Companion planting involves strategically placing different types of plants close to one another to create a mutually beneficial environment. This technique is effective in a raised bed where space and resources are limited, requiring careful planning. Understanding these symbiotic relationships allows a gardener to leverage interactions for better yields and healthier tomatoes. The goal is to maximize every square inch by encouraging plants to support each other’s growth, rather than compete for nutrients and water.
Companions for Pest Management and Protection
Specific companion plants offer a biological defense mechanism by repelling harmful insects or masking the tomato plant’s scent. French marigolds (Tagetes patula) are a documented example, as their roots release alpha-terthienyl. This natural nematicide suppresses root-knot nematodes, microscopic worms that attack and damage tomato roots.
The strong, aromatic oils produced by certain herbs act as a natural camouflage, confusing pests that rely on scent to locate tomato plants. Basil is a popular companion, as its odor helps deter the adult moth of the tomato hornworm. Similarly, the pungent scent from the Allium family, such as chives or onions, can repel aphids and other soft-bodied insects.
Other flowers and herbs attract beneficial insects that prey on or parasitize common tomato pests. Borage attracts bees for pollination while also drawing in small wasps that target tomato hornworms. Sweet alyssum or dill, when allowed to flower, provide nectar and pollen for beneficial insects like braconid wasps and ladybugs, which help control aphids and caterpillar pests. Nasturtiums serve as a trap crop, luring aphids away from the tomato foliage.
Companions for Soil Health and Growth Support
Certain plants are included in a raised bed to improve soil structure, enhance nutrient availability, or modify the microclimate around the tomato roots. Tomatoes are heavy feeders, demanding high levels of nitrogen, phosphorus, and potassium throughout the growing season. Nitrogen-fixing plants, such as beans or clovers, enter a symbiotic relationship with Rhizobia bacteria, which convert atmospheric nitrogen into a usable form.
The nitrogen fixed by legumes is not immediately available to the tomato, but its benefit is released when the roots or plant material decompose. Low-growing plants like white clover can be used as a living ground cover, helping to replenish nitrogen and keep the soil cool, which conserves moisture. Deep-rooted plants, known as dynamic accumulators, draw up minerals from deeper soil layers that shallow-rooted tomatoes cannot reach.
Comfrey is a dynamic accumulator, with deep taproots that mine potassium, an element essential for tomato flowering and fruiting. When the leaves are cut and dropped around the tomato base, they act as a nutrient-rich mulch, slowly releasing sequestered minerals back into the topsoil. This organic material helps shade the soil surface, reducing water evaporation and keeping the root zone consistently moist.
Plants to Avoid Growing Near Tomatoes
Planting certain species near tomatoes can be detrimental, leading to shared diseases, aggressive competition, or chemical inhibition. The primary concern involves plants from the same nightshade family (Solanaceae), such as potatoes and eggplants. These plants are susceptible to the same pests and soil-borne diseases, including early blight and late blight. Planting them together can rapidly multiply the devastation across the entire bed.
Other plants should be avoided due to aggressive competition for the limited resources in a raised bed. Large brassicas, including cabbage, broccoli, and Brussels sprouts, are heavy feeders that compete intensely with tomatoes for nitrogen and other soil nutrients. Corn should also be kept separate, as its height can quickly shade out sun-loving tomatoes. Both crops are targeted by the same pest, the corn earworm.
Some plants release allelopathic chemicals that inhibit the growth of nearby species. Fennel is a well-known allelopathic plant that should be grown in isolation or avoided entirely, as its compounds can stunt tomato growth. Even beneficial companions, like dill, can become harmful once they mature, potentially attracting tomato hornworms and developing allelopathic properties that suppress growth.
Maximizing Space in a Raised Bed
The finite volume of a raised bed requires strategic planting techniques to ensure high yields without overcrowding, which can lead to poor air circulation and disease. Vertical gardening is a primary strategy, involving the use of trellises, cages, or stakes to train indeterminate tomato varieties to grow upward. This vertical support minimizes the plant’s footprint on the soil surface, freeing up space for companion plants underneath.
Intensive planting principles, often referred to as square-foot gardening, allow plants to be placed closer together than in traditional row gardening. For supported indeterminate tomatoes, spacing is typically 1.5 to 2 feet apart, rather than the 3 to 4 feet required for unsupported plants. This close spacing is possible because vertical support reduces the canopy spread, but careful pruning is necessary to maintain adequate airflow.
Choosing compact, determinate tomato varieties is a space-saving measure, as they grow to a predetermined size and require less extensive support. The remaining ground space can be filled with low-growing companion herbs or leafy greens, planting them in areas shaded by the mature tomato canopy. This layering of plants utilizes different vertical strata, increasing the total harvestable area within the raised bed.