Plants started from seed can be inoculated at the earliest growth stage, offering seeds and seedlings the advantages of early mycorrhizal association.MOST ANNUAL AND perennial plants form arbuscular mycorrhizae, and many plants respond readily to inoculation. In fact, almost any nursery-grown plant or start forms arbuscular mycorrhizal associations when planted in the garden. Some plants have a difficult time thriving without mycorrhizae, especially trees and shrubs, and plants with thick, carrotlike roots. Again, plants that do not form mycorrhizal relationships are members of families Amaranthaceae, Brassicaceae, Caryophyllaceae, Chenopodiaceae, Polygonaceae, Portulacaceae, and Proteaceae.
Inoculating seed and/or soil with mycorrhizal fungi can be a cost-effective and conscientious way to minimize potentially polluting nutrients such as nitrogen and phosphorus in runoff water. Adding mycorrhizal propagules to soils greatly increases the uptake of water and mineral nutrients, and mycorrhizae can protect plant roots from attack by pathogens. Despite these and other practical benefits of mycorrhizal inoculation, however, many nursery-grown seedlings are planted in sterile media that is heavily fertilized.
Mycorrhizae do not establish in beneficial numbers if ample phosphorus is already available in the planting media. Nurseries often use premixes that contain enough phosphorus and other nutrients for the plant to grow at the nursery, and at home, for a while. Soils are sterilized and fumigated to prevent the spread of pathogens, and some of the peat used in planting mixes does not contain and support growth of the appropriate types of mycorrhizal fungi.
Special mycorrhizal formulations and delivery systems have been developed to make inoculation easier and more cost-effective in a nursery setting, and a great number of studies are confirming the benefits of inoculating nursery plants:
Plants started from seed can be inoculated at the earliest growth stage, offering seeds and seedlings the advantages of early mycorrhizal association.
For plants that are propagated using tissue culture, adding mycorrhizal propagules to the soil mix before planting can help the tiny cuttings establish strong root systems so healthy plants can be transplanted earlier.
A plant with established mycorrhizal root associations has a stronger connection to the planting media and more readily accepts nutrients. It can also handle stresses such as lack of water and excessive heat.
Plants with mycorrhizal associations can better withstand damage by common root pathogens, such as nematodes and mites.
Seedlings grown with mycorrhizal fungi exhibit a uniformity that is often required in horticultural displays and landscaping beds. Because all seedlings in a flat can share the same mycorrhizal matrix, they can share nutrients and other benefits.
Nursery inoculation helps ensure that strong plants will survive and thrive in their next location, resulting in fewer returns and increased customer satisfaction.
Both arbuscular and ectomycorrhizal fungi inoculants are available to nursery wholesalers, retail nurseries, and home growers. (You can also make your own inoculants, as you’ll learn later in this book.) Two general types of commercial inoculants are available: a grab bag of fungal species that may include other kinds of microbes, and propagules of one specific kind of mycorrhizal fungus.
Most blends of mycorrhizal fungi are available as spores or hyphal fragments mixed with various media such as sand, peat, clay, gel, or water to ensure delivery to the plant’s root system. Many potting mixes and composts also add mycorrhizal fungi to their formulations. A list of mycorrhizal species and their concentrations is usually included on the packaging of products containing propagules. Some manufacturers list them simply as propagules, while others add more specific information. It is always a good idea to read the package ingredients list to identify the type of fungi (endo- or ectomycorrhizal) and the species included, so you can match the fungi to the plant. Also useful is the percentage of spores and/or propagules that will germinate.
In some countries there is governmental regulation of commercial formulations of mycorrhizal fungi. In the United States, for example, spore counts and identification of the mycorrhizal fungi in the formulation must be included on the product package. This information says nothing of the fungi’s viability, however, which can be affected by the age of the propagules and how they have been handled. Although extreme cold and some heat does not affect their viability, extreme heat can kill propagules, and moisture can trigger premature germination (which proves that even fungi can experience dysfunction). In general, you can rely on viable spores lasting a year or two and root and hyphal fragments to last a year or more.
How much to use? A fungus can regenerate from just a tiny amount of a hypha. Determining the appropriate number of propagules to apply is not easy, though some manufacturers do offer recommendations. It depends on the plant, the fungi, the soil conditions, the delivery media, and other factors. Experiment and review the literature. Fortunately, it seems that you cannot apply too many mycorrhizal propagules to plants. Different nursery conditions mandate different usage depending on fertilizer and pesticide application practices. Nurseries can inoculate seed and transplants with liquid and granular mixes, and liquid formulations can be added to drip systems.
The timing of inoculation is important to the fungi’s effectiveness. Apply the inoculant too late, and its benefits are lost. It is best to inoculate as early as possible in the plant’s life to ensure that it gets the maximum benefits from the mycorrhizal association. Roll or spray seeds in mycorrhizal formulations before they germinate.
