Mycorrhizas: The Underground Internet

Most people are familiar with fungi that cause itchy toes and moldy bread. What if I told you that there are a multitude of fungi in an “Underground Internet” that wraps around the roots of almost every plant in the your landscape? Not only that, but also these fungi thrives by sucking energy from the plants to sustain themselves? Yes, these fungi are lurking in the soil of your landscape right now! Is this some Friday the 13th scary story intended to raise the hair on the back of your neck? No! These fungi are real and they are everywhere! They are mycorrhizas (pronounced my-CORE-rye-zuhs). This not-so-glorious association between plants, soil, and fungus is fundamental to plant establishment and growth.

There are about 5,000 different fungal species that form a mycorrhizal relationship with over 300,000 plants. This association is an absolute necessity for the establishment of many plants. The absence of mycorrhizas often reduces the successful transition from nursery to soil of newly planted trees and shrubs. This is particularly true when the plants are added to the landscape where the soil is unprepared or depleted of organic matter and nutrients.

importance of mycorrhizal fungi (endomycorrhizas that don’t form mushrooms) has been evident for at least 500 million years. Early fossil records indicate their presence during the Devonian period about 350 million years ago (mya). A more complex mycorrhiza (ectomycorrhizas that form mushrooms) arose during the Middle Cretaceous Period about 144 mya. Scientists believe that the marriage between fungi and plants was an essential step in the evolutionary process that brought plants from the marine environment to the terrestrial environment. What would the world be like today without the lowly fungi?

Benefits of Mycorrhizal Fungi
Soil fungi form a root-fungus structure (mycorrhizas) that invades the roots of 85% to 90% of all the trees, shrubs, flowers, and vegetables in the landscape and forest. The mycorrhizal association is beneficial to both the plants and the fungus. The plant may supply up to 80% of its net energy (carbohydrates) and other growth nutrients to the fungus. The fungus increases water and mineral uptake (particularly phosphorus) of the host plant by increasing the total absorptive area of the root system.

Mycorrhizal fungi are able to absorb and transfer all of the 15 major macronutrients and micronutrients necessary for plant growth. They release powerful chemicals into the soil that dissolve hard to capture nutrients such as phosphorous, iron, and other tightly bound soil nutrients. This  explains why nonmycorrhizal plants require high amounts of fertilizer to maintain their health.

Mycorrhizal fungi benefit the plants in other ways. The extensive network of fungal mycelium in the soil is important not only to nutrient uptake but also water uptake and storage. During drought conditions, mycorrhizal plants are under far less stress compared to non-mycorrhizal plants.

Disease and pathogen suppression is another benefit for a mycorrhizal plant. These plants have a sheath around the root hairs that acts as a physical barrier against the invasion of root disease. Mycorrhizal fungi attack pathogen and disease organisms entering the root zone. Excretions of specific antibiotics produced by these fungi immobilize and kill disease organisms. Some mycorrhizal fungi protect some plants from  Phytophora, Fusarium, and Rhizoctonia diseases.

Mycorrhizal fungi also improve soil structure. Mycorrhizal mycelium produce humic compounds and organic glues (polysaccharides) that bind soils into aggregates and improve soil porosity. Soil porosity and soil structure positively influence the growth of plants by promoting aeration, water movement into soil, root growth, and distribution. In sandy or compacted soils, the ability of mycorrhizal fungi to improve soil structure may be as important as seeking out nutrients.

Mycorrhizas are vibrant and live in close association with native plants. Estimates of the amount of mycorrhizal filaments (mycelium) present in healthy soil are amazing. Several miles of mycelium can be present in less than a thimbleful of soil underneath vigorously growing native plants.

Soils in natural settings are full of beneficial soil organisms including mycorrhizal fungi. Artificial landscapes affect the plant growth and survivability in two fundamental ways:

  • They isolate the plants from beneficial mycorrhizal fungi found in natural settings, and
  • They increase plant stress and the need for water, nutrients, and soil structure that is mediated by the presence of mycorrhizal fungi.

Many common practices can reduce or eliminate beneficial soil fungi and place undue stress on plants. The home gardener should use practices that build a healthy soil and are beneficial to organisms, including mycorrhiza. Try to:

  • Avoid unnecessary roto-tilling or cultivation, as it will destroy the mycorrhizal web and the natural soil structure.
  • Only amend the soil if it is necessary to improve the soil structure (heavy clay soils).
  • Prevent soil erosion or the removal of topsoil.
  • Do not leave the soil bare for extended periods. Cover the bare soil with mulch.
  • Mulch the soil with organic materials (compost) that are a food source for soil organisms.
  • Avoid over-irrigation since waterlogged soils will be harmful to beneficial soil organisms.
  • Avoid unwarranted use of high phosphate fertilizers.
  • Avoid over-fertilization and over-watering since they reduce mycorrhizal populations.
  • Avoid unwarranted use of pesticides, fungicides, and herbicides. Fungicides can kill mycorrhizas.
  • Use organic mulches for weed control and to help reduce soil compaction forces that lower soil oxygen levels needed by beneficial soil organisms and roots.

