Gardening Articles: Flowers :: Annuals

Helping Plants to Help Themselves (page 2 of 3)

by William Bryant Logan

How to use beneficial fungi

The fungi are not harmful to plants or animals, and typically you only have to inoculate once, at planting time, by mixing the fungal inoculant into the planting hole. For dosage, follow product recommendations.

Two main types of mycorrhizal fungi are available: ectomycorrhizal and endomycorrhizal. (Mycorrhizae is the symbiotic association of the fungi and plant roots.)

Ectomycorrhizae form mostly outside a plant's roots. They colonize the roots of many common trees and shrubs including alder, arborvitae, aspen, basswood, beech, birch, chestnut, chinquapin, eucalyptus, fir, hazelnut, hemlock, hickory, larch, oak, pecan, pine, poplar, spruce, and willow. If they appear aboveground at all, it is usually in the form of small puffballs or mushrooms with dusty brown spores. But if you carefully dig into the root zone, you will likely find masses of white or orangish "roots" growing out of the plant's roots. These are the ectomycorrhizae, and in exchange for the plant's feeding them with sun-derived carbohydrates, these fungal roots exponentially increase the plant's ability to draw what it needs from the soil. Indeed, it is now thought that without mycorrhizae, many species of trees and shrubs might never have survived at all.

Endomycorrhizae form mostly inside a plant's roots. This is the most widespread type of mycorrhizae. The list of plants they live with is long, reaching from bamboo to camellia, and from apple trees to fescue grass. Their hyphae do grow extensively out of the roots. For the most part, endomycorrhizae are invisible to the naked eye, though if you managed to remove the soil from around the roots of plants host to these fungi, you could see them with a microscope. (They would look similar to the fine roots on the plant.)

Vesicular-arbuscular mycorrhizae (VAM), a subgroup, occurs naturally in 90 percent of all vascular plants, including agricultural crops, grasses, desert plants, flowers, citrus, and shrubs. Other subgroups are more specialized and grow only with orchids, for example, where they are necessary for orchid seeds to germinate. Still others associate only with ericaceous plants (heaths and heathers).

Mycorrhizae provide the most benefit to long-lived trees, shrubs, and perennials planted in less intensively maintained soils. In heavily fertilized perennial or vegetable beds, mycorrhizae are of least benefit. Nitrogen and phosphorus fertilizers in particular inhibit their growth.

Adding VAM fungi to soil has been shown to improve growth of some slower-growing annuals and vegetables. The fungi penetrate the plant?s root cells, resulting in bushier plants with higher yields of fruits or vegetables.

For established trees, shrubs, and perennials, lightly cultivate mycorrhizal inoculant into soil within the drip line. Alternatively, dig holes around the drip line and backfill with inoculant, or inject it into the root zone.

Mycorrhizae are not a miracle cure -- any more than nuking plants with nitrogen is nourishing them -- but using these fungal roots is an important step toward healthy plants that live off the soil rather than doses of fertilizer.

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