Companion planting is a horticultural practice where different plant species are grown in close proximity to maximize health and yield. This strategy is particularly valuable for asparagus, a perennial crop that occupies the same garden space for decades. Since the asparagus crown and root system remain undisturbed, introducing compatible annuals and herbs creates a stable, mutually beneficial ecosystem. Chosen companions can deter pests, improve soil quality, and suppress weeds, contributing to the productivity of the asparagus bed.
Beneficial Companion Plants
One of the most well-documented beneficial pairings is the relationship between asparagus and nightshade family members, particularly tomatoes. Tomatoes emit solanine, a chemical compound that acts as a natural deterrent against the destructive asparagus beetle. In a reciprocal benefit, asparagus releases a compound that repels root-knot nematodes, which attack the root systems of tomato plants. This creates a symbiotic relationship where both species protect the other from specific soil-borne and foliar threats.
Aromatic herbs and certain flowering plants offer advantages to the asparagus patch. Herbs like basil and parsley release strong aromatic compounds that confuse and repel the asparagus beetle. Basil, in particular, attracts beneficial predatory insects, such as tiny parasitic wasps, which prey on the asparagus beetle and other harmful garden insects. The shallow root systems of these herbs prevent them from competing with the deep asparagus crowns for water and nutrients.
Flowers such as marigolds and nasturtiums function as excellent pest deterrents and beneficial insect attractors. Marigolds release substances into the soil that suppress root-knot nematodes. Nasturtiums serve as a trap crop, drawing in pests like aphids and providing a food source for natural predators like ladybugs. Low-growing plants like strawberries also act as a living mulch, helping to suppress weed growth and conserve soil moisture around the asparagus crowns.
Plants That Inhibit Asparagus Growth
Certain plants should be kept away from the asparagus bed because they compete aggressively or introduce harmful pathogens. The entire Allium family, including garlic, onions, leeks, and chives, is a poor companion for asparagus. These plants are heavy feeders, competing with established asparagus crowns for nutrients and resulting in stunted growth for the perennial crop.
Growing alliums near asparagus is discouraged because they can host or introduce Fusarium spp., a fungal pathogen that causes crown and root rot. Once this disease is present, it can significantly reduce the long-term viability and productivity of the asparagus bed. Furthermore, planting deep-rooted alliums like garlic can physically disrupt the delicate structure of the asparagus crowns during their annual growth cycle.
Root vegetables that require significant soil disturbance or have aggressive, deep root systems should be avoided. Potatoes are a poor choice because their tubers develop deep in the soil, competing directly with the asparagus crown for space and moisture. Harvesting potatoes involves aggressive digging, which is likely to damage the permanent root structure of the asparagus. Similarly, harvesting carrots requires pulling from the soil, disturbing the undisturbed environment that asparagus prefers for long-term health.
Placement Considerations for Companion Crops
Integrating companion plants requires managing their physical arrangement to avoid competition with the permanent asparagus crown. Since asparagus has an extensive, deep root system, it is best to select shallow-rooted annual companions. Herbs like basil, parsley, and dill can be planted directly between the rows or near the base of the asparagus plants, utilizing the surface soil layer without interfering with the deep crown.
Taller, deeper-rooted companions, such as tomatoes, should be planted strategically on the perimeter or at the end of the asparagus rows. This placement minimizes direct root competition while still allowing the beneficial solanine compounds to deter pests in the immediate area. Placing these heavier feeders slightly farther away ensures they do not deplete the nutrients immediately surrounding the perennial crown.
Light management is another factor, as asparagus requires full sun exposure for its fern-like foliage to photosynthesize effectively throughout the summer. It should not be shaded by other plants, meaning extremely tall companions must be positioned carefully to the north of the bed. Conversely, the mature asparagus fern can provide partial shade for heat-sensitive, low-growing companions like lettuce or cilantro during the hottest months. Maintaining the proper distance ensures that the asparagus receives the necessary light to store energy for the next season’s harvest.