Mycorrhizal Inoculants
Nursery grown plants are often deficient in mycorrhiza. Plants raised in most nurseries receive intensive care. These artificial conditions, high levels of water and nutrients, and sterile soils keep certain soil borne diseases to a minimum and produce large quantities of plants for sale. Unfortunately, this intensive care often discourages the plant from producing an extensive root system necessary for transplantation and sustainable life. When transplanted these plants must be weaned, left to fend for themselves, or be inoculated with mycorrhizal fungi.

Applying commercially available inoculants is slowly becoming a more prevalent practice with nurseries, landscapers, and homeowners. Adding mycorrhizal inoculants to the soil can lower mortality rates and reduce water and fertilizer consumption. There is, however, much uncertainty concerning where to obtain the fungi, what type to use, the quality of commercially available products, and the most effective application methods.

A trip to the Internet reveals a boatload of companies trying to sell mycorrhizal inoculants. They may contain varying amounts of different species of fungi, different percentages of viable spores, as well as additives such as fertilizers, hydrogels, and a gimmick ingredient called “biostimulants.” Some inoculants contain spores specific to particular fungus species while others contain a broad mixture. The first order of business is to determine a specific plant’s needs and the present soil conditions. For example, some conifers (spruce and fir trees) usually prefer ectomycorrhiza while maple trees, camellias, marigolds, and others prefer endomycorrhiza (see A Bit of Dirt, Winter Edition, 2007).

Mycorrhizal inoculum can come from native topsoil or may be obtained commercially. Buying commercially produced mycorrhizal inoculants can be expensive but is much less labor-intensive than collecting forest soil. Removing “donor” soil from natural areas may cause existing plants to suffer from root disturbance as well as opening areas to invasion of weed species. Native forest “donor” soil might come laden with weeds,  pathogens, and non-native pathogens such as Sudden Oak Death.

Home Grown Mycorrhizal Inoculum
To make your own inoculum, you’ll need some supplies and a recipe for success.

  1. Containers: One gallon or larger pots, with drainage in the bottom, can be used to produce a “pot culture.” Sterilize the container using a 10% solution of household bleach (1 part by volume of bleach in 9 parts by volume of water)
  2. Growth Medium: A good growth medium is sterilized quartz (coarse construction sand can be substituted). Rinse the sand well to remove the finer dust particles. Sterilized sandy soil can be used if it has good drainage (to sterilize heat at 200 degrees F for 60 minutes and then cool).
    An alternative medium is a mixture of 1 part by volume of peat moss with 3 parts by volume of either horticultural grade vermiculite or perlite. Do not use a medium that contains phosphorus (P) fertilizer.
  3. Fungi. Collect fine roots or soil from the root zone of native vegetation from habitat you are restoring or from specific pants that are likely to be mycorrhizal (See A Bit of Dirt, Winter Edition, 2007 for a list of mycorrhizal plants).
  4. Host Plant Seeds: Select seeds from a fast-growing native plant species for the sacrificial “host” plant. Corn or sorghum seeds work well for endomycorrhizal or AM inoculum. Soak the seeds in 10% solution of household bleach for about 5-10 minutes to sterilize their surface. Germinate the seeds by placing them between moistened paper towels on a plate placed in a loosely sealed plastic bag. Corn seeds will germinate in about 2-3 days. Ectomycorrhizas are associated mostly with trees and the seeds need moist-chilling called stratification (Refer to The Reference Manual of Woody Plant Propagation, Dirr and Heuser, Varsity Press, Athens, GA).
  5. Low Phosphorus Fertilizer: For best results use a slow release, low Phosphorus fertilizer (17- 6-10). Apex® Foliage Fertilizer (19-5-15) is an example. You can make your own liquid fertilizer by mixing 2 gallons of water with 1-1/3 teaspoons of Peters® Professional 15-0-15 fertilizer,
    1/5 teaspoon of Epsom salts (MgSO4), and 1.5 milliliters (a milliliter measuring spoon is available in most drug stores) of a concentrated phosphate fertilizer solution called Quick Start (4-12-4) made by Miracle Gro®.


