Mycorrhizae (mycorrhizal fungi) are a group of soil fungi that live in symbiosis with plants through their roots: the fungus feeds the plant water and nutrients, and the plant pays it back in sugars. There's no single fungus called "mycorrhiza"; the term covers the whole crew of fungi that partner up with a plant's roots, and they need direct contact with those roots to do it. Add them to your cannabis from the start and you can see the difference in how the plant grows.

Living in symbiosis means it's a two-way street: the fungi do something for the plant, and the plant does something for the fungi. Mycorrhizal fungi live alongside cannabis roots and supply nutrients like phosphorus and nitrogen, plus water. In return, the plant hands over carbohydrates and vitamins it makes through photosynthesis.

This partnership goes way back in plant evolution. Roughly 400 million years ago, when plants first moved from the ocean onto land, they had no root systems. They relied on fungi to pull in the nutrients and water they needed to survive. In a sense, those fungi were the plants' roots before plants ever grew roots of their own.

The relationship stuck around long after plants developed root systems. Some plants can't grow properly without mycorrhizal fungi at all; that's called obligate mycotrophy. Cannabis isn't one of them: it can grow without mycorrhizae (it's a facultative mycotroph), but partnering with these fungi helps the plant get much closer to its full genetic potential.

Types of mycorrhizae in cannabis

Growers usually talk about two main types of mycorrhizae: arbuscular mycorrhizae (AM) and ectomycorrhizae (ECM). Both form symbiotic relationships with plant roots, meaning both sides benefit, but only one of them really matters for weed.

1. Arbuscular mycorrhizae (AM): The most common type across land plants, and the one cannabis actually forms. Arbuscular fungi grow structures called arbuscules inside the plant's root cells, which lets them pass along nutrients like phosphorus and other minerals in exchange for organic compounds the plant produces. AM fungi are especially important for phosphorus uptake, an essential nutrient for healthy cannabis growth.

2. Ectomycorrhizae (ECM): A different type, mostly associated with trees like pines, oaks and birches; cannabis doesn't typically form them, which is why you'll see them listed in some blends but not doing much for your plants. Ectomycorrhizal fungi build a web of hyphae around the roots without getting inside the root cells. Instead, they form a dense sheath around the roots that improves nutrient uptake, especially nitrogen. In the plants that host them, they can also help with disease resistance and coping with stress.

Bottom line: when you buy a mycorrhizal inoculant for cannabis, look for arbuscular species (Glomus and Rhizophagus are the usual suspects). They can have a real positive impact on growth and plant health by boosting nutrient uptake and resistance to disease and environmental stress. Keep in mind that how present and active mycorrhizae are in your soil depends on your specific growing conditions and medium, so it pays to understand what these fungi need in the context of your grow.

 

How does mycorrhizal symbiosis work?

Mycorrhizal symbiosis is a mutualistic relationship between plant roots and certain mycorrhiza-forming fungi, a partnership where both organisms come out ahead. The plant feeds the fungi sugars and other organic compounds made through photosynthesis, while the fungi supply the plant with essential nutrients like phosphorus, nitrogen, zinc and other minerals that are hard to pull straight out of the soil.

The partnership kicks off when the fungus's hyphae (thin, thread-like filaments) grow into the plant's roots, forming mycorrhizal structures inside the root cells or around them. Those hyphae reach far beyond the roots themselves, so the plant effectively gets a much bigger root zone. 

So, to sum it up: the plant gets essential nutrients from the fungus, and the fungus gets organic compounds made by the plant. This symbiosis is a big part of how nutrients cycle through living soil, and it can play a significant role in how well plants feed and grow, cannabis included.

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