The worst tomato companions can do more than just compete for space; they can actively sabotage your harvest, invite devastating diseases, attract destructive pests, and even poison the soil around your prized plants. For every gardener who dreams of pulling plump, sun-ripened tomatoes from the vine, understanding the principles of companion planting is a game-changer. This ancient horticultural practice involves pairing plants that mutually benefit one another, creating a thriving, self-sustaining ecosystem right in your backyard. But just as important as knowing what to plant together is knowing what not to.
This concept, often called antagonistic planting, is the crucial other half of the equation. Certain plants are simply bad neighbors. They engage in underground chemical warfare, host the same blights and mildews, or compete so fiercely for resources that one or both will fail to thrive. Ignoring these hostile relationships is a common mistake that can lead to a summer of frustration, with stunted growth, yellowing leaves, and a disappointing yield.
To help you cultivate the healthiest, most productive tomato patch possible, we’ve compiled a definitive list of the 11 worst companions for your tomatoes. By understanding why these pairings fail, you can design a garden that works in harmony, ensuring your tomatoes have the best possible chance to shine.
1. The Cabbage Family (Brassicas)
This is a large and popular family of vegetables, including broccoli, cauliflower, cabbage, Brussels sprouts, kale, and kohlrabi. While they are staples in many gardens, they should be given a wide berth from your tomato plants.
Why they are poor companions: The primary issue is resource competition. Brassicas are incredibly “heavy feeders,” meaning they require a significant amount of nutrients from the soil, particularly nitrogen and calcium, to produce their dense heads and leaves. Tomatoes are also notoriously hungry plants. Planting them together creates a fierce, invisible battle for food beneath the soil. This intense competition often results in stunted growth and reduced fruit production for your tomatoes, which may also become more susceptible to issues like blossom end rot due to calcium deficiency.
What you’ll see: Your tomato plants might look smaller than expected, with pale or yellowing leaves. Fruit development may be slow or the tomatoes may be smaller than their variety’s potential.
A better choice: Plant your brassicas in a separate bed or at the opposite end of the garden. Give your tomatoes companions like basil or borage, which don’t compete for the same resources.
2. Fennel
Fennel is a beautiful, aromatic herb with feathery leaves and a delicious anise-like flavor. Unfortunately, its beauty masks a dark secret: it is one of the most unfriendly plants in the entire garden and a notorious tomato antagonist.
Why it’s a poor companion: Fennel practices a form of biological warfare known as allelopathy. It excretes a substance from its roots (anethole) that inhibits the growth of most other plants around it, and tomatoes are particularly sensitive to this chemical. It actively suppresses seed germination and stunts the development of nearby plants. In short, fennel doesn’t like to share its space with anyone.
What you’ll see: Tomato plants near fennel will exhibit severely stunted growth, appearing weak and spindly. They will produce very little, if any, fruit.
A better choice: Give fennel its own isolated pot or a lonely corner of the garden where it can’t bully its neighbors. For a beneficial herb near tomatoes, plant parsley or chives instead.
3. Corn
A classic summer pairing on the dinner plate, corn and tomatoes are a disastrous duo in the garden bed. This is a common mistake for new gardeners who envision a picturesque vegetable patch.
Why they are poor companions: This rivalry exists on two fronts: pests and resources. First, both plants are hosts to the exact same and very destructive pest: the corn earworm, which is also known as the tomato fruitworm. Planting them together is like setting up a buffet and sending out a formal invitation to these hungry caterpillars. They will happily move from your corn silks to burrow into your ripening tomatoes. Second, both are tall, sun-loving, and heavy-feeding plants, leading to a battle for sunlight, water, and nutrients.
What you’ll see: Holes bored into your developing and ripe tomatoes, often surrounded by frass (caterpillar droppings). You might also notice your tomato plants leaning or struggling as they compete for sunlight blocked by the towering corn stalks.
A better choice: Separate these two crops as much as possible. Plant tomatoes near shorter companions like carrots and lettuce that won’t compete for sunlight.
