By Jeff Floyd
Certified horticulturist and arborist
Not all things that attack plants are bad for them. Our soils are filled with germs, viruses and bugs of all sorts. One of the most important soil organisms is a type of fungi called mycorrhizae (my-core-rie-zee). Trees and shrubs tend to survive the rigors of our tough growing conditions better when their roots form symbiotic relationships with these fungi. But is there any value in adding mycorrhizae to the soil when a new tree or shrub is planted?
Mycorrhizae are found in all soils that have plants growing in them. They grow over the surface of roots and increase their surface area as much as fifty percent. The fungi benefit from nutrients that naturally leak from fine roots. In return, they help roots take up minerals, especially phosphorus.
The association can be somewhat specific. For example, the roots of a Burford holly may support one type of mycorrhizae while those of a live oak support a different type. If the live oak fungi are placed around the roots of the holly, neither the plant nor the introduced mycorrhizae is likely to thrive.
Whenever we plant a new tree or shrub, we tend to make the planting hole as rich as possible. People often add fertilizers, root stimulators or mycorrhizae before the plant goes in. Indeed, mycorrhizae are packaged for sale. These additions are generally not helpful and may be harmful if done poorly. Fertilizers are salts. They may burn tender young roots, setting back the establishment time of the new plant.
The substances in root stimulators are often already in soil. Likewise, mycorrhizae are abundant. The newly planted tree or shrub roots will certainly be colonized by existing soil fungi. Adding mycorrhizae to the soil will not likely increase the benefit.
Instead of adding amendments, backfill the hole with original soil and save your money to buy another plant or two. There are rare circumstances that require amendments. Therefore, it’s a good idea to consult a certified arborist when planting. Arborists will recognize any special needs that the soil cannot provide.
