Over 460 million years ago, plants and mycorrhizal fungi formed a beneficial relationship below the soil surface that nurtured and protected plants while feeding the fungi and other organisms in the root zone of the plants.1 That relationship exists today for over ninety percent of terrestrial species in natural settings, allowing plants to survive harsh conditions of drought, diseases, temperature extremes, poor soil, and competition.2 They are the dominant microbial life form in undisturbed soils, accounting for sixty to eighty percent of the microbial mass of the soil. Without them, for instance, large oaks, which use hundreds of gallons of water per day in the summer heat, would not thrive or grow to mature size on the dry hills of California.
Although the root-fungus association has been known since classical Greek times, when Theophrastus traced the mycelium of a mushroom back to oak tree roots, it has only been in the last ten years that this relationship has been utilized for agriculture, forestry, golf course management, and ornamental horticulture. Commercial production of mycorrhizal fungi has been available to the green industry for a decade, but only recently in small quantities for home gardeners. This article is intended to educate the home gardeners and landscape contractors, who can make use of this essential resource to bring long term health and beauty to their gardens, while weaning their properties of unsustainable practices dependent upon the use of chemical fertilizers, pesticides, and herbicides.
What are Mycorrhizae, and How Do They Work?
The word “mycorrhizae” comes from the Greek words for fungus and root, and refers to the symbiotic relationship that exists between plant roots and certain fungi. In natural settings, these mycorrhizal fungi are present in the soil in association with plant roots. The fungi colonize by attaching to the surface of the root (ectomycorrhizal) or to the inside of the root cells (endomycorrhizal). Then they send their filaments (called mycelium) into the surrounding soil, effectively extending the plant’s roots and root absorbing capacity ten to 1000 times—far beyond what the plant can do alone.3 Several miles of these ultra-fine filaments can be present in less than a thimbleful of soil. Mycorrhizae supply the water and nutrients needed by the plant for establishment and survival, and, in return, receive from the plant roots sugars and other compounds needed by the fungus. Mycorrhizae are much smaller than roots, so they can easily penetrate into smaller spaces between soil particles, where they release powerful enzymes that dissolve tightly bound minerals like phosphorus, sulfur, iron, and all the major and minor nutrients known to be used by plants. The nutrients are retained by the mycorrhizae and become available for use by the plants.
Mycorrhizae provide many other benefits to plants. The fungal filaments take up and store water, decreasing drought stress during dry periods. The fungal filaments bind soil particles into larger aggregates with organic glues such as humic compounds; the resulting soil structure allows air and water movement into the soil, encouraging root growth and distribution. Mycorrhizal fungi attack disease organisms that enter the root zone in several ways. Some produce antibiotics that immobilize or kill disease organisms. Others trap root-feeding nematodes and kill them. Some protect roots against attacking insects with a thick “net” of their mycelium. Some protect the plants from disease-causing fungi, such as Fusarium, Phytophthora, and Rhizoctonia. These activities result in improved survival, enhanced top and root growth, and increased production of flowers and fruits, plus protection from disease and an improved soil structure. The plants can better compete against invasion by weed plants and can better survive drought conditions.
When Do We Need to Apply or Replace Mycorrhizae in Our Soil?
Most of our man-made environments were built using practices that destroy the soil conditions supportive of beneficial soil organisms. In many cases of landscape projects in disturbed urban environments, it has been documented that the new plants had not formed mycorrhizal relationships many years after planting, and were surviving only through “intensive care”—fertilizers, pesticides, and lots of water. Where there has been compaction, erosion, grading, topsoil removal, overgrazing, tillage, fertilization, paving, pollution, and the use of soilless planting mixes in nurseries, mycorrhizal fungi have been eliminated completely. Many mycorrhizal fungi do not disperse their spores in the wind, but must grow from root to root, or be dispersed by animals, so a close proximity to healthy and undisturbed natural sites is necessary to repopulate a disturbed soil. In lieu of that, mycorrhizae need to be provided as soil additives at the time of planting.
