Beneath every plant root lies an invisible world of fungal filaments that plays a decisive role in plant life in both urban and natural environments. Endomycorrhiza refers to the symbiosis in which fungal hyphae literally penetrate the root cells and form a dense network of exchange structures there. This symbiotic relationship is not merely a curiosity of basic research, but one of the most ecologically and planning-wise significant interactions in the soil; understanding it is becoming increasingly indispensable for vegetation engineering, open-space planning, and urban greening.

• What endomycorrhiza is and how it differs from other types of mycorrhiza
• Which fungal groups and plant species are involved in the symbiosis
• How the fungal hyphae penetrate the root cells and what structures are formed in the process
• What physiological benefits endomycorrhiza provides to the host plant
• Why urban soils often inhibit mycorrhizal activity and what this means for planning
• How endomycorrhizal inoculants are used in landscaping and open-space planning
• What mistakes are commonly made when using mycorrhizal preparations
• How endomycorrhiza fits into the broader context of soil ecology and climate adaptation

What is endomycorrhiza? Definition, types, and distinction

Mycorrhiza, derived from the Greek words for “fungus” and “root,” generally refers to the symbiosis between soil-dwelling fungi and the roots of higher plants. Within this widespread symbiotic relationship, the scientific literature makes a fundamental distinction between ectomycorrhiza and endomycorrhiza. In ectomycorrhiza, the fungal hyphae envelop the root from the outside and form what is known as a Hartig’s network, but do not penetrate the plant cells. Endomycorrhiza, on the other hand, is defined by the fact that the fungal hyphae penetrate the cell walls of the root epidermis and cortex and form specialized structures intracellularly—that is, within the living plant cell. This intracellular penetration is the defining and functionally crucial characteristic.

By far the most significant group within endomycorrhiza is arbuscular mycorrhiza, or AM for short. Its name derives from the so-called arbuscules, tree-like, branched hyphal structures that develop within the root cells and function as the primary exchange organs between the fungus and the plant. In addition, many AM fungi also form vesicles, bubble-like structures that serve to store fats. Arbuscular mycorrhiza is formed by fungi of the division Glomeromycota, a phylogenetically ancient group of fungi whose fossils date back to the Ordovician period. This evolutionary depth underscores just how fundamental this symbiosis was to the colonization of the land by plants.

In addition to arbuscular mycorrhiza, other types of endomycorrhiza exist, though they play a minor role ecologically and in conservation planning. Ericoid mycorrhiza is specialized for plants of the order Ericales—that is, heath family plants—and is relevant for bog restoration. Orchid-type mycorrhiza is essential for the germination and survival of many orchid species and plays a role in the reintroduction of these plants in nature conservation practice. When endomycorrhiza is discussed below, the focus is on arbuscular mycorrhiza, as it is by far the most common and, from a planning perspective, the most relevant form, associated with an estimated eighty percent of all terrestrial plant species.

Morphology and Physiology: How the Symbiosis Works

The infection process of arbuscular mycorrhiza begins with the germination of resting spores in the soil, which are chemically stimulated by root exudates from the host plant. The fungal hypha grows toward the root surface and forms an appressorium there—a flat adhesive structure that initiates entry into the root. The hypha then penetrates the cell wall into the epidermal cells without breaking through the plasma membrane of the plant cell. Instead, the plasma membrane folds around the invading hypha, forming a new membrane layer known as the periarbuscular membrane. This membrane is the actual site of material exchange and has a greatly enlarged surface area.

The arbuscules, which develop within this membrane sheath through repeated branching of the hyphae, increase the contact area between the fungus and the plant cell many times over. The central exchange processes take place across this interface: The fungus primarily supplies the plant with phosphate, as well as nitrogen and other minerals, which it mobilizes from the soil via its extensive external hyphal network. In return, the fungus receives photosynthetically fixed carbon from the plant in the form of sugars and lipids. According to estimates, a host plant can invest up to twenty percent of its photosynthetically produced carbon in its fungal partner. This investment is not parasitism, but a true mutualistic symbiosis, since the plant receives nutrients in return that it could absorb far less efficiently from the soil on its own.

The external hyphal network of the AM fungus, which extends far beyond the rhizosphere into the soil, has a diameter of only a few micrometers. This allows it to access soil pores that are inaccessible even to roots. This is particularly crucial for phosphate supply: Phosphate is not very mobile in the soil and quickly accumulates in the immediate root zone because the plant absorbs it faster than it can be replenished by diffusion. The fungal network bridges this depletion zone and taps into phosphate reservoirs located farther away from the root. In phosphate-poor soils—such as those found on virgin soils, mine tailings, restoration sites, and in many urban substrates—this mechanism is often crucial for plant growth.

