Tomatoes are one of the most popular garden crops due to their versatility, flavor, and productivity. But growing healthy, high-yielding tomato plants requires more than just sunlight, water, and soil. One crucial—but often overlooked—factor is companion planting.
Companion planting is the practice of positioning plants to support each other in growth, pest control, and nutrient use. While some plants benefit tomatoes, others can cause problems by attracting pests, competing for nutrients, or spreading disease.
In this guide, we’ll explore what not to plant near tomatoes, why these combinations fail, and how to avoid common companion planting mistakes that can impact your garden’s health and harvest.
Why Companion Planting Matters for Tomatoes
Tomatoes are nutrient-hungry, sun-loving plants with specific soil, water, and space requirements. Nearby plants can affect their growth both positively and negatively.
Poor companions may:
- Compete for water, nitrogen, or root space
- Harbor pests that feed on tomatoes
- Spread fungal or bacterial diseases
- Interfere with pollination or air circulation
- Stunt tomato growth through chemical interactions (allelopathy)
By knowing which plants to avoid near your tomatoes, you can prevent issues before they start and encourage a healthier garden.
1. Corn
Corn is a surprising but important plant to keep away from tomatoes.
Why It’s a Bad Companion:
- Both are vulnerable to the corn earworm, also known as the tomato fruitworm
- These pests can easily spread between corn stalks and tomato vines
- Corn also grows tall and can shade tomato plants, reducing their productivity
- Competes heavily for soil nutrients, especially nitrogen
If you want to grow both crops, plant them in separate areas to minimize pest migration and competition.
2. Potatoes
Potatoes and tomatoes are part of the nightshade family (Solanaceae) and share many diseases and pests.
Problems with Potatoes:
- Increased risk of blight, particularly early and late blight
- Root competition as both are heavy feeders
- Can spread nematodes and fungal spores through the soil
- Harvesting potatoes may disturb tomato roots
Avoid planting tomatoes and potatoes in the same bed or rotating them back-to-back in the same soil. This minimizes disease buildup.
3. Fennel
Fennel has a reputation for being an unfriendly companion to most garden plants—and tomatoes are no exception.
Why Fennel Doesn’t Work:
- Releases allelopathic chemicals that inhibit growth of neighboring plants
- Competes aggressively for nutrients and space
- Can stunt or delay fruiting in tomato plants
If you grow fennel, give it its own dedicated space, preferably in a container or isolated bed far from tomatoes and other vegetables.
4. Cabbage and Brassicas
Members of the brassica family—including cabbage, broccoli, kale, and cauliflower—are poor companions for tomatoes.
Issues with Brassicas:
- Compete heavily for nutrients, especially in fertile garden beds
- Tomatoes and brassicas have different soil pH preferences
- Can attract pests like cabbage worms, which may move to tomatoes
- Brassicas tend to suppress tomato growth
Keep brassicas in a different part of the garden and avoid interplanting them with tomatoes in raised beds or containers.
5. Dill (When Mature)
Dill can be a good companion for tomatoes in its early stages but becomes problematic as it matures.
Why Mature Dill is a Problem:
- Older dill plants can inhibit tomato growth through root excretions
- Can become invasive and compete with tomatoes for space
- Attracts tomato hornworms if left unchecked
If you plant dill near tomatoes, be sure to harvest it before it flowers and remove mature plants before they affect growth.
6. Eggplant
Eggplant and tomato are both nightshades and should be kept apart when possible.
Similar Risks as Potatoes:
- Susceptible to the same diseases, such as verticillium wilt and blight
- Share insect pests like flea beetles and spider mites
- Compete for space and soil nutrients
Rotating eggplant and tomato locations between seasons is also recommended to reduce soil-borne disease risks.
7. Peppers
Although peppers and tomatoes are both garden staples, they don’t always thrive side-by-side.
