Alfalfa (Medicago sativa) is a perennial herbaceous plant cultivated globally, primarily for its high-protein forage value. Because of its long lifespan, strategic planting decisions are crucial for long-term productivity. Companion planting involves growing different plant species in close proximity to encourage mutual benefits, such as improved nutrient uptake or pest management. Understanding these synergistic and antagonistic relationships is key to optimizing field health and maximizing agricultural output. This article explores how to select beneficial partners for alfalfa and identifies the species that should be avoided.
Alfalfa’s Contribution to Soil Health
Alfalfa possesses a remarkable root system that provides substantial benefits to soil structure. Its deep taproot can penetrate the subsoil to depths of 15 to 20 feet, accessing water and nutrients unavailable to shallow-rooted annual crops. This characteristic breaks up compacted soil layers, improving soil aeration and water infiltration. The complex root network also stabilizes the soil, helping to reduce surface erosion from both wind and water, especially on sloping ground.
Alfalfa is also a highly effective source of natural soil fertility through symbiotic nitrogen (N) fixation. Specialized Rhizobia bacteria reside in nodules on the roots, converting atmospheric nitrogen gas into a plant-usable form. This fixed nitrogen is released into the soil as the roots and crowns decompose, providing a fertility credit for subsequent crops grown in rotation. Alfalfa enhances organic matter and creates beneficial pathways that future crop roots can easily follow.
Beneficial Companion Plant Categories
The most widely utilized companion plants for alfalfa are cool-season grasses, forming an alfalfa-grass mixture. Grasses like orchardgrass and smooth bromegrass offer structural support and improved erosion control during the vulnerable establishment phase. Their fibrous root systems occupy the upper soil profile, capturing surface nutrients and moisture. This complements the deep-reaching alfalfa taproot, allowing for efficient utilization of soil resources across different depths.
Selecting grass varieties that mature later allows for better synchronization with alfalfa’s harvest timing, ensuring high forage quality. The inclusion of grass stems aids in faster drying rates of the harvested forage. It also protects the alfalfa crowns from damage caused by heavy field traffic and offers protection against frost heaving.
Small grains, such as oats, barley, and wheat, are often used as a cover crop during initial establishment. These cereals provide rapid canopy cover, suppressing weeds and shielding young alfalfa seedlings from soil erosion in the first year. This strategy is valuable on sandy or highly erosive soils where alfalfa’s slow top growth leaves the ground exposed. To minimize competition, these companions must be seeded at a low density and harvested early, ideally at the boot stage before grain formation begins.
In garden or small-scale settings, flowering and herbaceous plants can manage pest populations. Plants like nasturtium function as a trap crop, attracting pests such as aphids away from the alfalfa while drawing in beneficial predatory insects. Herbs, including lemon balm, produce compounds that deter common garden pests. This approach leverages biodiversity to create a more resilient micro-ecosystem and naturally disrupt pest colonization.
Antagonistic Plants and Specific Avoidances
A primary concern when managing alfalfa stands is autotoxicity, a form of allelopathy where the plant inhibits the growth of its own species. Alfalfa releases water-soluble inhibitory substances from its roots and leaves that severely reduce the germination and growth of new seedlings. This chemical effect means that re-seeding alfalfa directly into an existing or recently terminated field will likely result in poor stand establishment and reduced yields.
New alfalfa plants established close to older plants, within about eight inches, will likely fail or show significant stunting. To successfully re-establish alfalfa, it is recommended to wait for a period, often a year or more, and rotate the field with a non-legume crop, such as corn or a grass species. This rotation allows time for the autotoxic compounds to degrade and leach out of the soil profile.
Certain plants should be avoided because they aggressively compete for shallow nutrients and sunlight. If small grain companions like oats or barley are seeded at high rates or allowed to mature for grain, they transition from a beneficial cover to a severe competitor. This unmanaged competition robs the young alfalfa of light, water, and essential nutrients, resulting in thin, weak stands that struggle to persist. Applying nitrogen fertilizer in these mixed stands should also be avoided, as it disproportionately stimulates the growth of the competing grass, further disadvantaging the alfalfa.
Some plant families, including specific brassicas like rapeseed, can exhibit allelopathic effects that may inhibit alfalfa growth and should be used with caution. Additionally, specific clover varieties can act as disease vectors or share susceptibility to the same pests that target alfalfa. Interplanting species that share common diseases or insect pests creates a bridge for pathogens, increasing the risk of widespread infestation or infection. Careful selection is necessary to prevent the buildup of pest and disease pressure that could shorten the lifespan of the alfalfa crop.