Grasses inoculated with mycorrhizal fungi are generally healthier.HOMEOWNERS AND GROUNDSKEEPERS alike usually fertilize their lawns and turf several times a year, adding pounds of expensive chemical fertilizers with each application. Lawn fertilizers can include high amounts of phosphorus and nitrogen, both important nutrients for grasses. But nutrient runoff has caused many pollutants to drain into rivers, lakes, and streams. In response, many American states have banned the use of phosphorus in lawn fertilizers.
The solution is arbuscular mycorrhizae. The quality of grasses grown in lawns, golf courses, and recreational fields can be enhanced by maintaining a healthy network of arbuscular mycorrhizal fungi. This practice can totally replace the application of fertilizers—in particular, the use of chemical phosphorus—because the mycorrhizal network mobilizes both phosphorus and nitrogen.
Grasses are divided into two physiological groups according to the number of carbon molecules produced during photosynthesis. Cool-season grasses are C3 grasses, and warm-season grasses are C4 grasses. The latter generally grow in tropical climates and are more mycorrhizae-dependent than cool-season grasses; in fact, bahiagrass (Paspalum notatum), a C4 grass, is the preferred host in the production of commercial propagules. But all true grasses (Poaceae or Gramineae), including cereals, bamboo, and the grasses of lawns, golf courses, and grasslands, readily form arbuscular mycorrhizae.
Associations with arbuscular mycorrhizal fungi benefit grass plants in the same ways they benefit trees, shrubs, and most other colonized plants:
Grasses inoculated with mycorrhizal fungi are generally healthier.
Tests show that lawn grasses inoculated with mycorrhizal fungi contain more chlorophyll with improved photosynthesis.
Root systems grow larger, denser, and faster as a result of mycorrhizal association, which can mean total coverage, without the need to replant.
Mycorrhizae improve soil structure as fungal hyphae extend and explore surrounding soils. Mycorrhizae bind soil particles, allowing for better air and water movement throughout the soil.
Mycorrhizae improve soils damaged by compaction, particularly soils supporting grass on playing fields and parks.
Mycorrhizae improve drought resistance by reaching deep into the soil to access water resources.
Mycorrhizae lessen infection rates from bacterial wilt, parasitic nematodes, and other pathogens.
Providing water to large expanses of grass can be expensive, time-consuming, and wasteful. Research has demonstrated that when mycorrhizae are established in lawns and turfgrass, less water is required than in areas with low colonization. In fact, in some climates, grasses with mycorrhizal associations need no irrigation. Simply put, mycorrhizae store water, but they also explore for it.
Grasses with mycorrhizal associations are also better able to resist the ravages of drought. Arbuscular fungi form tremendous extraradical networks in the soil that increase grass roots’ access to water, especially in response to drought. Healthy mycorrhizal grasses with extensive root systems are better equipped to face the stresses of drought, and grasses bounce back much faster when water again becomes available. The complex mantle formed around roots by ectomycorrhizal fungi holds water, which enhances storage and the plants’ ability to interact with surface water. Arbuscular mycorrhizal fungi also form abundant vesicles within grass roots that stay hydrated and spongy to protect roots from desiccation during drought.
Endomycorrhizal fungi deposit sticky glomalin into the soil, improving soil structure. Glomalin-enriched soil particles stick together, creating pores, tunnels, and reservoirs for greater water retention. Mycorrhizal colonization also increases the number of root aquaporins, the embedded cell membrane protein channels that transport water.
Mycorrhizae benefit grasses in other ways as well. For instance, they impact gas exchange in plants, helping them expel oxygen and take in carbon dioxide for photosynthesis. Mycorrhizae impact the hydraulics of the host grass plant as it balances the water absorbed through its roots and released through its stomata. Stomata pores open and close to regulate the amount of gasses and water vapor that are expelled from or held within the plant; the stomata close as water becomes scarce and open again to allow respiration.
In a freshly seeded lawn, the best way to discourage weed growth is to encourage the quick germination of grass seed—and mycorrhizal inoculation helps grass seeds germinate faster and establish quicker. But allowing a few weeds to grow in a lawn, golf course, or playing field is not necessarily a bad idea. For one thing, monocultures, areas planted in a single crop, are magnets for problems; if a fungal or nematode infection occurs in one place, it is likely to spread throughout a lawn planted in a single species of grass. And some weeds actually add nutrients to a lawn. Clover, for example, is an excellent addition. This nitrogen-fixing plant also forms mycorrhizae, and because mycorrhizae deliver nitrogen to grass roots, including clover in the lawn is a great way to achieve a healthy lawn without adding fertilizers. The presence of mycorrhizae also increases the number of nitrogen-fixing root nodules. Clover also holds water longer than most lawn grasses, and it shares the water with other organisms in the soil food web.
Even the dreaded yellow dandelion has its uses. It is so proficient at mining minerals with its mycorrhizal partners that it is sometimes used as a cover crop. Plantings in the following year will benefit because the soil contains more phosphorus. Because of pollution issues associated with the use of fertilizers in the United States and other parts of the world, most lawn fertilizer mixes no longer contain phosphorus. Perhaps weeds may someday take the place of the middle number in the N-P-K fertilizer trilogy.
