In Michael Phillips work ‘The Holistic Orchard’, he introduces the reader to this term: fungal duff. In the context of the orchard, we learn about the importance of managing the crown understory for the role it plays in tree growth and pest/disease resistance. For the urban forester and arborist, it is an interesting view on root zone management, and there may be potential for applications outside the orchard. 

According to Phillips, the fungal duff is the system of living plants, decaying organic material and fungal activity located within the drip line of the tree and just beyond. In the fungal duff system, it is necessary to forgo superficial aesthetics like neatly mowed grass in return for nurturing a functional network of plants, decaying organic matter and beneficial fungal partnerships interacting to provide optimal growing conditions for the fine feeder root flushes of the tree in spring and fall. These ephemeral root flushes are nurtured by the fungal duff. But the aesthetic is more wild than what we are accustomed to in a more conventional ornamental design. The wild aesthetic of such a system is more attractive anyway, creating a more diverse environment that is self servicing and sustainable.

“The case has been made that fungal-dominated biology supports orchard health. We enable subtleties that keep pest and disease pressure in balance by managing the ground beneath our tree and berries as fungal duff. Compost, deciduous wood chips, raked leaves, rotting hay and taproot plant allies set the scene,” (Phillips, 108).

Phillips is an orchardist, and he explains that in his orchard systems he aims to replicate a forest edge habitat. Habitat managers will recognize this as a soft edge, where annuals and perennials combine with fruit trees to create a transition zone between forest and meadow.

Regular mowing is an enemy of good fungal duff development. According to Phillips, mowing grass in the critical root zone of a tree can increase the root density of sod up to twenty times that of a taproot herb understory (think dandelions and comfrey). High levels of carbon dioxide build up from dense mats of mowed grass roots and discourage tree feeder roots from developing. High CO2 levels drive fine tree feeder roots deeper into the soil profile away from the organic soil/humus layer, away from where mycorrhizal relationships thrive in the upper humus layer. 

Phillips introduces us to the idea of ‘biological mowing.’ This mechanical cultural concept is all about timing our mowing applications in the critical root zone to coincide with fine feeder root development and fruit set in the spring, and again with the second flush of fine feeder root development in the fall. Mowing after fruit set in spring will encourage the ‘white root’ flushes to support fruit development due to more pore space in the humus of the root zone. Laying herbaceous material down as a mulch is another benefit of biological mowing, adding organic matter to the tree root system. Aside from supporting healthy feeder root development in the spring by encouraging more pore space in the soil, high grass and the right herbs under trees can play a role in suppressing disease pressure as well: high grass can discourage scab ascospores from releasing up into the canopy after a rain (110). Not only this, but high grass also encourages insect pests to refrain from migrating upward into the crown of trees.

Leading up to fruit set, not mowing supports biological fitness in the understroy: a ‘carbonized understory’ manifests in tall grasses and wildflowers. According to Phillips, soluable K levels are abundant from good mulching and composting practices. The un-mowed understory attracts and supports pollinators, which has an obvious effect fruit set and biodiversity.

“The mowing tool itself has relevance as well for this first orchard cut-and yes, once again, it’s for the benefit of fungal allies. Grasses and wildflowers are laid down with a scythe as drip line mulch, and thus not chopped to bits by a machine, in order to suppress understory growth in two respects. This single mowing causes root mass in perennial plants to shed just as the spring feeder root flush begins. This enhances access to new nutrient zones for the tree-with the help of mycorrhizal fungi, of course-for the purpose of sizing the fruit in the month ahead. The swaths of carbon-rich organic matter left behind by the swoosh of a sharp blade suppress the pace at which understory plants recover as well,” (Phillips, 110).

Scything grass will most likely not be the approach many landscape managers take in attempting to aid the fungal duff system, although there may be some rogue scythers out there. Scything aside, this critical root flush timings can offer windows for adding natural mulches in the form of remedial wood chips, or other forms of compost, especially during the fall feeder root flush when trees will be building up carbohydrate stores for bud development and the following year’s leaf and twig growth flush.

It is a misconception that all annual and perennials plants cause unnecessary competition for tree roots. Sure, a regularly mown lawn can be a real adversary. But Phillips gives us several examples of herbaceous plants that actually aid in creating pore space as well as organic return in the understory of a edge habitat: dandelions, taproot herbs like comfrey and even bulbs like daffodils. The latter actually prove disagreeable to voles, good information for an anyone planting or managing young trees that are susceptible to this type of critter damage. A list of plants in one of Phillips illustrations consists of the following polyculture: Asian pear, buffalo cherry, comfrey, lovage, horseradish, pea shrub, daffodil, Marguerite, and lemon balm; all topped with wood chip ground cover. Permaculturalists will recognize this as a plant guild. Arborists and horticulturalists will recognize it as an illustration of plant diversity. It is a picture of fungal duff in action, an ongoing cycle of growth and nutrients cycled back into the the root zone.

There are no exact formulas for the fungal duff recipe. Countless approaches can be investigated depending on the type of site being managed. Instead it is a concept for encouraging fungal based biology in the critical root zone of trees. Instead of mindlessly removing plants through exhaustive mechanical practices, we can utilize them in an educated and selective way, by allowing them to grow and add to the soil food web.  We know that organic matter return to the soil is a crucial ingredient for encouraging the institution of mycorrhizal fungi. The timing of our cultural practices such as mowing and mulching can influence the sustenance of that relationship in a major way. Understanding and implementing the fungal duff is about a process of working smarter, not harder.