Nasturtiums are celebrated for their vibrant appearance and practical uses, as both the flowers and leaves are edible. These popular annuals are often introduced into vegetable gardens as companion plants. However, their vigorous growth means that not all pairings are beneficial. Careful consideration of nasturtium’s growth habits, resource requirements, and pest interactions is necessary, as planting them alongside the wrong species can undermine a garden’s health and productivity.
Plants That Suffer From Intense Resource Competition
Nasturtiums possess a vigorous growth habit and a deep, spreading root system, allowing them to thrive even in less fertile soil. This adaptability makes them formidable competitors for nearby plants, especially those that are heavy feeders or slow to establish. When given rich soil, nasturtiums prioritize leaf growth and rapidly consume available water and nutrients.
Tuber crops, such as potatoes, are sensitive to this competition. Both nasturtiums and potatoes have similar nutritional needs and dense root zones, leading to a fight for resources like phosphorus and potassium, often resulting in smaller yields. Legumes like peas and beans, which rely on soil microbes to fix nitrogen, also struggle when nasturtiums aggressively deplete surrounding nutrients.
Young seedlings and delicate herbs with shallow root systems are easily overwhelmed by nasturtium’s fast-spreading nature. The dense foliage quickly shades out smaller plants, reducing their ability to photosynthesize and compete for soil resources. To avoid stunted growth and poor yields, separate nasturtiums from any crop that requires consistently rich soil or high nitrogen supplementation.
Crops Vulnerable to Nasturtium’s Trap Cropping Effect
Nasturtiums are famously used as a “trap crop” because they attract specific pests, primarily aphids and whiteflies, away from valuable vegetables. This strategy works by luring insects to the nasturtium foliage, which can then be treated or removed. However, if nasturtiums are planted too closely to equally susceptible crops, the intended diversion effect fails and can become an amplification problem.
The most problematic pairing involves the Brassica family, including kale, broccoli, cauliflower, and cabbage. These crops are highly vulnerable to aphids, the same pests nasturtiums attract in large numbers. Instead of drawing aphids away, nasturtiums act as a high-density reservoir, allowing pest populations to explode and spill over onto adjacent Brassicas.
Nasturtiums create a localized pest hotspot, providing an ideal breeding ground for insects that migrate to the main crop. For this strategy to work effectively, the trap crop must be planted at a sufficient distance from the protected crop. Alternatively, the nasturtium plants must be regularly monitored and destroyed when they become heavily infested. Planting nasturtiums directly alongside sensitive leafy greens or young brassica plants risks concentrating the infestation.
Managing Physical Space and Shade Conflicts
Nasturtiums come in two main types: bushy (mounding) and vining (trailing). Vining varieties are prone to creating physical conflicts in the garden. The long, sprawling stems can rapidly cover the ground or climb nearby supports, leading to the smothering of low-growing plants. Slow-maturing crops, such as carrots or parsnips, can be completely shaded out by the nasturtium’s broad leaves before they establish.
Dense foliage significantly reduces air circulation around neighboring plants. This creates a humid, stagnant microclimate conducive to the development and spread of fungal diseases, such as powdery mildew. Planting nasturtiums near other vining crops, like squash or cucumbers, can exacerbate this issue. The combined dense canopy traps moisture and restricts airflow, leading to widespread disease.
A specific incompatibility arises with fennel, which releases chemicals into the soil that inhibit the growth of many other plant species, including nasturtiums. While the nasturtium suffers from this pairing, it illustrates a fundamental conflict in shared space. To maintain plant health and prevent disease, provide ample space between nasturtiums and any plant that requires intense sunlight or suffers from high humidity.