If you’d rather not roll out the weed welcome wagon, studies show that the use of arbuscular mycorrhizal fungi can help control weed growth in established lawns and turfgrass; scientists hypothesize that the fungi either discourage the growth of the weeds or help the desired perennial grasses outcompete them.
Mixing spores and propagules into the soil might seem like a great way to benefit grass plants, but the fungal mix works both ways and can also give weeds a boost, because many of them form mycorrhizae as well. For this reason, inoculating lawn soils is not recommended unless a new lawn is being created on soils laid bare by recent construction. The addition of mycorrhizal fungi in this case is almost mandatory: the construction process severely damages soil structure and removes organic materials. Many of the important soil organisms that once thrived are no longer present. Mycorrhizae can help restore the soils and rebuild the food web, which benefits grasses and other plants. Golf greens benefit from the addition of mycorrhizal fungi because they are constructed mostly atop sand, which lacks organic matter and holds few nutrients; these grasses can also be bolstered by adding fungal spores. Grasses in recreational fields are exposed to all manner of stress factors, from compaction to wet and dry conditions, which can damage existing mycorrhizae. Adding propagules can replace the depleted fungi.
New lawns are usually established from seed or by laying sod. To prevent the establishment of weeds at the early stages of planting, inoculate grass seed with the appropriate mycorrhizal propagules before planting. Seed can be rolled in arbuscular mycorrhizal mixes designed for lawns, or the inoculum can be placed directly in the spreader, along with the seed, before being dropped onto the soil. If squares or rolls of sod are used, the soil side can be dusted with mycorrhizal mixes or sprayed with a liquid formula. Inoculated sod grows into the substrate faster, and the grass is more firmly established in the soil because of the extended root systems provided by the mycelial matrix.
After grass has been planted and established, fungal spores can be easily replenished; some powder mixes are small enough to wash into the root system with regular watering, but applying liquid-based inoculants can be even more efficient.
Arbuscular mycorrhizae are particularly sensitive to soil compaction in areas where heavy traffic is the norm. The solution is aeration, which can be accomplished by hand tool or machine. If you aerate the grass prior to applying liquid or granular mycorrhizal propagules, the aeration holes offer direct routes down through the soil to the root zone.
Some grasses welcome associations with many different types of fungi; for example, in a Canadian study using soils of varying fertility, researchers found 17 species of arbuscular mycorrhizal fungi in root zones of velvet bentgrass (Agrostis canina) and 15 in creeping bentgrass (A. stolonifera).
Rhizophagus intraradices has been used to colonize Kentucky bluegrass (Poa pratensis), red fescue (Festuca rubra), and perennial ryegrass (Lolium perenne). Adding the less effective Claroideoglomus etunicatum seemed to provide growth diversity, though it was not as effective when used alone. Many other grasses, including weed grasses, have been studied for their dependency on and association with mycorrhizal fungi and for the impact these fungi can have on their growth and appearance.
Annual bluegrass (Poa annua) This quick-growing, opportunistic weed grass has shallow roots and is not a preferred host for mycorrhizal fungi. Experiments in golf courses have successfully used arbuscular mycorrhizae to act as a biocontrol for it; the aforementioned Canadian study found 14 species of arbuscular mycorrhizal fungi in its various root zones. Studies indicate that annual bluegrass does not thrive if mycorrhizae colonization is extensive in other, more favored, perennial grasses.
Bermuda grass (Cynodon dactylon) This tough grass is used in greenbelts, playing fields, and lawns. Inoculation with mycorrhizal fungi improves all aspects of growth, from drought resistance to nematode and disease resistance. Less fertilizer is also required.
Creeping bentgrass (Agrostis stolonifera) and Kentucky bluegrass (Poa pratensis) Creeping bentgrass and Kentucky bluegrass inoculated with Funneliformis mosseae, Glomus aggregatum, or Rhizophagus intraradices showed 60 percent higher colonization under low phosphorus conditions than controls. Creeping bentgrass showed twice as much established mycorrhizae.
Perennial ryegrass (Lolium perenne) After inoculation with arbuscular mycorrhizal fungi, ryegrass plants contained up to 29 percent more chlorophyll than uninoculated plants. They also showed increased biomass and better visual quality, and they contained more phosphorus, potassium, and zinc concentrations than controls. Rhizophagus intraradices proved to be a better colonizer than Funneliformis mosseae.
St. Augustine grass (Stenotaphrum secundatum) This grass is natively associated with Rhizophagus intraradices. It is susceptible to infection by brown patch (Rhizoctonia solani) and take-all (Gaeumannomyces graminis), which affect its roots. In studies, mycorrhizae did not seem to protect it from infection by these fungi.
Zoysia (Zoysia spp.) Zoysia grasses do not appear to be the subject of studies, but some commercial advertisements suggest that inoculation with arbuscular mycorrhizal fungi is useful and helps the grass survive drought.