  1. Add 1 cup of fine roots per gallon of sand growth medium and mix thoroughly. Add 1 teaspoon of slow release fertilizer for every gallon of potting soil. Set up several pots to produce sufficient inoculum.
  2. Sow 4-6 “host” plant seeds in you container. The object is to maximize root development in the pot so that the medium is completely filled with a mass of roots.
  3. After seedlings emerge, if you are not using slow release fertilizer, fertilize weekly with 1 cup of your homemade, low-P liquid fertilizer solution per gallon of potting soil. Water in between fertilizer applications.
  4. Let plants grow for about 16 weeks (mycorrhizal fungi should form in about 6 weeks). After week 14 stop watering to allow the plants and potting mixture to dry slowly and completely in a greenhouse or cold frame for a period of 2-3 weeks.
  5. Remove the aboveground portion of the “host” plant and discard.
  6. Dump the medium and roots from the pots onto a clean, sterilized tray. Cut up the roots using shears or scissors and mix the fragments into the growing medium. This root sand mixture is your crude but concentrated fungal inoculum.
  7. The inoculum can be stored for six months (one year maximum) in plastic containers and kept in a cool, dry area like a refrigerator.

Use this inoculum to produce more “pot cultures,” add to potting mix to raise mycorrhizal plants, and
inoculate non-mycorrhizal plants that are being transplanted.


ST. AUSGUSTINEIt is August and we are still experiencing drought conditions. As I walk the neighborhood, it is no surprise to find a lot of ugly lawns. Obviously, some of my neighbors have been watering and trying to keep their lawns healthy, while others seem to have given up and accepted the fact that mankind was not really meant to be surrounded by a green carpet of thousands of little high maintenance plants.

My biggest surprise has been discovering how the various types have faired. As expected, most fescue looks pretty bad at this time of year, especially those lawns that were planted in the spring. Centipede and Zoysia have held up pretty well when in full sun, and only seem thin and
weak as they experience shade and competition from trees. Bermuda can take the drought but seems to have taken a beating from diseases this year.

The surprise has been St. Augustine. There are several such lawns in my neighborhood and, whether in sun or shade, every one looks great. The days of “St. Augustine just can’t grow well this far north” are long gone and evidence shows we should accept it as an option when planting a new lawn. Sorry Al – the ability to grow St. Augustine has nothing to do with global warming. There are some great new cultivars that have no problem with winter cold and grow well up into North Carolina.

Origin and Distribution: St. Augustine grass is primarily of tropical origin and is native to sandy beach ridges, fringes of swamps and lagoons, salty and fresh water marshes and shorelines. It gradually moved inland to naturally open sites such as stream banks, lakeshores, and other moist sites on the other side of the sand dunes.

ST. AUSGUSTINEDescription: St. Augustine grass, Stenotaphrum secundatum (Walt.) Kuntze, is a perennial robust grass widely used for pastures and lawns. OK, if you want to get technical, it is a coarse textured, stoloniferous species that roots at the nodes. Unlike bermudagrass, St. Augustine grass does not have rhizomes. Its stems (stolons) and overlapping leaf sheaths are generally compressed; leaf blades generally folded, abruptly contracted at the base, rounded at the tip, and smooth; ligule is reduced to a short fringe of hairs; collar is petioled and the sheath greatly compressed and ciliate along the margins.

Adaptation and Use: St. Augustine grass is adapted to moist, coastal areas with mild winter temperatures. It is known to be tolerant of high summer temperatures, and retains its color at temperatures as much as 10° lower than those that discolor bermudagrass. Varieties and Cultivars: Since St. Augustine grass has been propagated vegetatively for 200 years. Only a few strains or varieties have evolved and none have been developed through grass breeding programs. Of the warm-season grasses, it exhibits the best tolerance to shade. Recommended St. Augustine grass cultivars for the Atlanta area include Raleigh, Palmetto and Mercedes.

Propagation: St. Augustine grass is propagated by vegetative means — stolons, plugs or sod. Propagation by seed has potential, but we are just not there yet. It is readily established from sod since the species is vigorous and spreads rapidly by creeping stolons. Sod plugs or stolons planted on 1 to 2 foot spacing can be expected to cover in one growing season. In typical lawn plantings, 2 to 4 square inch sod plugs are planted on 1 to 2 foot spacing. This grass can be successfully established from plugs anytime during the growing season if water is available.

