The practice of companion planting involves strategically placing different species near each other to create a mutually beneficial environment. However, not all neighboring relationships in an orchard are supportive, and some pairings can be detrimental to the health and productivity of one or both trees. Incompatibility can arise from direct chemical interference, the spread of shared pathogens, or intense competition for specific resources in the soil. Avoiding these planting errors is fundamental for maintaining a healthy and successful orchard environment.
Specific Incompatible Pairings
One severe incompatibility involves the black walnut (Juglans nigra), which should be isolated from almost all other fruit trees. Walnuts produce a toxic compound detrimental to the growth of many species, including apples and apricots, which may experience stunted growth or death if planted within the extensive root zone. This incompatibility extends beyond the canopy drip line, often requiring a separation distance of 50 feet or more to prevent root contact and chemical exposure.
Spacing is also a practical element of incompatibility, where highly vigorous species can out-compete smaller, less established trees. Planting a standard-sized apple tree too close to a dwarf plum or cherry will starve the stone fruit of sunlight and root space. Grouping closely related species, such as mixing varieties of peaches, plums, and cherries in a dense block, creates an easy “disease bridge” that allows pathogens to spread rapidly through the orchard.
Allelopathy Chemical Warfare Between Roots
A powerful biological mechanism driving incompatibility is allelopathy, where a plant releases biochemicals into the environment that inhibit the germination or growth of surrounding plants. The most prominent example in fruit production is the black walnut’s use of the chemical juglone to suppress competitors. Juglone is found in all parts of the walnut tree, including the roots, leaves, and fruit husks.
The tree stores this compound in a non-toxic form called hydrojuglone. When a walnut root is damaged or when the compound is shed into the soil, the hydrojuglone is oxidized into its highly toxic form, juglone. Once in the soil, juglone attacks the root systems of susceptible plants by interfering with cellular respiration and damaging the root hairs responsible for absorbing water and nutrients.
Shared Pests and Disease Vulnerabilities
Planting trees from the same botanical family in close proximity creates an environment for the rapid spread of shared pests and diseases. This lack of biological diversity allows a single pathogen or insect to thrive on a continuous food source. For example, the Prunus genus (peaches, plums, and cherries) shares a susceptibility to fungal diseases like brown rot and specific root rots. If one tree becomes infected, the proximity ensures the quick transfer of fungal spores or bacterial cells to its neighbors.
A specific instance of shared vulnerability involves the cedar-apple rust fungus, which requires two separate hosts to complete its life cycle. Planting apple or crabapple trees (Malus species) near junipers (Juniperus species) is discouraged because the juniper acts as the alternate host. This allows the fungus to overwinter and produce spores that infect the pome fruit in the spring, reducing tree vigor. Spacing different fruit families throughout the orchard acts as a biological buffer, slowing the progression of species-specific threats.
Conflicting Soil and Water Requirements
Incompatibility can also result from incompatible environmental needs that create resource competition. Fruit trees have varying tolerances for soil pH, drainage, and moisture levels, and mixing species with opposing needs results in poor health for at least one partner. For instance, most stone fruits, such as peaches, nectarines, and cherries, are intolerant of poorly draining or wet soils.
Planting these species in a location that favors a water-tolerant pear or quince tree will lead to root rot and eventual decline for the stone fruit. Similarly, most common fruit trees thrive in a slightly acidic to neutral soil pH, typically between 6.0 and 7.5. However, certain specialty fruits, like blueberries, require highly acidic soil (pH 4.5–5.5) to properly absorb necessary micronutrients. Placing a blueberry bush next to a cherry tree, which prefers higher pH, will result in one or both suffering from nutrient deficiencies due to the soil’s inability to satisfy two distinct requirements.