Creating a plant guild beneath fruit trees, known as underplanting, is a strategy that goes beyond simple ground cover. This method involves the ecological integration of multiple species to create a mutually beneficial system for the primary fruit tree. Understory plants are selected for specific functions that contribute to the health and productivity of the entire ecosystem. Establishing a functional understory helps gardeners reduce the need for external inputs like fertilizers and pesticides. The ultimate goal is to foster a self-regulating environment where the fruit tree benefits from improved soil structure and natural pest management.
Soil Health and Nutrient Cycling
A core function of a fruit tree guild is to improve the physical and chemical composition of the soil, directly benefiting the tree’s root system. This is often achieved by planting species that perform two distinct functions: nitrogen fixation and dynamic accumulation of minerals.
Nitrogen-fixing plants, primarily legumes, work in a symbiotic relationship with Rhizobium bacteria in their root nodules. These bacteria convert atmospheric nitrogen gas into ammonium, a usable form of nitrogen. Plants like clover, vetch, and perennial peas release this nitrogen into the soil as their roots and leaves decompose. This process effectively provides a slow-release natural fertilizer for the neighboring fruit tree. White clover, for example, is a common herbaceous choice that can significantly reduce the need for synthetic nitrogen fertilizers.
Dynamic accumulators are plants with deep taproots that act as “mineral miners,” drawing up nutrients from the subsoil that are typically beyond the reach of the fruit tree’s shallower feeder roots. When the leaves of these plants die back or are cut and left as mulch—a technique known as “chop-and-drop”—they decompose on the surface, making those deep-mined minerals available in the topsoil. Comfrey is a highly valued dynamic accumulator, known to concentrate potassium and calcium, both of which are important for fruit development and storage. Similarly, dandelion and chicory have deep roots that bring up calcium, magnesium, and phosphorus.
Managing Pests and Pollinators
The diverse understory can also be engineered to manage the insect population, reducing pest pressure and increasing fruit set. This is accomplished through two opposing strategies: planting aromatic repellents and flowering attractors.
Repellent plants release strong volatile compounds that confuse, mask, or deter common fruit tree pests. The aromatic oils of alliums like chives and garlic can help repel insects such as the plum curculio. Mint, with its intense scent, is sometimes used to deter pests like the codling moth, though its aggressive spreading nature requires careful management. Aromatic herbs like lavender and sage are also known to confuse pests that rely on scent to locate their host trees.
Attractor plants serve as “insectary plants,” providing a continuous source of nectar and pollen to support beneficial insects, including pollinators and natural pest predators. Tiny-flowered plants, particularly those in the carrot family (Umbelliferae), are effective at attracting small predatory wasps, hoverflies, and lacewings. Dill, fennel, and yarrow, with their accessible flower structures, provide a landing platform and food source for these beneficials. These beneficial insects then prey on aphids, caterpillars, and other common fruit tree pests. Planting a variety of these flowers ensures that beneficial insects have resources throughout the season, establishing a stable, resident population.
Practical Considerations for Successful Underplanting
Transitioning from theory to successful implementation requires careful attention to potential negative interactions, particularly root competition and light availability. The dense network of fruit tree feeder roots occupies the top 12 to 18 inches of soil, making resource competition a significant concern, especially for young or dwarf trees.
To mitigate root competition, the planting distance must be considered. Aggressive runners like mint and oregano should either be avoided or contained in submerged pots or with root barriers. Root crops like potatoes, carrots, and beets are also problematic because their growth and harvesting can disturb the shallow, sensitive root zone of the fruit tree. A common strategy is to leave a small, mulched buffer zone immediately around the trunk before the underplanting begins.
The canopy of a mature fruit tree reduces the amount of sunlight reaching the understory, meaning plants must be selected for their shade tolerance. Furthermore, understory plants must be suitable for the existing soil moisture conditions, as excessive competition for water can stress the tree. Aggressive weeds and turfgrasses should be suppressed with an organic mulch layer to conserve moisture and prevent them from outcompeting the tree for nutrients. Finally, certain allelopathic species must be strictly avoided, such as the black walnut, which releases the toxic chemical juglone through its roots, leaves, and nuts, inhibiting the growth of many fruit trees.