Establishing mycorrhizae in planted containers can take up to two months—a lot less time than it can take to colonize an agricultural field. This is because the conditions in pots are usually far more ideal and controllable than those in the field, and propagules are applied in much greater concentrations in pots. Container-grown mycorrhizal fungi also produce spores much more quickly. Sporulation is usually triggered by crowding in the pot as mycorrhizae develop. As soon as the pot becomes full of fungi, they begin the reproduction process. Container-grown mycorrhizal fungi can begin reproducing via spores within a year of inoculation.
Mycorrhizal fungi spores will not grow and thrive and hyphal fragments will not develop if they do not come in contact with the proper root exudates. To ensure proper propagule placement, make sure propagules are in physical contact with plant roots. The standard advice is to roll the roots of transplants in mycorrhizal mixes or sprinkle the mix directly on exposed roots. Once the roots are inoculated and the plant is transplanted, the extracellular hyphae grow from the roots and into the soil in the container.
Studies have shown that it is also good practice to mix propagules throughout the potting mix before transplanting. The spores in the soil will germinate, grow, and increase, and more mycorrhizae will form as they become exposed to the expanding network of roots. A larger mycorrhizal network results, with increased nutrient uptake and other benefits. As plants are transplanted to larger pots, the existing fungal network comes along, though adding more fungal spores to the transplant mix continues to increase the number of mycorrhizae.
Finally, when potted plants are transferred to the garden, gardeners can again supplement the mycorrhizal fungi at the grow location to bolster the plant and surrounding soils.
Growing mixes are critical to forming and sustaining arbuscular mycorrhizae. Some manufacturers of commercial potting and growing mixes sterilize their growing media, which renders them free from microbes, including mycorrhizal fungi. Other manufacturers add mycorrhizal propagules to their blends (quite a change after years of using mix sterility as a selling point).
Compost is free of mycorrhizal spores, because it contains no live roots to support mycorrhizae and because the temperatures used to produce compost kill mycorrhizal propagules. In addition, manures included in compost mixes can have very high levels of phosphorus. It is crucial that the levels of phosphorus be low enough in compost not to discourage the germination of inoculum and formation of mycorrhizae.
The best soil and compost blends ensure that pathogens will not prevail and that proper nutrients and microbes will be available to the plant and the mycorrhizal fungi. The needed microbes, along with mycorrhizal fungi, include nitrogen-fixing bacteria and phosphate solubilizers that produce the requisite hormones and control plant diseases. If mycorrhizal propagules have not been added to the mix, add them at planting time.
Thousands of studies on the horticultural uses of mycorrhizal fungi have been conducted by commercial entities, academic institutions, and home growers. The results of some of these are described here.
Cyclamen (Cyclamen spp.) Inoculated plants showed better resistance to infection by the fungal pathogen Cryptocline cyclaminis. Mortality rates decreased in inoculated plants.
Daffodil (Narcissus spp.) Bulbs inoculated with Funneliformis mosseae showed improved flower yield, stalk length, and quality.
Freesia (Freesia spp.) Inoculated plants produced larger daughter corms and more flowers. Plants also grew roots and shoots more quickly.
Geranium (Pelargonium spp.) Plants inoculated with Funneliformis mosseae and Rhizophagus fasciculatus were better able to tolerate drought stress.
Gerbera (Gerbera jamesonii) A mix of Rhizophagus intraradices and R. vesiculiferus increased leaf and root dry weight, and inoculated plants flowered significantly earlier than the controls.
Hosta (Hosta spp.) Use of commercial mycorrhizal fungi resulted in greater top growth, an important development for commercial growers as well as home gardeners.
Marigold (Tagetes erecta) and zinnia (Zinnia elegans) Seeds inoculated with Claroideoglomus etunicatum flowered faster and produced more flowers.
Petunia (Petunia ×atkinsiana) Inoculated plants had higher reproductive growth than nonmycorrhizal control plants, with a threefold increase in vegetative growth, and colonized plants flowered 15 days earlier than controls. In petunias colonized by Rhizophagus irregularis, nitrogen starvation resulted in formation of mycorrhizae even with a high availability of phosphorus, suggesting that mycorrhizae may form even when they are limited by nitrogen or phosphorus.
Poinsettia (Euphorbia pulcherrima) Cuttings showed improved growth as a result of inoculation with Gigaspora margarita and after adding spores to the plant misting system and potting mix.
Rose (Rosa spp.) Rhizophagus intraradices improved the growth of container-grown mini roses. For some varieties, using mycorrhizal fungi was as good as or better than using rooting hormone.
Snapdragon (Antirrhinum spp.) Plants inoculated with Claroideoglomus etunicatum outperformed controls with regard to size and blooms.
