From a botanical perspective, sweet peas (Lathyrus odoratus) are themselves nitrogen-fixing plants. They form a symbiotic relationship with Rhizobia bacteria in the soil, which convert atmospheric nitrogen into a form usable by plants. This process enriches the soil. Therefore, planting sweet peas with heavy feeders that require abundant nitrogen is highly beneficial. Ideal companions in this category are leafy greens like spinach (Spinacia oleracea) and lettuce (Lactuca sativa), or brassicas such as cabbage (Brassica oleracea) and kale (Brassica oleracea var. sabellica). These plants thrive on the extra nitrogen provided by the sweet peas, leading to lusher, more vigorous growth without competing for this crucial nutrient.
The growth habit of sweet peas is a key consideration. They are vigorous climbers, requiring vertical support. This creates a unique opportunity for companion planting. Low-growing, shade-tolerant species can benefit from the dappled light and microclimate created by the sweet pea foliage. Plants like radishes (Raphanus sativus) and carrots (Daucus carota subsp. sativus) are excellent choices. Their roots develop better in slightly cooler soil, and the partial shade provided by the sweet pea canopy can help prevent them from bolting (prematurely flowering) in warmer weather, allowing for a longer harvest window for these root vegetables.
Companion planting is also a strategic defense mechanism. Sweet peas can be susceptible to pests like aphids. Interplanting with aromatic herbs can create a confusing scent mask, deterring pests from locating their target. Strongly-scented herbs such as rosemary (Salvia rosmarinus) and oregano (Origanum vulgare) are effective for this purpose. Furthermore, to bolster the garden's ecosystem, planting companions that attract beneficial insects is wise. Alyssum (Lobularia maritima) draws in hoverflies, whose larvae are voracious predators of aphids. Similarly, nectar-rich flowers like calendula (Calendula officinalis) attract ladybugs and parasitic wasps, which provide natural pest control for the sweet peas and surrounding plants.
It is equally important to understand which plants are poor companions. The most significant group to avoid are alliums, including onions (Allium cepa), garlic (Allium sativum), and leeks (Allium porrum). These plants are believed to exude biochemicals that can inhibit the growth of legumes like sweet peas and peas, potentially stunting their development. Furthermore, it is advisable to avoid planting other vigorous climbers or tall, dense plants too close. Species like pole beans or sunflowers would compete intensely with sweet peas for sunlight and physical support, leading to reduced growth and flowering for all plants involved.
