Which plants harm tomatoes when planted nearby
Some plants actively compete with tomatoes for water and nutrients, while others attract pests that feed on tomato leaves and fruit. The worst companions are brassicas (cabbage, broccoli, kale), fennel, potatoes, and corn. Brassicas draw the same soil nutrients tomatoes need, especially nitrogen and potassium. Fennel releases chemicals into the soil that stunt tomato growth. Potatoes and tomatoes share the same diseases—particularly early blight and late blight—so planting them near each other spreads infection quickly between crops.
Beyond direct competition, some plants attract insects that move straight to your tomatoes. Dill and parsnips draw hornworms and spider mites. Walnut trees release a toxin called juglone that poisons tomato roots. If you have a walnut on your property, keep tomatoes at least 50 feet away. Spacing matters as much as the plant itself—even a good companion planted too close will steal water and light.
Key Takeaways
- Brassicas like cabbage and broccoli compete with tomatoes for the same nitrogen and potassium, weakening both crops.
- Potatoes and tomatoes share fungal diseases like early blight, so planting them within 10 feet of each other spreads infection between plants.
- Fennel and walnut trees release chemicals that actively harm tomato growth and should be kept at least 50 feet away.
- Dill and parsnips attract hornworms and spider mites that feed on tomato leaves and fruit.
Why brassicas stunt tomato growth
Brassicas—the family that includes cabbage, broccoli, Brussels sprouts, and kale—are heavy feeders that pull nitrogen from the soil faster than tomatoes can use it. Both crops need nitrogen to build leaves and stems, so when they compete for the same supply, both end up smaller and produce less fruit. The problem gets worse in smaller gardens where soil volume is limited.
Brassicas also take up significant space and shade. A mature cabbage plant spreads 18 to 24 inches wide, and if it sits on the south or west side of your tomato row, it blocks afternoon sun that tomatoes need for ripening. Plant brassicas at least 3 feet away from tomatoes, or better yet, in a separate bed. If your garden is small, choose one or the other for that season rather than crowding both in.
Potatoes and tomatoes spread disease to each other
Potatoes and tomatoes are both members of the nightshade family and catch the same fungal diseases. Early blight starts as brown spots on lower leaves and spreads upward; late blight causes water-soaked patches that turn the whole plant black and mushy. If you plant potatoes and tomatoes within 10 feet of each other, spores from one crop splash onto the other during rain or watering, and the disease moves between them.
The risk is highest in humid climates or during wet summers. Even if you space them 10 feet apart, avoid planting tomatoes where potatoes grew the previous year—fungal spores survive in soil and plant debris. Rotate nightshade crops to a different bed each season, and remove all potato and tomato plants (including fallen fruit) at the end of the season rather than leaving them to decompose in place.
Fennel and walnut toxins that poison tomato roots
Fennel is a common herb that looks harmless but releases a chemical called anethole into the soil through its roots and fallen leaves. This chemical slows or stops tomato seed germination and stunts the growth of young tomato plants. Fennel affects a wide radius—keep it at least 5 feet away from tomatoes, and do not use fennel leaves as mulch around tomato plants.
Walnut trees are even more aggressive. They produce juglone, a toxin that moves through the soil and kills or severely damages most plants within their root zone. Tomato roots are sensitive to juglone. If you have a walnut tree on your property, plant tomatoes at least 50 feet away—farther if the tree is mature and its roots have spread. Black walnuts are the worst offenders, but English walnuts also produce juglone, just in smaller amounts.
Insects that move from nearby plants to tomatoes
Dill and parsnips attract hornworms and spider mites, both of which feed heavily on tomato leaves. Hornworms are large green caterpillars that can strip a tomato plant bare in days. Spider mites are tiny and hard to see but cause yellowing and stippling on leaves. If dill or parsnips are flowering nearby, these pests lay eggs on them first, then move to your tomatoes as the herbs decline.
You do not have to remove dill entirely—it also attracts beneficial wasps that prey on hornworms. But plant dill at least 10 feet away from tomatoes, or plant it early in the season so it finishes flowering before tomato plants are large enough to attract pests. Parsnips are slower-growing and less commonly planted in home gardens, but the same spacing rule applies.
Corn and tomatoes compete for the same nutrients
Corn is a tall, heavy feeder that shades everything around it and pulls large amounts of nitrogen from the soil. Tomatoes planted near corn get shaded during the critical afternoon hours when they need direct sun for ripening, and they compete with corn for nitrogen. Corn also attracts corn borers and other pests that occasionally move to nearby plants.
If you want to grow both, plant corn on the north side of your garden so it does not shade tomatoes, and space them at least 4 feet apart. Better yet, plant corn in a separate bed or grow it in a different part of your yard. Corn matures and is harvested in 70 to 90 days, so you can also plant it in a different season—early corn in spring, tomatoes in summer—to avoid competition entirely.
How to plan your garden layout to avoid bad companions
Start by mapping where the sun hits your garden at different times of day. Tomatoes need 6 to 8 hours of direct sun, so plant them in the sunniest spot. Then place tall plants like corn and brassicas on the north or west side where they will not shade tomatoes during afternoon hours. Keep potatoes in a separate bed at least 10 feet away, or skip potatoes that year if space is tight.
Draw a straightforward sketch of your garden and mark distances. Fennel and walnut need 5 to 50 feet of clearance depending on the plant, so if your garden is small, these may not be practical to grow alongside tomatoes. Dill and parsnips can work if you plant them far enough away or time them to finish before tomatoes are vulnerable. Rotate crops each year—if tomatoes grew in bed A this year, plant them in bed B or C next year and put a different crop family in bed A.
Frequently Asked Questions
Can I plant tomatoes and basil together?
Yes. Basil is one of the best tomato companions—it repels some pests and does not compete for nutrients. Plant basil at the base of tomato plants or nearby. Basil also improves the flavor of tomatoes when both are harvested together for cooking.
What if I already planted potatoes near my tomatoes?
Watch both plants closely for brown spots on lower leaves (early blight) or water-soaked patches (late blight). Remove any affected leaves when ready and throw them in the trash, not the compost. Space them farther apart next season, or grow them in different beds. Fungal spores survive in soil, so rotate crops to a new bed the following year.
Is it safe to plant tomatoes where corn grew last year?
Yes, but add compost or fertilizer to replace the nitrogen that corn depleted. Corn and tomatoes do not share diseases, so disease carryover is not a concern. The main issue is soil depletion, which you can fix with soil amendment before planting tomatoes.
How far should tomatoes be from a walnut tree?
At least 50 feet away from a mature walnut tree. Walnut roots spread underground and release juglone, a toxin that damages tomato roots. If your property is smaller than 50 feet, plant tomatoes in containers on the opposite side of your house, or choose a different location entirely.
Can I use fennel leaves as mulch around tomatoes?
No. Fennel leaves release anethole as they break down, which harms tomato roots. Use straw, wood chips, or leaves from other plants as mulch instead. Keep fennel plants at least 5 feet away from tomatoes.