Benefits of endomycorrhiza: Nutrient supply, stress tolerance, and soil structure

Phosphate supply is the most thoroughly studied and quantitatively most significant benefit of arbuscular mycorrhiza. In mycorrhizal plants, phosphate uptake can be several times higher than in non-mycorrhizal control plants, although the extent depends heavily on soil type, fungal species, and plant species. In addition to phosphorus, the symbiosis also improves the uptake of nitrogen, potassium, zinc, copper, and other micronutrients, the availability of which in the soil is often limited by local depletion zones.

Mycorrhizal plants exhibit a significantly improved water supply under drought stress. The fungal network also taps into soil water reserves in finer pores that root hairs cannot reach. In addition, mycorrhiza improves the hydraulic conductivity of the root itself by influencing the expression of certain aquaporins—that is, membrane-bound water channels. For urban greening, where street trees and shrub plantings regularly suffer from drought stress, this aspect is highly relevant to planning.

The fungal hyphae produce the glycoprotein glomalin, a stable, humus-like macromolecule that binds soil particles into aggregates. This aggregate stability improves soil structure, increases water-holding capacity, and promotes soil aeration. In degraded urban soils, where soil compaction and a lack of humus severely limit the availability of water and nutrients to plants, glomalin is a key factor in soil regeneration. Glomalin is also relatively persistent and contributes to long-term carbon sequestration in the soil, which is becoming increasingly important in the context of climate adaptation and climate protection.

Endomycorrhiza also increases plants’ tolerance to biotic stress factors. Mycorrhizal plants often exhibit increased resistance to soil-borne pathogens such as Pythium and Phytophthora, presumably through a combination of the physical barrier effect of fungal hyphae, the activation of plant defense mechanisms, and competition for space in the rhizosphere. For vegetation management, this means that well-mycorrhizal plant populations are, under certain conditions, less susceptible to root diseases, which can reduce the use of fungicides.

Endomycorrhiza in an Urban Context: Obstacles and Planning Implications

Urban soils pose a particular challenge for arbuscular mycorrhiza. Compaction, soil sealing, soil replacement, high phosphate levels resulting from decades of fertilization, pesticide inputs, and the fragmentation of soil habitats significantly reduce the diversity and activity of AM fungi. Studies of urban soils consistently show a significantly lower spore density and species diversity of Glomeromycota compared to near-natural soils. This means that urban plants often grow in soil that does not support mycorrhizal symbiosis at all or only inadequately.

Particularly critical is the high phosphate content of many urban and horticultural soils. AM fungi have evolved to specialize in low-phosphate conditions: When phosphate levels in the soil are high, the plant actively reduces colonization by its fungal partner because the benefits of the symbiosis no longer outweigh the carbon costs. Soils that have been supplied with phosphate-rich fertilizers over a long period are therefore often poorly mycorrhizal, even if the fungi are present in principle. For planning plantings on such soils, this means that mycorrhizal inoculation alone is not sufficient if soil conditions do not support the symbiosis.

Systemic fungicides used in plant care can directly harm AM fungi, since they are fungi themselves. Certain herbicides and insecticides also negatively affect mycorrhizal activity. In management strategies for mycorrhiza-dependent plantings, the use of such agents should therefore be carefully evaluated. In contrast, copper-based products used in organic farming are also known to be mycorrhiza-toxic and should be avoided in mycorrhiza-sensitive plantings.

Not all plant species form arbuscular mycorrhizae. Important exceptions include the families Brassicaceae (cruciferous plants), Chenopodiaceae (goosefoot family), and Caryophyllaceae (carnation family), which generally form no mycorrhizal relationships or only weak ones. For plant communities dominated by such species, mycorrhizal inoculation is of little benefit. Street trees of the genera Tilia (linden) and Acer (maple), on the other hand, are highly dependent on mycorrhizae and have been shown to benefit from inoculation when planted in degraded urban soils.

Endomycorrhizal Inoculants: Use, Quality, and Common Mistakes

Commercial mycorrhizal preparations, known as inoculants, contain spores, hyphal fragments, or colonized root segments of AM fungi, often of the genera Rhizophagus (formerly Glomus), Funneliformis, or Claroideoglomus. They are available as powders, granules, gels, or liquids and are intended to accelerate mycorrhizal colonization of newly planted plants, particularly in soils with low natural fungal populations. Such products are widely used in landscaping, street tree planting, and the restoration of degraded areas.