So, Why Not Simply Rely on Fertilizers?
Fertilizers frequently stimulate top growth at the expense of root growth, producing lush plants that are more susceptible to drought and other stresses. If the soil is a confining hard clay, each planting hole may function like a container without drainage holes; plants whose roots are expanding to catch up with the top growth may become diseased or completely root-bound in a few years. Obviously, poor planting practices contribute to the problem, but mycorrhizae can improve the soil structure, stimulate root growth, and feed the plant. Fertilizers cannot improve soil structure, prevent root disease, or absorb and store water. Fertilizers also result in water pollution, excess salt in the soil, and worsened soil structure. Without mycorrhizae, plants will become dependent on fertilizers for survival and will not tolerate environmental stresses well.
Thousands of studies have proven the superior growth and health of plants inoculated with mycorrhizae. One study, in particular, showed that mycorrhizae-inoculated seedlings of a bent grass (Agrostis), grown without time-release or soluble fertilizers in a harsh, denuded, high-elevation site, not only survived but had higher leaf concentrations of major nutrients than did non-inoculated seedlings grown with slow release fertilizers. In addition, one hundred percent of the inoculated seedlings survived, compared to only 26.4 percent of the seedlings grown only with slow-release fertilizers.4
If this information has not yet convinced you that mycorrhizae are a good thing, consider the shock suffered when a plant from the nursery-watered daily and fertilized weekly in a protected environment—is planted out into the elements with only a tiny root system and expected to fend for itself without the daily infusion of water or weekly feeding. It is suddenly like a toddler without its mother; its roots are unable to reach the “refrigerator of food” that may exist just beyond the planting hole. The situation is worse if the plant is placed in a cleared construction site lacking topsoil or native vegetation nearby. Mycorrhizal fungi and other beneficial soil organisms are the only solutions that can nurture that young plant.
Where Does One Find Mycorrhizae, and How Does One Choose the Right Type?
According to Michael Amaranthus of Mycorrhizal Applications in Grants Pass, Oregon, “inoculum containing a variety of mycorrhizal fungi species often gives the best response. . . . Not all mycorrhizal fungi have the same capacities and tolerances to the wide variety of soil temperatures, soil types, climate, and biotic conditions characterizing man-made environments.” Therefore, “a diversity of fungal species may increase plant survival in a range of chemical and physical conditions.” Conifers, oaks, and other hardwoods generally have relationships dominated by ectomycorrhizal fungi, whereas most other horticultural and agricultural plants form mostly endomycorrhizal relationships. The average home garden needs a combination of both mycorrhizal fungi groups.
Mycorrhizal fungi are sold as spores, the “seeds” of fungi. Propagules, or root fragments, last only a couple of weeks away from their hosts, whereas spores are viable for at least eighteen months after produced. The spores come in different forms. Granular forms can be mixed into potting soil, mixed with water and drenched into rocky porous soil, applied as a root dip gel on bare root plants, or injected into the root zone of existing plants using a soil probe. The goal is to get the spores in physical contact with the roots that they will colonize. Application is easy and inexpensive, and requires no special equipment. For small plants, cost is only a few pennies per plant.
Mycorrhizal products often contain nutrients and other ingredients to enhance their growth. Organic matter, such as kelp meal, provides nutrients, stimulates microbial activity, and improves soil structure for root growth. Water-absorbing gels help spores adhere to feeder roots, and help retain soil moisture for mycorrhizal fungi growth. Stress vitamins, such as B1, may increase nutrient uptake and root growth.
Learning about the role of mycorrhizae, and the conditions that inhibit their presence in the soil, is the first step toward sturdier plants and healthier landscapes. The next step is to add the fungi to the root zone when transplanting nursery grown plants, and when restoring soil that was recently stripped of millions of years worth of nutrient and microorganism development. If you feed the soil with composted vegetation, and protect the soil organisms from major disturbances and toxins, they will, in turn, feed and protect your plants better than any method we have ever developed—and with so little effort on your part!
