Companion planting works both ways — some plants help tomatoes thrive, while others compete for nutrients, attract the same pests, or release chemicals that stunt growth

The plants you grow next to tomatoes matter as much as the ones you choose to grow with them. Certain combinations create problems: brassicas like cabbage and broccoli draw the same nutrients tomatoes need, fennel actively inhibits tomato growth through its roots, and potatoes invite the same diseases that destroy tomato crops. Others straightforward crowd out your tomatoes or make pest control harder. Knowing what to avoid saves you space, water, and the frustration of watching plants fail for reasons that had nothing to do with sun or soil.

A raised bed is a contained space where plant roots compete directly for the same water and nutrients. That closeness makes bad pairings worse than they would be in open ground. This guide walks you through the plants to keep separate from tomatoes and why each one causes trouble.

Key Takeaways

  • Potatoes and tomatoes are both nightshades and share the same diseases, especially early blight and late blight, so planting them together nearly guarantees one will infect the other.
  • Brassicas — cabbage, broccoli, kale, and Brussels sprouts — compete with tomatoes for nitrogen and potassium, leaving both plants stunted.
  • Fennel releases a chemical from its roots that actively suppresses tomato growth, and this effect spreads through the soil of a raised bed.
  • Corn, dill, and kohlrabi also inhibit tomato growth through root secretions or nutrient competition, so they belong in a separate bed.
  • Walnut trees and black walnut roots release juglone, a toxin that kills tomatoes, so check whether a walnut grows near your raised bed's location.

Potatoes: The disease-sharing nightshade

Potatoes and tomatoes are both nightshades, which means they are vulnerable to the same fungal diseases. The two most destructive are early blight and late blight. Early blight starts as brown spots with concentric rings on lower leaves and spreads upward; late blight causes water-soaked patches that turn gray-green and collapse the entire plant in wet weather. If one plant carries the disease, the other will catch it — the spores travel through soil moisture and air.

Planting potatoes and tomatoes in the same raised bed means you are essentially guaranteeing that if disease strikes one, it will strike both. You lose the crop twice over. Even if you rotate crops year to year, the spores can survive in soil, so use a different bed for potatoes or wait at least three years before planting tomatoes where potatoes grew.

Brassicas: Nitrogen and potassium competitors

Brassicas — the family that includes cabbage, broccoli, kale, cauliflower, and Brussels sprouts — are heavy feeders that pull nitrogen and potassium from soil at the same rate tomatoes do. In a raised bed with limited soil volume, both plants end up undernourished. Tomatoes show the stress first: leaves yellow, fruit stays small, and yields drop. The brassica may look fine while it is stealing the nutrients your tomato needs.

This is not a case of one plant poisoning the other; it is straightforward competition. If your raised bed is large enough (4 feet by 8 feet or bigger) and you amend heavily with compost before planting, you might get away with it. But in a standard 4-by-4 bed, keep brassicas out. Grow them in a separate bed or in the season when you are not growing tomatoes.

Fennel: A root chemical that stunts tomato growth

Fennel is unusual because it does not just compete — it actively poisons the soil around it. The plant releases a chemical called anethole from its roots that suppresses the growth of many plants, including tomatoes. This is not something you can wash away or work around; it is a built-in defense mechanism that fennel uses to reduce competition.

In a raised bed, fennel's roots and the toxin they release are confined to a small space with your tomatoes. Even if you pull the fennel out, the chemical lingers in the soil. If you have grown fennel in your raised bed, wait a season before planting tomatoes there, or replace the top 6 to 8 inches of soil. Fennel is worth growing — it attracts beneficial insects and the bulb is edible — but it needs its own bed.

Corn, dill, and kohlrabi: Other growth inhibitors

Corn and dill both release compounds from their roots that inhibit tomato growth, though the effect is less dramatic than fennel's. Corn also grows tall and shades tomatoes, which need six to eight hours of direct sun daily. Dill is a smaller plant but still competes for nutrients and can harbor pests that move to tomatoes.

Kohlrabi, a brassica relative, competes for the same nutrients as tomatoes and takes up significant space in a raised bed for a relatively small harvest. If you want to grow any of these, give them a separate bed or plant them in a different season. Dill is sometimes recommended as a companion plant, but that information works better in open ground where roots are not confined; in a raised bed, the negatives outweigh any benefit.

Walnut trees and juglone toxicity

If a black walnut tree grows within 50 feet of your raised bed, the tree's roots may be releasing juglone, a toxin that kills tomatoes and many other plants. Walnut roots spread wide and deep, and juglone leaches into the soil. You will not see the walnut's leaves in your bed, but you will see tomatoes wilt and die despite adequate water and sun.

Before you build or plant a raised bed, check whether a walnut tree is nearby. If one is, move the bed farther away or choose a location on the opposite side of your property. If the tree is already there and you cannot move the bed, you can line the bottom with heavy plastic to block root contact, though this is not foolproof. The safest option is to grow tomatoes in containers or a different part of your yard.

Spacing and timing: When separation is not enough

Even plants that do not actively inhibit tomatoes can cause problems if they crowd them. Tomatoes need air circulation to prevent fungal diseases, and they need space for their root systems to spread. In a raised bed, a good rule is to give each tomato plant at least 18 to 24 inches of space in all directions. That means you can fit only two or three plants in a standard 4-by-4 bed, depending on variety.

If you are tempted to squeeze in a companion plant to save space, choose one that grows low and does not compete for nutrients — basil, parsley, or marigolds are safe choices. Avoid anything that grows tall, spreads wide, or has heavy nutrient demands. When in doubt, use a separate bed or plant the problematic crop in a different season.

Frequently Asked Questions

Can I plant tomatoes where potatoes grew last year?

Not safely. Early blight and late blight spores can survive in soil for up to three years. Wait at least three seasons before planting tomatoes in a bed where potatoes grew, or replace the top 8 to 12 inches of soil with fresh compost. If disease was visible on the potatoes, replace more soil or use a different bed entirely.

Is it okay to plant fennel far away from tomatoes in the same bed?

No. Fennel's root toxin spreads through the entire soil volume of a raised bed, so distance does not help. The chemical will reach your tomatoes' roots regardless of where you plant the fennel. Use a separate bed for fennel or grow it in a container.

What if I already planted brassicas with tomatoes?

Your tomatoes will likely be stunted, but they will survive. Feed them with a balanced fertilizer every two weeks to offset the nutrient competition. Next season, move the brassicas to a different bed. If your raised bed is large and well-amended, the damage may be minimal.

Can I grow dill near tomatoes if I keep it trimmed back?

Trimming does not stop the root competition or the growth-inhibiting compounds dill releases into the soil. Grow dill in a separate bed or in containers. If you want the dill for cooking or to attract beneficial insects, it is worth the extra space.

How do I know if a walnut tree is affecting my tomatoes?

Tomatoes affected by juglone wilt despite adequate water, leaves turn yellow or brown, and the plant may die suddenly. If you see this pattern and a walnut tree is nearby, move the bed away from the tree's root zone. Juglone toxicity is hard to reverse once it is in the soil.