Companion planting involves growing specific plants near one another for mutual benefits, such as pest control or improved growth. Planting dill with tomatoes can be highly beneficial, but this relationship depends entirely on careful timing and management. Successful interplanting relies on understanding the distinct phases of dill’s growth and how they impact the sun-loving tomato plant.
Compatibility: The Primary Reasons for Planting Dill with Tomatoes
Dill acts as an insectary plant, attracting a range of beneficial insects to the garden. The herb’s distinctive umbrella-shaped flower heads, known as umbels, attract tiny predatory species like parasitic wasps and ladybugs. These insects are natural predators of common tomato pests, including aphids and various caterpillars.
Dill is particularly valued for controlling the tomato hornworm, a destructive pest. Dill can function as a “trap crop,” with hornworms sometimes preferring its foliage, making them easier to spot and remove. The umbel flowers also attract parasitic wasps, which lay eggs on or inside the hornworm larvae, neutralizing the pest population.
The early-stage dill plant has a light, airy structure that minimizes competition for sunlight near young tomato seedlings. This initial growth provides pest deterrence and attracts beneficial insects without creating significant shade or demanding excessive resources. This early companionship offers ecological support before the dill matures.
The Critical Timing Issue: Dill’s Growth Stages and Tomato Health
The beneficial relationship between dill and tomatoes is temporary, becoming competitive as the dill plant matures. Dill is an annual herb that quickly reaches maturity, often accelerated by warmer temperatures, causing it to “bolt.” Bolting occurs when the plant shifts its energy from producing foliage to forming flowers and seeds.
Once dill bolts, it becomes a tall, dense plant. This rapid increase in height and leaf mass creates significant shade, directly competing with the tomato for sunlight. Since tomatoes require abundant sunlight for fruit production, shading can impede the tomato plant’s growth and ultimately reduce its overall yield.
Competition also occurs below the soil surface, where mature dill develops a deep taproot system. This aggressive root growth competes directly with tomato roots for water and nutrients in the immediate vicinity. Because both plants require consistent moisture and feeding, this root competition can stunt the growth of the developing tomato.
Mature dill may also release biochemicals into the soil, a phenomenon known as allelopathy, that can inhibit the growth of nearby plants. Laboratory studies using concentrated dill extract have demonstrated an inhibitory effect on tomato seedling growth, particularly on root elongation and shoot length. This inhibitory effect is most pronounced as the dill plant reaches full maturity.
Practical Strategies for Successful Interplanting
The initial benefits of planting dill with tomatoes can be secured while mitigating later risks through specific management techniques. Proper spacing is a fundamental strategy, requiring dill to be planted far enough away to minimize root and light competition as the plants mature. Plant the dill about 12 to 24 inches away from the tomato base; this allows beneficial insects to access the flowers while separating the competing root zones.
Gardeners can employ succession planting, which involves planting dill early alongside young tomatoes, and then proactively removing the dill as it begins to bolt. Removing the dill before it becomes tall and begins to set seed prevents shading and intense resource competition that can harm the tomato crop. This allows tomatoes to benefit from early pest control without suffering the negative effects of the mature herb.
Aggressive pruning and harvesting of dill foliage can also delay bolting and manage its size. Cutting back the dill regularly prevents excessive height, reducing its shading effect on the tomatoes. This method allows the gardener to continue harvesting the herb while delaying the growth phase that is most detrimental to its companion.
To eliminate root competition entirely, grow the dill in separate containers placed near the tomato plants. Dill thrives in pots, provided the container is at least 12 inches deep to accommodate its taproot. This method allows dill to attract beneficial insects nearby, but the container prevents its roots from directly competing with the tomato’s established root system for water and nutrients.