorus rises. The growth response ranges from strongly positive to negative depending on conditions.

Is mycorrhizal inoculation worth it for cannabis?

In cannabis, inoculation paid off strongly in poor and sterile soil, while in coco with full feeding it helped only some cultivars. In sterile, phosphorus-poor soil without fertiliser, THC rose by 60 % and CBD by 33 % compared with uninoculated hemp. In coco with weekly full feeding, three of five cultivars produced a higher dry flower yield with a commercial inoculant. Cannabinoid levels rose or fell depending on the cultivar. A gain in every plant, as advertising promises, does not show in cannabis.

When and how does the inoculant reach the roots?

The inoculant belongs directly on the roots at the first potting-on. Watered in, the spores hardly get there, because they settle quickly in the water and stay on the surface. Inoculants for watering in are mostly powders, which can also be applied dry to the roots instead of in water.

  • When potting on, sprinkle the powder dry around the root ball before backfilling the pot.
  • When sowing straight into the final pot, mix the powder into the substrate; this takes more of it.

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