3 Herbs You Should Avoid Growing Near Tomatoes In The Garden

Gardeners and houseplant parents have long thought of our plants as being more than, well, plants. Sometimes we treat them like they have feelings and personalities. In fact, plants have been known to have a sense of smell, and you've likely heard that they respond well to music. Although the verdict is mixed on those findings, plants do have preferences around who they hang out with. 

As you're planning your garden this year, be mindful of which plants, including herbs, shouldn't be too close to your tomatoes. This guidance is based on companion planting and how your garden benefits or suffers when certain plants are grown near each other. Herbs like fennel, dill, and rosemary don't work well with tomato plants, and can leave you with a stunted plant and smaller harvest. 

One main reason for this is allelopathy, which refers to the release of chemicals (allelochemicals) from plants that can either be harmful or beneficial to surrounding plants. Allelochemicals are stored in nearly every part of a plant, including its roots and seeds, but are primarily found in leaves and roots. The chemicals can be transported through the air or the soil, and even through fallen decomposing leaves. 

The relationship between tomato and fennel plants is a perfect example of allelopathy. Growing fennel in close proximity to tomatoes affects seed germination, fruit production, and growth habits. If your tomato plant produces fruit, they'll likely be smaller than if you'd grown the fennel elsewhere. If you choose to grow fennel near other plants, consider doing so in a pot, since allelochemicals from fennel are emitted from its roots. But be aware that even when planted in a pot, this herb is known to attract aphids, making your tomato plant susceptible to infestation. 

Mature dill plants also have a similar effect on tomato plants. A 2019 study published in "Ecologia Balkanica" discovered that dill seeds possess allelochemicals that result in suppressed seed germination, a weaker root system, and smaller seedling size among tomato plants. In addition, dill and tomato plants both crave nutrient-rich environments, and sharing a close planting plot may mean insufficient resources for everyone. The two plants also compete for height, with the tomato plant's sprawling limbs potentially blocking the dill's access to full sun and, vice versa, the dill's wispy fronds creating shade over the tomatoes. 

The dynamic between rosemary and tomatoes is a bit more complicated, with mixed opinions about their compatibility. These two plants have different needs, creating challenges for both plants to flourish in the same space. Rosemary shrubs prefer dry soil and have low-maintenance nutrient needs compared with tomato plants, which require consistent moisture and fertile soil. 

However, according to a 2019 study in Scientia Horticulturae, applying rosemary oil to the leaves and soil of tomato seedlings actually boosted root growth, which in turn improved the plant's ability to take in nutrients. The strong scent from a rosemary shrub may also overpower that of an adjacent tomato plant, making it invisible to tomato-loving pests and, therefore, increasing its chances to thrive. With all of this in mind, growing tomatoes in its own pot — with its preferred soil and watering needs — next to a rosemary bush allows you to reap the benefits of this relationship.

So, what can you plant near your tomatoes? There are many reasons you'll want to add basil to your garden, one of them being that it is an ideal companion for tomatoes, in the garden and on your plate. This tasty herb complements tomatoes by helping them to produce more fruit and develop stronger roots, stems, and leaves. They effectively repel pests like thrips that target tomatoes and attract bees, which are beneficial to a tomato plant's growth and fruit production. If you're looking for another beneficial companion plant, consider planting parsley near your tomatoes.