Landscape
And
Nursery
Dialog
Mary Ann Rose
Commercial Landscape
& Nursery Specialist
The Ohio State UniversityApril, 1998
Mycorrhizae: Potential in the Landscape
Keywords: alternative substrates, fertility, fungus, new products, tree health, soil amendments
I n last monthās Buckeye I described mycorrhizae and their basic functions. Briefly, mycorrhizae are an association of certain types of fungi with the roots of plants. Mycorrhizal associations can improve plant nutrition by increasing their absorptive capacity for water and nutrients. In this article Iāll explore the significance and potential of mycorrhizae in the landscape.
Some plants will not survive more than a few months without mycorrhizae, including the genera Abies, Larix, Picea, Pinus, Carpinus, Fagus, and Quercus. Itās an interesting historical note that attempts to establish pine plantations in parts of the world where they were not native were unsuccessful until some soil from the native habitat was transferred to the planting site. The soil of course contained mycorrhizal inoculum. On the other hand, while most plants in their native habitats occur in association with mycorrhizae, many are capable of living without them. Conventional agricultural practices tend to work against mycorrhizal colonization. Soil sterilization or fungicide use eliminates the fungus. Since many types of mycorrhizal fungi have wind-borne spores, usually this is a temporary problem. Fertilization, particularly phosphorus fertilization, raises the levels of nutrients in the host plant root tissue and inhibits mycorrhizal colonization. Essentially fertilizers reduce the host plant needā for mycorrhizae. As a rule, the poorer the soil, the greater the potential for mycorrhizal colonization, and the greater the dependence of host plants on mycorrhizae.
Products containing ecto- and/or endomycorrhizal fungi are currently being marketed to the green industry; apparently they are also being advertised to homeowners. The question is, will these additions really help? The products are relatively expensive, so the question is an important one.
What research shows. There is irrefutable evidence that mycorrhizal inoculation has aided in establishment of certain species of plants in extreme environments. The classic work in this area was done by Dr. Donald Marx and others in acid coal spoils where soil conditions were so bad (pH between 2.0 and 3.0!) that nothing would grow. Reclamation of these sites was made possible through mycorrhizal inoculation. For those tree species with an absolute requirement for mycorrhizae, research also suggests that there is an economic benefit to inoculating fumigated forest nursery soils.
Companies that are producing mycorrhizal products are now suggesting that the urban landscape is the next frontierā for mycorrhizal inoculation. This is not surprising since landscape horticulture is potentially a huge market. Wouldnāt it be great if mycorrhizae could improve transplant survivability and growth in poor landscape soils as was proven for acid coal spoils?
What does nursery/landscape research show? In preparing for this article, I examined articles from about the last 12 years in the Journal of Environmental Horticulture, the premier journal for landscape horticulture, as well as the last several years of the Journal of Arboriculture. I found that most studies examined the potential benefits of mycorrhizae to container-grown plants. There is a certain economy of scale to using mycorrhizae in production (and even in propagation) rather than in the landscape, since much less inoculum is required per plant. Whether or not nurserymen want to bear this expense is another question. Researchers have shown that mycorrhizae can survive, and often improve container plant growth when moderate fertilizer rates are used in container production. Unfortunately, relatively few studies have dealt with the performance of inoculated plants once they reach the landscape, or with introduction of mycorrhizae into the transplanting site. Several researchers that transplanted mycorrhizal container plants into the landscape saw no differences between inoculated and uninoculated plants. Typically research results are mixed: e.g., in one study, growth in the landscape was enhanced in one out of four species where mycorrhizal inoculum had been introduced in the field nursery.
These studies donāt make a clear-cut argument for mycorrhizal inoculation, but they do suggest a need for more unbiased research on the benefits of mycorrhizal inoculum in the landscape. Experiments with mycorrhizae in problem landscape soils would be of most benefit. Unfortunately, some of the studies carried out by companies selling products are not very scientific; for example, these studies may lack controls or replications (repetitions of experimental treatments). Furthermore, many of the commercial mycorrhizae products contain fertilizer. Even if benefits of the product are demonstrated, how can we determine whether the fertilizer or the mycorrhizae produced growth enhancement? Plain old fertilizer is a lot cheaper than fertilizer plus mycorrhizae!
I am interested in experimenting with these products, and certainly endorse further experimentation with them in the landscape. Weāre just not at the stage yet to recommend putting these products into landscape specifications. At Ohio State, Dr. Dan Struve will begin a study this spring with a variety of materials that may enhance transplanting success. Mycorrhizal inoculum, biostimulants, and composts are among the treatments he has planned.
Some points to consider if you want to try mycorrhizal inoculum in the landscape:
- Does the product contain a species of mycorrhizal fungus that will actually colonize the plant you are inoculating? For example, ectomycorrhizal fungi only colonize certain tree genera.
- Has the site been heavily fertilized, particularly with high levels of phosphorus? Mycorrhizal colonization may be poor under these circumstances.
- Does the site have good soil and are landscape plants already thriving in this site? Chances are new transplants may not derive much benefit from mycorrhizae.
- Are other plants of the same genus growing nearby and doing well? In this case, chances are the site is already colonized by native mycorrhizal fungi. Although there is speculation that superā strains of mycorrhizal fungi may provide greater benefits than native fungi, there isnāt much proof of this in the landscape.
- Mycorrhizal fungi are living organisms that will not survive indefinitely in the product. Is the product dated and is shelf-life information provided?