For effective use, several basic conditions must be met:

• The inoculum must contain viable propagules. Storage at high temperatures, exposure to frost, or direct sunlight can destroy the spores’ viability.
• The inoculum must be brought into direct contact with the plant’s fine roots, as AM fungi do not undergo free spore germination over long distances.
• The soil must not have excessively high phosphate levels, as the plant will otherwise actively suppress the symbiosis.
• The plant must belong to a mycorrhiza-forming species, which should be verified before planning.
• Systemic fungicides must not be applied at the same time.

A common mistake in practice is to apply inoculants to soils with naturally healthy AM fungal populations. In such cases, inoculation offers no added benefit, as the introduced fungal strains compete with the already established local populations and often fail to establish themselves. Inoculants are primarily useful for degraded, sterile, or heavily disturbed soils—that is, for virgin soils, spoil heaps, construction site soils, commercially available substrates, and heavily compacted urban soils. On near-natural, undisturbed soils, inoculation is generally unnecessary and can even impair local fungal diversity if non-native fungal strains are introduced.

Quality control of commercial inoculants is a well-known problem. Independent studies have repeatedly shown that some of the products available on the market contain no viable propagules or only very small amounts. Professional planners should therefore rely on products that have been verifiably tested for germination capacity and colonization potential using standardized procedures. Relevant requirements for quality certification of mycorrhizal inoculants can be found in technical publications by the Research Association for Landscape Development and Landscape Construction (FLL) as well as in scientific assessment frameworks of the European Research Community.

Endomycorrhiza in Restoration and Climate Adaptation Plantings

In restoration sites, post-mining landscapes, landfills, and bare soil sites, the natural mycorrhizal population is often completely absent because the original soil has been removed or covered by deposits. Here, targeted inoculation with AM fungi is an established method for supporting the establishment of pioneer plants and accelerating the succession process. Mycorrhizal plants show significantly higher survival rates and growth performance at such sites than non-inoculated control plants, as documented in numerous field trials.

In the context of climate adaptation, endomycorrhiza is gaining importance as a planning tool. Urban trees facing increasing drought stress benefit from a functioning mycorrhizal symbiosis, which improves their water uptake and ensures their nutrient supply in compacted soils. Planting concepts for “climate-resilient trees”—that is, tree species with increased drought tolerance such as Gleditsia triacanthos, Zelkova serrata, or Quercus pubescens—should take into account these species’ dependence on mycorrhizae and design soil preparation accordingly. Incidentally, Quercus species are both ectomycorrhizal and capable of forming arbuscular mycorrhizae, which helps explain their adaptability to different soil types.

For perennial plantings in public green spaces established on soils with poor structure, inoculation at the time of planting can shorten the establishment phase and reduce maintenance requirements in the first few years. In particular, species from drought-prone habitats—which are increasingly used in modern perennial landscaping, such as Salvia nemorosa, Echinacea purpurea, or Pennisetum species—are highly dependent on mycorrhizae and benefit from early fungal colonization. Integrating mycorrhiza promotion into maintenance strategies—for example, by avoiding phosphate-rich fertilizers and reducing fungicide use—is a simple and cost-effective way to improve the longevity and resilience of plantings.

Endomycorrhiza as the Foundation of Sustainable Vegetation Management

Arbuscular mycorrhiza is not an optional feature that can be included in planting plans at one’s discretion. It is a fundamental component of soil life on which the vast majority of terrestrial plants depend evolutionarily. Anyone who damages soils through compaction, phosphate overfertilization, or the use of biocides also damages the very foundation on which plants thrive. Conversely, promoting mycorrhizal activity through soil-conserving planning, appropriate fertilization, and targeted inoculation represents an investment in the long-term functionality of vegetated areas.

For landscape architects and open-space planners, this has a clear implication: soil quality and soil biology must be given equal weight as planning parameters alongside substrate selection, irrigation concepts, and species selection. Endomycorrhiza serves as both an indicator and a tool in this context. Soil that supports a rich population of AM fungi is generally living, well-structured, and functional. Soil that has lost this population sends a signal indicating degradation that goes beyond the mycorrhiza itself.

The growing availability of high-quality inoculant products, increasing research on fungus-friendly substrate mixtures, and the integration of soil biology into vegetation management guidelines demonstrate that this topic has become established in professional practice. What is still missing is the consistent incorporation of these findings into bidding standards, maintenance plans, and training programs. Endomycorrhiza is not a niche topic for soil ecologists, but rather core knowledge in modern vegetation engineering that should be integrated into every qualified plan for planting areas.