What Not to Plant Next to Tomatoes

Some plants actively harm tomatoes when grown nearby, either by competing for nutrients, attracting the same pests, or releasing chemicals that stunt growth. The worst companions are brassicas (cabbage, broccoli, cauliflower), fennel, potatoes, and corn. Planting these within a few feet of tomatoes will reduce your yield and invite disease. Avoid them entirely in the same bed, or space them at least three to four feet apart if your garden is small.

Other plants are less harmful but still not worth the space. Dill, for instance, attracts hornworms — the same pest that devastates tomato plants. Walnut trees release a chemical called juglone that poisons many garden plants, including tomatoes, so keep tomatoes at least 50 feet away if you have one. Knowing what to avoid is as important as knowing what to plant nearby, because a poor companion can undo months of care.

Key Takeaways

  • Brassicas (cabbage, broccoli, cauliflower) and potatoes compete with tomatoes for the same soil nutrients and attract similar diseases, so plant them in a different bed or at least three to four feet away.
  • Fennel and corn stunt tomato growth through chemical release or root competition, and should not share a tomato bed under any circumstance.
  • Dill and other herbs in the carrot family attract hornworms, the most destructive tomato pest, so keep them separate if hornworms are a problem in your area.
  • Walnut trees release juglone, a toxin that kills tomatoes and many other plants, requiring at least 50 feet of distance between the tree and your tomato patch.
  • Spacing poor companions three to four feet away is a compromise when garden space is limited, but a separate bed is always safer.

Brassicas and Potatoes: Direct Competition

Brassicas — cabbage, broccoli, Brussels sprouts, cauliflower, and kale — are heavy feeders that pull nitrogen and potassium from the soil at the same rate tomatoes do. When both are in the same bed, neither gets enough, and both produce smaller fruit or heads. More damaging, they attract the same fungal diseases, especially early blight and septoria leaf spot, so planting them together creates a disease reservoir that spreads between crops.

Potatoes are worse. They are in the same family as tomatoes (Solanaceae) and share every major pest and disease: late blight, early blight, verticillium wilt, and Colorado potato beetles. Growing potatoes near tomatoes guarantees that if one plant gets sick, the other will follow within days. If you must grow both, plant potatoes in a completely separate area of your garden, at least 10 feet away, and do not plant tomatoes in that bed for at least two years.

Fennel and Corn: Growth Suppressors

Fennel is allelopathic, meaning it releases a chemical from its roots and leaves that inhibits the growth of nearby plants. Tomatoes are particularly sensitive to this chemical, and will grow slowly, produce fewer flowers, and yield less fruit when fennel is within two feet. Even fennel planted at the edge of a bed can reduce tomato productivity by 20 to 30 percent. The effect is not when ready — it builds over weeks — so you may not notice until late summer when your tomatoes lag behind.

Corn competes aggressively for water and nitrogen, especially in the first six weeks of growth. Tomatoes planted within four feet of corn will be stunted and may not recover even if the corn is removed later. Corn also grows tall and can shade tomatoes, reducing the light they need to ripen fruit. If you grow corn, give it its own section of the garden and plant tomatoes at least four feet away on the sunny side.

Dill and Carrot-Family Herbs: Pest Attraction

Dill, fennel, and parsnips attract hornworms — large green caterpillars that can strip a tomato plant bare in a few days. These insects lay eggs on dill and fennel first because they prefer them, but once the plants are eaten, the caterpillars move to tomatoes. If you grow dill for cooking or pickling, plant it far from tomatoes or in a separate part of the garden. Basil, by contrast, repels hornworms and is a good tomato companion.

Cilantro and parsley are less aggressive than dill but still attract some of the same pests. If hornworms are common in your area (they are in most of the United States), keep all carrot-family herbs at least six feet from tomatoes. If you have never seen hornworms in your garden, the risk is lower, and you can plant these herbs closer.

Walnut Trees: Juglone Toxicity

Black walnut and butternut walnut trees produce juglone, a compound in their roots, leaves, and fruit that kills or severely damages most garden plants, including tomatoes. The toxin persists in the soil for years after a tree is removed, so even if you cut down a walnut, you cannot plant tomatoes in that spot for at least three to five years. Juglone spreads through the soil and can affect plants up to 50 feet away from the tree, depending on soil type and moisture.

If you have a walnut tree on your property, plant tomatoes on the opposite side of your house or garden, as far away as possible. If your garden is small and a walnut is nearby, grow tomatoes in containers filled with fresh potting soil, which isolates them from the contaminated ground. Container growing is the only reliable way to grow tomatoes near a walnut tree.

Spacing and Bed Separation as Compromises

If your garden is small and you cannot avoid planting poor companions, spacing is a partial solution. Three to four feet between tomatoes and brassicas or corn reduces — but does not eliminate — competition and disease spread. For fennel and dill, six feet is the minimum safe distance. These distances assume you have good air circulation and are watering and fertilizing adequately to make up for the competition.

A better approach is to use separate beds or sections of the garden. Raised beds make this straightforward: put tomatoes in one bed and brassicas in another, with a path between them. This also makes crop rotation simpler, since you can move tomatoes to a different bed each year and reduce soil-borne disease. If you have the space, separation is always safer than spacing.

What Happens If You Plant Them Together Anyway

If you accidentally plant a poor companion near tomatoes, the damage will show up gradually. Tomatoes may grow slowly, produce fewer flowers, or develop yellow leaves (a sign of nitrogen deficiency if brassicas are nearby). Leaves may spot or curl, especially if disease is spreading from a nearby plant. Fruit may be smaller or ripen later than usual. The longer the poor companion stays in the bed, the worse the effect.

If you notice these signs, remove the poor companion when ready — do not wait until the end of the season. Pull out the plant, roots and all, and dispose of it in the trash, not the compost pile (especially if disease is present). Water and fertilize your tomatoes well to help them recover. Removing the problem plant mid-season will not fully restore your yield, but it will prevent further damage.

Frequently Asked Questions

Can I plant tomatoes and basil together?

Yes. Basil is one of the best tomato companions. It repels hornworms and some other pests, improves flavor when grown nearby, and uses different soil nutrients. Plant basil at the base of tomato plants or in the same bed a few inches away.

What about planting tomatoes near lettuce or spinach?

Lettuce and spinach are safe companions. They are shallow-rooted and do not compete with tomatoes for deep soil nutrients. They also shade the soil, which keeps it cooler and reduces water loss. Plant them under or around tomato plants without worry.

Is it okay to plant tomatoes near onions or garlic?

Yes. Onions and garlic repel some tomato pests and do not compete significantly for nutrients. They are among the best companions for tomatoes and can be planted in the same bed or nearby.

How far away should I plant tomatoes from a walnut tree?

At least 50 feet, though farther is safer. Juglone spreads through soil and can affect plants even at that distance. If you cannot get 50 feet away, grow tomatoes in containers with fresh potting soil instead of planting them in the ground.

Can I compost plants that had disease if they were growing near tomatoes?

No. If a plant near your tomatoes had early blight, late blight, or another fungal disease, throw it in the trash, not the compost pile. Compost piles do not always get hot enough to kill disease spores, and you risk spreading the disease to next year's garden.