Do Mycorrhizal Fungi Actually Help Your Plants? (What the Science Says)

Watch the episode · 4:42

Under every healthy plant there is a second root system, and it isn't a plant. It is a fungus. Mycorrhizal fungi partner with about seven in ten land plants, cannabis included, and growers buy them by the bag. But a big review in 2024 found that most commercial products barely colonise roots at all. So do they actually help?

Source · Brundrett & Tedersoo, New Phytol. (2018)

Roots hit a wall ▶ 0:31

Roots are surprisingly bad at finding phosphorus. Phosphorus is vital for roots, flowers and energy, but phosphate barely moves through soil, so a root can only take up what is right next to it. It soon empties a thin shell around itself, just a millimetre or two wide: the depletion zone. To find more, it has to keep growing.

Source · Smith & Read, Mycorrhizal Symbiosis (2008)

The handshake ▶ 0:57

So the root calls for help. It leaks chemicals called strigolactones into the soil. They wake a resting fungal spore and make its threads, the hyphae, branch towards the root. The fungus answers with signals of its own, called Myc factors, which tell the root to let it in.

Source · Akiyama et al., Nature (2005)

Then the fungus moves in, right inside the root's own cells. The hypha pushes through the root's outer layer and spreads between the cells. Inside some of them it builds an arbuscule: a tiny branching tree wrapped in the cell's own membrane. This is the trading post. Each one lasts only a few days; then the cell clears it away and a new one forms.

Source · Kobae & Hata, Plant Cell Physiol. (2010)

The trade ▶ 1:39

Meanwhile, outside the root, the fungus spreads. Its hyphae are just a few thousandths of a millimetre wide, thin enough to slip into pores no root can enter, and they reach centimetres past the depletion zone. A single gram of healthy soil can hold metres of them. They take up phosphate, along with nitrogen, zinc and copper, and carry it all back to the root, where the fungus hands it over at the arbuscules.

But nothing is free. The plant pays in carbon: sugars made in its leaves, and fats. These fungi cannot even make their own fatty acids, so the plant makes them and passes them across.

Source · Luginbuehl et al., Science (2017)

A big bill, but it buys a much bigger root system. The plant now draws on far more soil than its roots could ever reach alone, and the fungus, well fed, starts making new spores for the next generation.

Do products work? ▶ 2:35

Sometimes, a lot. In a 2022 hemp trial, plants given the best fungus grew nearly twice as tall as unfed plants, with more than three times the leaf mass and roots. But a second fungus did no better than ordinary fertiliser. The species matters.

And now the catch. In 2024, researchers reviewed 302 trials of commercial products. Only about one in nine showed both real root colonisation and better growth. Lab-grown fungi managed it in 63%.

Over 80% of products failed to colonise the roots in any meaningful way. The fungi were often dead, or there was just a single species. And when plants did grow better, it was often thanks to other ingredients in the mix, not the fungus.

Make it work ▶ 3:27

If you do try them, a few simple rules make all the difference. Put the spores where the roots are, in the planting hole or on the roots at transplant; sprinkled on top, they may never meet one. Go easy on phosphorus. Give it weeks, because colonisation is slow. And keep the fungus alive, so skip fungicide drenches.

Why phosphorus? Because when phosphate is plentiful, the plant itself shuts the door. In one study, petunias fed lots of phosphate switched down the genes the partnership needs and let in far less fungus. Heavy bloom boosters can do the same.

Source · Breuillin et al., Plant J. (2010)

Roots empty a thin zone around themselves; mycorrhizal fungi reach far beyond it, trading minerals for sugar and fat. Most bagged products failed in trials, so if you use one, put the spores on the roots and go easy on phosphorus.