A Resource Guide for Mycorrhizae
For Further Reading
Hayes, Virginia et al. Lessons from Lotusland: Sustainability in the Garden. Pacific Horticulture 64:2 (April 2003), 22-28.
Ingham, Elaine and Hendrikus Schraven. Break-through in Soil Technology: Microbes, Minerals, and Methods. Course manual available from Soil Dynamics, PO Box 1289, Issaquah, WA 98027, 425/392-1200, www.soildynamics.com.
Soil & Water Conservation Society with USDA Natural Resources Conservation Service. Soil Biology Primer. 2000. Available from SWCS, 7515 Northeast Ankeny Road, Ankeny, IA 50021, 800/THE-SOIL x24, www.swcs.org.
Producers/Suppliers of Mycorrhizal Inoculants
These are mostly growers or wholesalers who sell mycorrhizal inoculants to agriculture or landscape industries in large quantities for specific projects, such as wild land restoration, agriculture, or forestry. Check with each for local distributors. For home gardeners, mail order catalogues may offer inoculants in small sizes and smaller retail quantities. For example: the smallest size package of “Tree Saver Transplant” mycor-rhizae from Plant Health Care is three ounces, and minimum order is a box of fifty packages. Home gardeners who are planting only a few plants can order by mail a one-pound bag, or a four-ounce bag of mixed mycorrhizae types, with nutrients, from Territorial Seed Company.
Bio-Organics, 601 Daily Drive #118, Camarillo, CA 93010, 800/604-0444. www.bio-organics.com
Bioscientific, Inc, 4405 South Litchfield Road, Avondale, AZ 85232, 602/932-4588.
BioTerra Technologies, Inc, 9491 West Pioneer Avenue, Las Vegas, NV 89117, 702/256-6404. www.bioterra.com
First Fruits, LLC, RD 1, Box 156, Triadelphia, WV 26059, 888/489-0162.
Grow-Power, Inc, 15065 Telephone Avenue, Chino, CA 91710-9614, 909/393-3744. www.gro-life.com
Mikko-Tek Labs, PO Box 2120, Timmons, Ontario, Canada P4N 7X8, 705/268-3536.
Mycorrhizal Applications, Inc, PO Box 1181, Grants Pass, OR 97528, 541/476-3985. www.mycorrhizae.com
Plant Health Care, 440 William Pitt Way, Pittsburg, PA 15238, 800/421-9051. www.planthealthcare.com
Plant Revolution, Premier Enterprises LTD, 326 Main Street, Red Hill, PA 18076, 800/424-2554. www.plantrevolution.com
Reforestation Technologies International, 875 Airport Road, Unit R, Monterey, CA 93940, 800/784-4769. www.reforest.com
Roots, Inc, 3120 Weatherford Road, Independence, MO 64055, 800/342-6173. www.rootsinc.com
Tree of Life Nursery, PO Box 736, San Juan Capistrano, CA 92693, 714/728-0685. www.prtcl.com/tol/products.htm
Tree Pro, 3180 West 250 North, West Lafayette, IN 47906. www.nlci.com/treepro/
Target Specialty Products, 1155 Maybury Road, San Jose, Ca 95133-1029, 800/767-0719. (Wholesale to industry only)
Totally Organic, A Garden Product Corporation, 1355 North Tenth Street, San Jose, CA 95112. 408/971-6749. www.totallyorganic.org (Wholesale to industry only)
Down to Earth Distributors, Inc, PO Box 1419, Eugene, OR 97440, 800/234-5932. www.downtoearthdistributors.com
(Garden and Housewares Wholesale Catalog)
Mail Order Sources for Home Gardeners
Territorial Seed Company, PO Box 158, Cottage Grove, OR 97424-0061, 541/942-9547. www.territorialseed.com
Fungi Perfecti LLC, PO Box 7634, Olympia, WA 98507, 360/426-9292. www.fungi.com/mycogrow/index.html
GardenRoots LLC, 291 Whitney Avenue, New Haven, CT 06511, 203/777-4753. www.gardenroots.com