Management: After establishment the success of SA as a lawn grass depends largely on management. Mowing, fertilization and supplemental watering are required to maintain a dense, green, weed-free turf. Mowing heights may range from 1 to 3 inches depending on the frequency of mowing and the degree of shade present. Go easy on the fertilizer. St. Augustine grass is responsive to nitrogen fertilizer in terms of color and growth rate. On sandy soils this grass requires about 1 pound of nitrogen per 1,000 square feet per month during the growing season to maintain satisfactory color and density. So long as fertility and drainage are adequate, St. Augustine grass tolerates a wide range of soil types. St. Augustine grass grows satisfactorily at a pH range from 5.0 to 8.5

Insects and Diseases: Several insect pests can cause damage to St. Augustine grass lawns. Fortunately, the dreaded chinch bug isn’t a problem north of Macon, Ga. We do have white grubs that can feed on SA during the summer months. Sod webworms, armyworms and cutworms can also feed on leaves and can cause damage when infestations are heavy. All of the leaf-feeding insects can be easily controlled by insecticides or possibly BT. Brown patch and gray leaf spot are the most serious diseases caused by fungi attacking St. Augustine grass. Although these diseases rarely kill St. Augustine, they can weaken and thin the grass to the degree that the lawn is unsightly.

Weeds: A healthy St. Augustine lawn effectively crowds out most weeds. If is not properly maintained or is weakened by insects or disease, it can be invaded by grassy and broadleaved weeds. Cool season weeds such as henbit, chickweed and clover can be a problem in dormant St. Augustine grass. These weeds can be controlled with commonly available herbicides in early spring.

As possibly the laziest gardener in Gwinnett County, I am always on the lookout for grass, shrubs, and trees that will perform consistently well with as little care from me as possible. The last two summers have convinced me that St. Augustine is now a viable option for North Georgia. As I watch my last green blade of fescue start to fade to yellow (after about a million gallons of water), I am already making plans to plant St. Augustine in the spring.

The Winter Garden Planning and Planting for the Southeast – Book Review

The Winter Garden Planning and Planting for the Southeast
By Peter Loewer and Larry Mellichamp

Perquisites for growing a beautiful winter garden are planning and choosing the plant species for impact or subtleness. Walk through The Winter

Garden with authors Peter Loewer and Larry Mellichamp. See the beauty of opting for planting trees because of their bark texture, color, or  structure that will catch your eye in the bleak months of winter. Choose pods, fruits, plumes, and berries to add sparkle to the dreary landscape and pizzazz to holiday centerpieces and wreaths.

You will not miss the profusion of summer flowers when you select from a large variety of winter bulbs or winter-blooming herbaceous perennials. No garden would be complete without fragrant plants for “…fragrances can reach across the decades like a physical link, reminding us of an eventful time now long forgotten.”

There is no excuse not to have an array of winter blooming trees, shrubs, and vines. Many of these are the heralds of spring! They will lift your spirits and brighten the dark days of winter. It has been said that the “glitter of green leaves is like the sunlight on water.” Evergreen foliage with a  glossy look would do just that…shimmer and reflect light.

There is an abundance of trees and shrubs, herbaceous plants, ornamental grasses, and ferns from which to ponder and make your selection. Start by making a list to plan your garden. Add color, visual structures, garden ornaments, living sculpture, night lighting, and water. Voile! The Winter Garden with interest and appeal!

Oh yes, don’t miss reading this book for all its
wonderful descriptions and glorious pictures.

A Bit of Dirt – Fall 2007

The full pdf copy of this edition is available here.

By Glenn Parsons
This morning I began to work in my garden before 9:00 am with the hope that I would avoid the oppressive heat and humidity. Unfortunately, my plan was unsuccessful. To make matters worse I was attacked by a formation of B-52’s, or should I say some very large Georgia mosquitoes! My garden is progressing slowly but seems to be handling the drought conditions fairly well. However, the outlook for the rest of August and the entire month of September is not good. Have you noticed that even against all adversity the weeds continue to thrive?

I am trying a few members of the Agastache species in the hummingbird garden this year. There are about four-color varieties with tubular flowers, which hummingbirds cannot resist. In fact, one common name for these plants is Hummingbird Mint. Agastache species have a long blooming period and should be great for the hummer’s late summer migration. I am sure many of you have been growing one or more of this plant variety for years. I have had one more gardening related success that was a surprise. Remember the two gentlemen who spoke to us about their birdhouses and feeders? Well, I purchased one of their large “fly-through” feeders and placed it in the backyard according to instructions. After about one month, there are routinely fourteen species of birds visiting that feeder. This is very pleasing indeed.

Before closing I want to let you all know that the Gwinnett County Master Gardeners Association is doing very well. We have had a very successful year at fund raising. Our membership is active and attendance at our monthly meetings continues to increase. There are ample funds available for qualifying community projects that have been challenged this year by  the ongoing drought. This month your elected officers and I will be working on revisions to our Constitution and By-laws in an effort to ensure we remain a strong and well-organized group. All things considered, the “State of the Garden” is healthy and growing.

Other articles in this issue:
The Winter Garden Planning and Planting for the Southeast – Book Review – By Marlene Gillman
ST. AUSGUSTINE GRASS – By Robert Brannen
Mycorrhizas: The Underground Internet – By Dan Willis