Mycorrhizae inoculation


I am in a condo in downtown Toronto. We have a long row of cedars that was put in about 4 years ago along the top / back of terraced beds of annuals. The cedars are now about 6′ – 8′ tall and look good. Our tree service has been fertilizing and doing a mycorrhizae inoculation every spring. I thought mycorrhizae inoculation was normally a 1 time at planting thing. It either takes or it will never take. Is an annual application standard practice in this situation?

Thank you for any insight you can offer.


Thank you for contacting the Toronto Master Gardeners about mycorrhizae inoculation.

The use of mycorrhizal inoculant if used, is used at the time of planting and based on our research, not annually. There seems to be two views on the issue that I articulate below.

The first point of view is that there are some benefits to the use of mycorrhizae inoculation. While it is not used universally in agriculture, the agriculture business is investigating its benefits as noted in the next paragraph.  Agriculture has different issues such as deep tilling and compaction to the soil that do not apply to residential gardening.

As noted by the Ontario Soil and Crop Improvement Association, mycorrhizae are a type of fungi that are found naturally in soils that form mutually beneficial networks with the roots of plants. As these fungal networks expand, they allow the plant to access minerals, nutrients and water that, through its roots alone, it would not be able to reach. This relationship between plant and fungi stimulates root development and overall plant growth, resulting in healthier and more vigorous plants and therefore greater production and disease resistance.

In addition to plant health, mycorrhizae improve soil structure, increase soil organic matter, and play a positive role in soil aggregation. Soil disturbances, especially frequent deep tilling, break up these fungal networks and reduce their positive effects.

There is also another point of view that the use of mycorrhizal innoculants are not necessary. As noted by Robert Pavlis, an author, Master Gardener and blogger, he thinks the use of mycorrhizae is a waste of money. See an portion of his article:

Facts About Mycorrhizae

  1. Garden soil already contains vast quantities of mycorrhizae, or at least it does if any kind of plants grow in it. Adding more is a waste of money, since they are already in your soil.
  2. Manufactured mycorrhizae consists of 2 or 3 types of fungi. It turns out that there are hundreds if not thousands of different kinds in your soil and some of these are very specific to certain types of plants. How do you know your plants will benefit from the 2 or 3 types you buy?
  3. Manufactured mycorrhizae are heat sensitive. If the container is left in a greenhouse, or a transport truck too long, the heat kills the fungi. There is no way for you to know that the product you buy contains active fungi – they could all be dead.
  4. Manufactured mycorrhizae are of a specific species, almost certainly from an area that is foreign to your soil. It is possible that these foreign species inhibit the growth of your native species. You have no way of knowing but it is never a good idea to import foreign species of any kind into your garden.

Conclusion: There is just no good reason to buy mycorrhizal fungi for your garden.

So it would appear that your assessment is correct. The tree service does not need to provide an inoculation each year.

Hope this helps.