Short answer, in plain words. Some bottled microbes really do help, mostly by guarding the roots against rot, not by feeding the plant. The best-supported are a few fungi and bacteria (Trichoderma, Gliocladium, Bacillus, Streptomyces) that crowd out or attack the bugs behind Pythium and Fusarium root rot [1][2][3][4]. Mycorrhizae help mainly in poor soil and often don't take at all [10]. "Feed your plant" microbe blends have the weakest evidence in a well-fed room [8]. In the founder's words: use them "within reason. Some really do help keep root rot and whatnot away, but not in the way many think or are told, and the good ones may surprise you." (founder's field experience) [11]
How do the good ones actually work?
By protecting roots, four ways. Established (plant pathology).
- Crowding out (competitive exclusion): they colonise the root surface first, leaving less room and food for pathogens [1][2].
- Chemical weapons (antibiosis): Bacillus species make antifungal lipopeptides and other compounds [2].
- Attacking the pathogen (mycoparasitism): Trichoderma coils around other fungi and digests them [1].
- Priming the plant's own defences (induced systemic resistance) [1][2].
That's why the good ones "surprise" people: a fungus in a bottle protecting roots from other fungi, or a soil bacterium (Streptomyces) that makes antibiotics, isn't what most labels sell.
The label
Bigger yields, more nutrients
- −'Feeds your plant' or 'boosts nutrient uptake'
- −Weakest evidence in a fully fed room
- −Results vary by product and strain
The evidence
Root protection
- +Reduced Pythium and Fusarium root rot in greenhouse trials
- +Some products registered for cannabis in Canada
- +Works best as prevention
Bodyguards, not chefs.
The tier list
Ranked by the strength of evidence for what growers actually need: healthy roots. Supported; tiers are a GenetixBay summary of the sources.
| Tier | What | Evidence | Best use |
|---|---|---|---|
| A | Trichoderma harzianum | Suppresses Pythium and Fusarium by competition, mycoparasitism and induced resistance [1]; products listed for cannabis in Alberta [6] | Preventive root protection in soil, coco, rockwool |
| A | Gliocladium (Clonostachys) catenulatum | Reduced Pythium and Fusarium root rot severity in greenhouse rockwool [3]; listed for cannabis in Alberta [6] | Seedlings, clones, rockwool and coco |
| A | Bacillus subtilis / amyloliquefaciens | Antibiotic lipopeptides and induced resistance against many plant pathogens [2] | Root zone and reservoir protection |
| A– | Streptomyces griseoviridis | Reduced Pythium and Fusarium root rot severity in greenhouse rockwool [3] | Root zone protection |
| B | Beneficial-microbe reservoir blends | House-preferred approach for recirculating hydro; evidence is product-specific (founder's field experience) [11] | RDWC and DWC, instead of a half-hearted sterile reservoir |
| C | Mycorrhizae (arbuscular) | Raised root length 34% and P content 24% in a cannabis seedling trial [9]; most of 28 commercial products failed to colonise roots [10] | Low-P soil and living soil; little point in fully fed hydro or coco |
| D | "Nutrient-booster" microbes (N-fixers, P-solubilisers) | Promising in research, but cannabis data are limited and benefits shrink when the plant is fully fed [8] | Low-input soil experiments |
| D | Compost tea | Inconsistent disease control, no cannabis trials, food-safety risks with molasses brews (see does compost tea work?) | Skip it |
For cannabis specifically, Punja's lab has tested biological control agents against Fusarium damping-off and Pythium root and crown rot on cannabis plants [5]being reviewed, after documenting how these pathogens attack hydroponic cannabis [7].
- ATrichoderma, Gliocladium, BacillusRoot-rot protection
- BBeneficial reservoir blendsHouse-preferred in hydro
- CMycorrhizaeLow-P soil only
- D'Nutrient boosters', compost teaWeakest evidence
Source: Tier table above [1][2][3][6][8][9][10]; house preference [11].
Rules for using them well
- Prevention, not cure. Start at planting or from the first fill; they don't rescue an established infection [1][3].
- Don't mix with sterilants. Hydrogen peroxide and chlorine kill the good with the bad. House position: a beneficial-microbe reservoir is preferred, but a sterile reservoir is valid if you commit fully (founder's field experience) [11].
- Fix the basics first. Root rot loves warm, low-oxygen water: keep reservoirs cool and well aerated (see sanitation and prevention).
- Use registered products where your jurisdiction requires it for cannabis [6].
- Don't feed the reservoir sugar (see molasses and sugars).
Common mistakes
- Buying microbes to "feed" a plant that's already fully fed.
- Adding microbes and hydrogen peroxide to the same reservoir.
- Expecting a bottle to cure established root rot.
- Paying extra for mycorrhizae in coco or hydro.
A genus of oomycete water moulds, such as Pythium myriotylum and P. aphanidermatum, that cause brown, slimy root rot, especially in warm hydroponic systems.A soil-borne fungal disease, chiefly Fusarium oxysporum, that blocks water-conducting tissue, causing one-sided yellowing, wilting and brown streaks inside the stem.Mycorrhizae are symbioses in which fungi colonise plant roots and extend hyphae into the soil, trading water and nutrients such as phosphorus for the plant's sugars.Decay of roots by pathogens such as Pythium, Fusarium, Rhizoctonia or Phytophthora, usually triggered by low oxygen, heat and waterlogging around the roots.Pest and disease prevention by changing how the crop is grown, such as humidity, airflow, spacing, watering and cultivar choice, so pests and pathogens struggle to establish.Coco coir is a growing medium made from processed coconut husk, prized for holding water and air at once; it binds calcium and magnesium unless buffered.Rockwool is an inert growing medium spun from melted basalt rock into fibres, sold as plugs, cubes and slabs, that holds water and air in fixed proportions.The root zone is the volume of medium or solution where a plant's roots live, including its water, air, temperature, pH, EC and microbes.A beneficial-microbe reservoir deliberately inoculates hydroponic solution with helpful bacteria and fungi so they occupy the root zone before pathogens can; GenetixBay prefers this approach.A recirculating (closed) system returns drained nutrient solution to the reservoir to be reused, saving water and nutrients but needing more monitoring and sanitation.DWC (deep water culture) suspends a plant's roots in an aerated nutrient solution, typically in a bucket with a net pot lid and an air stone.A sterile reservoir keeps hydroponic solution as free of microbes as possible with oxidising sanitisers or UV, a valid strategy that only works if applied consistently.Compost tea is a liquid made by steeping compost or worm castings in water, often aerated with food sources, to extract and multiply microbes for the root zone.Molasses is a sugar-rich by-product of sugar refining added to soil feeds; its sugars feed soil microbes, not the plant, and do not sweeten buds.Using living natural enemies, such as predators, parasitoids, nematodes and microbes, to suppress pests and diseases instead of, or before, chemical sprays.Hydrogen peroxide (H₂O₂) is an oxidising sanitiser used to clean hydroponic equipment and reservoirs; it breaks down quickly into water and oxygen and kills beneficial microbes too.
