Following is the crib-notes recipe for producing beneficial AM fungus inoculum on-farm. For more details on the when, why and how, start here.


The following list will produce 16 “Grow Bags” of inoculum, enough to make 200 or 400 ft3 of inoculated greenhouse potting media depending on the dilution ratio (1:9 or 1:19) of inoculum:potting media used in the final step.

Bahiagrass seed

Conical plastic pots  (we use RLC-4 Pine Cells from Stuewe and Sons, Corvallis, OR 97333)

Coarse sand such as swimming pool filter sand (240 in3 for 80 seedlings)

Ground cover fabric (16 Grow Bags fit on a 1.2 m x 3.6 m or 4 ft x 12 ft section)

16-7 gallon “Grow Bags” (one source is Worm’s Way, Bloomington, IN 47404)

4-4 ft3 bags of vermiculite

4 ft3 of compost


In order to maximize mycorrhizal proliferation and colonization of the host plant, the inoculum bags should be setup outside as soon as possible after the last frost. Some work is necessary before this date. The finished inoculum will be ready for use the following spring.

4 months before the predicted last frost

Germinate bahiagrass seeds (or other host seeds) in vermiculite or seed starter.

Order any needed materials.

3 months before the predicted last frost

Transplant bahiagrass seedlings into conical plastic pots filled with 1:3 soil:sand mixture (volume basis). In order to avoid introducing pathogens, we suggest using sterilized field soil. Another option is to use soil from a natural area of the farm or from a field that has not been used within the past 2 years to grow the crop that will be inoculated.

As soon as possible after the last frost

Set up the inoculum production area by covering an area with the ground cover fabric. This will provide a clean, open area that makes maintenance easy. It will prevent weeds from growing around the bags and contaminating the inoculum with weed seed.

Set up the grow bags:

Mix compost and vermiculite in chosen dilution. A basic recommendation for yard clippings compost from municipal facilities is a 1:4 compost:vermiculite ratio (volume basis).

Fill bags ¾ full with mixture. Roll the lip of the bag down to just above the level of the mix.

Add 100 cm3of field soil as the “inoculum starter” and mix well.

Pool 4-5 soil samples taken from the surface to 10cm (4 in) deep. Sieve out roots and rocks.

To avoid introducing pathogens and to obtain a diverse sample of AM fungi, take samples from a natural area of the farm or from a field that has not been used within the past 2 years to grow the crop that will be inoculated.

Transplant 5 bahiagrass or host plant seedlings into each bag.

During the growing season

Weed and water the bags as needed. The mycorrhizae will proliferate as the plants grow.

Frost will kill the bahiagrass and the mycorrhizae will overwinter naturally outdoors in the bags.

The following spring

Harvest the inoculum:

To keep the inoculum clean, cut off the dead bahiagrass leaves and discard.

Shake the compost and vermiculite mix from the root ball into a bin. This mix will contain the mycorrhizal spores and hyphae.

Cut the roots into short segments (less than 1cm or ½ inch) and mix into bin. The roots contain the mycorrhizal vesicles.

Mix the inoculum into your potting medium:

Use a 1:9 inoculum:media mix (volume basis) for flats with cells of 50 cm3 or smaller. For larger cells a 1:19 mixture should be sufficient.

Amend your greenhouse fertilization regime to avoid P-sufficient plants that will resist colonization:

Conventional farmers: Try to achieve a P addition of 3 ppm or less for no more than three fertilizer applications per week. Apply P-free solutions at other times if necessary.

Organic farmers: If your potting medium requires additional fertilization, use a low P source. If your potting medium contains all the nutrients needed during the greenhouse culture phase, no modifications are recommended at this time.