Why Peppers Can Be a Risk:
- Attract the same pests: aphids, hornworms, whiteflies, and spider mites
- Vulnerable to bacterial spot and wilt, which can spread between them
- Close planting can reduce airflow, increasing humidity and disease risk
- Similar nutrient needs, leading to potential competition
You can grow them in the same garden, but space them well apart and monitor carefully for signs of shared pest problems.
8. Walnut Trees (Juglone-Producing Plants)
Black walnut and other juglone-producing trees like butternut and pecan release a chemical that is toxic to tomatoes.
Effects of Juglone:
- Causes wilting, yellowing leaves, and poor growth
- Can lead to sudden death of tomato plants, even in raised beds if roots are nearby
- The chemical remains in soil and leaf litter
Do not plant tomatoes within 50 to 80 feet of a black walnut tree, and avoid using walnut tree mulch or compost near garden beds.
9. Rosemary and Sage
These herbs are often recommended for other parts of the garden but not next to tomatoes.
Why They Don’t Pair Well:
- Thrive in dry, less fertile soil, unlike tomatoes, which need rich, moist conditions
- Can compete for space and water in smaller beds
- May affect tomato flavor or growth when planted too closely
Plant rosemary and sage in separate herb gardens or containers to avoid conflicting cultural needs.
10. Sunflowers
While beautiful, sunflowers can interfere with tomato success.
Reasons to Avoid Pairing Them:
- Compete for light and nutrients due to their size and root spread
- Exude allelopathic compounds from roots that may affect tomatoes
- Attract aphids and leafhoppers, which can transfer to tomatoes
If you grow sunflowers, plant them at least 10–15 feet away from your tomatoes to minimize impact.
Tips to Avoid Companion Planting Mistakes
Knowing what not to plant is just as important as knowing what works. Here’s how to avoid common pitfalls:
Practical Guidelines:
- Separate heavy feeders like tomatoes, potatoes, and corn
- Avoid planting within the same family (e.g., nightshades) in close proximity
- Rotate crops annually to prevent soil-borne diseases
- Use containers or raised beds for incompatible plants
- Observe spacing guidelines to maintain air circulation and reduce fungal risks
A little planning can save your garden from unnecessary struggles.
Good Companions for Tomatoes
To balance the picture, here are plants that do well with tomatoes and can help boost yields or deter pests:
Great Tomato Companions:
- Basil: Improves growth and flavor, repels hornworms
- Marigolds: Deter nematodes, whiteflies, and aphids
- Chives and onions: Repel aphids and spider mites
- Nasturtiums: Act as trap crops for aphids
- Carrots: Loosen soil without competing heavily for nutrients
- Lettuce and spinach: Good for filling in shade under tall tomato plants
These plants complement tomatoes and contribute to a diverse, productive garden.
Conclusion
Companion planting is both art and science, and knowing what not to plant near tomatoes is just as essential as choosing the right allies. By avoiding plants that compete, spread disease, or stunt growth—like corn, potatoes, fennel, and mature dill—you set your tomatoes up for success.
With careful planning and awareness of plant interactions, your garden can thrive. Avoiding common companion planting mistakes helps protect your tomatoes from pests, improves yields, and promotes a healthier, more balanced growing environment.
Frequently Asked Questions (FAQs)
Can I plant tomatoes and cucumbers together?
Yes, but ensure proper spacing and support structures. They share some pests but have similar growing conditions and can coexist if managed well.
Is it okay to plant basil near tomatoes?
Absolutely. Basil is one of the best tomato companions, known to improve flavor and deter pests like hornworms.
Can I grow tomatoes in the same bed every year?
It’s not recommended. Rotating crops yearly helps reduce the buildup of soil-borne diseases like wilt and blight.
Will herbs like mint harm tomato growth?
Mint can become invasive and should be grown in containers. It doesn’t directly harm tomatoes but can outcompete them for space and water.
How far apart should tomatoes be from bad companion plants?
Aim for at least 3 to 5 feet of distance, or grow incompatible plants in entirely separate beds or containers.