4. Potatoes
While they are both members of the nightshade family (Solanaceae), planting tomatoes and potatoes together is one of the riskiest moves a gardener can make. This familial tie is precisely what makes them such terrible neighbors.
Why they are poor companions: They are both highly susceptible to the same devastating fungal diseases, particularly early blight and late blight. Late blight (Phytophthora infestans) is the infamous pathogen behind the Irish Potato Famine and can wipe out an entire crop of both potatoes and tomatoes in a matter of days. Planting them near each other creates a “disease corridor,” allowing the blight to establish itself on one crop and then quickly spread to the other, doubling its destructive potential. Furthermore, Colorado potato beetles, a major pest of potatoes, will sometimes move over to feast on tomato leaves if their primary food source is nearby.
What you’ll see: Dark, water-soaked lesions on leaves and stems, which quickly turn brown and brittle. Fruits may develop greasy, brown spots and rot. This is the tell-tale sign of blight.
A better choice: Strict crop rotation is key. Never plant tomatoes where potatoes (or eggplant) were planted the previous year. Keep them in completely separate sections of your garden.
5. Walnuts (Specifically, Black Walnut Trees)
The danger here isn’t another vegetable but a large, imposing tree. If you have a black walnut tree on your property, you must plan your garden layout very carefully.
Why it’s a poor companion: Black walnut trees are the masters of allelopathy. They produce a powerful herbicidal chemical called juglone, which leaches into the soil from their roots, leaves, and nuts. Juglone is highly toxic to many plants, and tomatoes are one of the most severely affected. It prevents them from taking up water and nutrients, effectively poisoning them from the roots up.
What you’ll see: Tomato plants within the root zone of a black walnut (which can extend 50-80 feet from the trunk) will suddenly wilt, yellow, and die, often for no other apparent reason.
A better choice: Simply never plant a tomato garden anywhere near the drip line or known root zone of a black walnut tree. There is no companion plant that can protect them from juglone’s effects.
6. Eggplant
Like potatoes, eggplant is another member of the nightshade family, and the same rules of disease susceptibility apply.
Why it’s a poor companion: While they don’t compete for resources quite as fiercely as potatoes, eggplants are also highly vulnerable to early and late blight, as well as verticillium wilt. When planted next to tomatoes, they can easily catch and spread these soil-borne and airborne diseases, creating a hotbed of fungal issues that can persist in your soil for years.
What you’ll see: Similar to the issues with potatoes, look for signs of blight (leaf spots, stem lesions) and wilt (sudden wilting of lower leaves, often starting on one side of the plant).
A better choice: Practice crop rotation and keep your nightshades separated to minimize the risk of a widespread disease outbreak.
7. Dill (When Mature)
This one is a bit tricky, as dill’s relationship with tomatoes changes throughout its lifecycle. It’s a classic “frenemy” in the garden.
Why it’s a poor companion: When dill is young and small, it can be a beneficial companion. Its aroma can help repel some pests, including spider mites. However, once dill matures, flowers, and begins to set seed, it releases allelopathic compounds that can significantly stunt the growth of nearby tomato plants.
What you’ll see: If mature dill is too close, your tomato plants may seem to slow down their growth and fruit production later in the season.
A better choice: If you love dill, plant it nearby but plan to harvest it completely before it reaches full maturity. Alternatively, plant it in a separate container where its root secretions won’t affect your tomatoes.
The Science Behind the Worst Tomato Companions
Understanding these antagonistic relationships isn’t about memorizing a list; it’s about grasping the core principles that govern a garden’s ecosystem. The problems caused by the worst tomato companions generally fall into three scientific categories:
1. Extreme Resource Competition: Plants need sunlight, water, and soil nutrients. When you pair two “heavy feeders” like tomatoes and corn, or two plants with similar root structures, they are locked in a zero-sum game for the same limited resources, and neither will reach its full potential.
2. P
