Rule #1: The plants must survive
Rule #2: Rule #1 always takes precedence.
Mycorrhizal fungi are obligate symbionts: without a living host plant, the culture stops. Conversely, as long as the plant survives, the fungus established in symbiosis appears highly resilient. Under all circumstances, the host plant’s survival is essential.
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Lighting
Lighting drives photosynthesis and ultimately the fungus’s nutrition. The plants are lit 16 hours a day using General Electric LED lamps, model « GE Grow Light 40W Balanced Spectrum », and Sunblaster Horticultural Lighting fixtures. The lamps are placed about 15 cm above the containers, which ensures high light intensity without excessive heat output.
A simple principle of physics applies here: the light intensity received is inversely proportional to the square of the distance. A plant placed 30 cm from its source receives only 25% of the intensity received at 15 cm. This is a determining factor. For this reason, the Petri dishes are rotated at every watering: those at the bottom move up and those at the top move down, so that all of them receive equivalent lighting.
Stacking
The Petri dishes are stacked and placed under a tin can (796 ml) with an opening cut into it, to keep the roots and the polymer in the dark. Any other opaque cover with an opening for the foliage will also work. Black corrugated plastic (Coroplast) discs are added on top of the Petri dishes to ensure total darkness. Each box sits on a Coroplast base to make handling easier, and labels are used to record relevant information.
Irrigation
The goal is to reproduce natural conditions: roots need water, minerals, and air (oxygen, CO2, etc.). The substrate must therefore alternate between a « soaked » state and a « lightly moist » state, particularly on the vermiculite side.
Watering is done exclusively with the nutrient solution. The practical rule is to weigh the Petri dishes after preparation and add more solution once they have lost about 5 g (= 5 ml). Watering can also be managed visually. As a routine, for mature plants, watering takes place roughly every four days.
How to read the slider image: On the left, immediately after watering, the vermiculite is soaked with liquid — the medium looks like soil after rain. On the right, all the liquid has been absorbed; the vermiculite is still moist, but no liquid is visible on the floor of the Petri dish. This is the time to water, before the hyphae start to dry out.
Watering frequency varies depending on light intensity, ventilation, and the plant species. There is therefore no absolute rule.
Starting a culture with a seedling
At initial setup, about 8 ml of nutrient solution is added to the vermiculite to soak it thoroughly. With a young plant that only has two leaflets, another watering is generally not needed for 8 to 10 days. At the start of the culture, while the symbiosis is becoming established, moisture is critical: the chemical signals exchanged between the plant and the fungus are carried by water. Any drying out should be avoided. When in doubt, a slight excess of liquid is preferable to a shortage.
Growing mature plants
The ideal time to water is when almost all the free solution has been absorbed: the roots and hyphae have then had access to oxygen, and the Petri dish can be turned over for observation under the microscope.
After the “ideal” moment, the polymer grains will slowly start to dehydrate and shrink. If dehydration continues for too long, the shrinking grains will cause the hyphae bridging several grains to break. Numerous empirical observations show that fungi tend to “permanently close off” certain hyphae exposed to prolonged dryness.
In practice, watering is done every four days, on both sides of the Petri dish (vermiculite and polymer), adding enough solution to return to the initial weight. It is also advisable to keep the foliage to a maximum of three healthy leaves (Plantago lanceolata) to limit evapotranspiration and space out waterings.
Note, however, that once the plant is well established and the symbiosis is well developed, the pair becomes very resilient, as described in the “Rough Handling” section.
Growing older plants, over 3 months.
After several months, the polymer rehydrates less well and the roots become very abundant. The weight of the Petri dish can even exceed its initial weight because of the mass of roots — at this stage, weight-based tracking is dropped in favor of watering by eye, every four days, on both sides. Petri dishes can be maintained this way for more than 12 months.
It is possible to replace the old polymer with fresh polymer: most species recolonize the new substrate quickly, each at its own pace. Old grains containing many spores can often be used as inoculum for new cultures.
Vacations and resistance to rough handling
Periods of “voluntary neglect” for vacation reasons have made it possible to test the robustness of the cultures. Here are the results:
14 days without maintenance: The plants were trimmed down to three leaves and thoroughly watered before departure. Kept in a closed room under 16 h/day lighting, all the plants and their fungi survived. Upon return, new hyphae were quickly produced from the roots.
30 days without maintenance: In December 2025–January 2026, 25 well-colonized Petri dishes were placed in sealed ziploc bags in the refrigerator at 4 °C in total darkness. Thirty days later, all 25 plants still had turgid leaves. After replacing part of the old polymer and returning them to light, Rhizophagus irregularis, Funneliformis geosporus, and Funneliformis mosseae were producing new hyphae within 10 days.
