List of Companion Plants To Avoid When Growing Tomatoes


If plants fall in the same class, compete for the same nutrients, and suffer from the same pests or diseases, it remains scientifically correct to grow them separately to minimize plant disease and pest transfer risk.

Tomatoes farming often comes with various possible setbacks, ranging from fungal diseases and rotting blossom to insect pests. While the internet abounds with companion plants that attract natural predators that feed on non-beneficial pests in a tomato garden, one foes crop can significantly affect output.

Fortunately, this plant companion guide comprises extensive research on up to 9 crops, including cabbage and potatoes that should be separated from tomatoes. These plants can cause stunted growth of tomatoes, or at worst, they remain susceptible to diseases and pests affecting tomatoes. Here we go!

Why you should carefully choose the companion plants of Tomatoes

While some plants remain companions in the gardens and sauces, others cannot be grown together if you wish to maximize output.

The common reason farmers find companion planting beneficial is the ability of the plants to improve soil nutrients or keep away pests from the primary crop. Unfortunately, other attract pests that feed on non-beneficial pests in the garden.

Generally, companion plants act as natural protectants for each other while complimenting.

However, the relationship is not always beneficial. For instance, planting crops such as cabbage from the Brassicaceae family may lead to stunted growth in tomatoes.

Besides competing for soil nutrients, other plants harden the ground, making it difficult for companion plants to survive.

With that said, the following plants cannot thrive together with tomatoes due to different plant incompatibilities:

Bad Companion plants for Tomatoes

1. Fennel are bad companion plants to tomatoes because of an increase in toxicity levels

Fennel is one of the plants that remain incompatible with most crops, including tomatoes. While the plant has herbal benefits, growing it in companion with tomatoes could affect the growth and development of the latter.

This short-lived plant produces growth-inhibiting chemical substances, which will affect tomato productivity.

While companion crops are often planted with tomatoes to enhance growth and productivity, fennels cause adverse effects that remain visible in small tomato fruits and stunted growth of the tomato plants.

Due to such high toxicity levels produced by this plant, it is advisable to plant the fennels with dill, which is the only plant that can thrive under these conditions.

Otherwise, allot a piece of the garden away from tomatoes and other farms’ produce for the fennel.

Fennels are bad companion plants to tomatoes because they are susceptible to pests

Besides, fennel plants are susceptible to pests such as cutworms that can migrate to tomatoes eventually. Besides corn and lettuce, fennel remains among the cutworm hosts in the garden.

These warm often feed on stems of sprouting seedling of transplants. Similarly, cutworms are prone to damaging young tomatoes, making the two incompatible companion plants.

Besides this disadvantage, fennel is also beneficial to tomatoes. This crop may be a companion if a gardener only needs to manage pests; however, productivity remains a priority for most farmers.

Although fennel can attract beneficial insects such as parasitic aphidius wasps that eliminate aphids from tomatoes, the plant’s adverse effects disqualify it from being an effective companion for tomatoes.

2. Another bad companion plant for tomatoes would be Black Walnuts

Like a funnel, walnut produces a growth-inhibiting substance that makes it the wrong tomato companion for your garden. The black walnut has juglone from its leaves’ inner bark or roots and releases it to the adjacent crops. The chemical leaches into the soil when it rains.

These allelopathic chemicals (juglone) found in black walnut limits optimal fruit production when tomatoes grow under or near them.

Besides stunted growth, the effects may also exhibit through small-sized tomato fruits. Besides, tomatoes grown as companion crops to walnut trees can be afflicted with walnut wilt. This disorder affects plants such as tomatoes, potatoes, and asparagus. However, plants such as carrots remain resistant to juglone.

Uptake and effects of juglone on tomatoes depend on the crop’s proximity to the walnut. Plants can quickly wilt and die depending on the juglone toxicity levels, with tomatoes growing under the back walnut severely affected.

Common symptoms of walnut wilt in tomatoes, making them bad companion plants to tomatoes.

Like for verticillium wilt and fusarium wilt, walnut wilt in tomatoes remains identifiable with the following indicators:

Symptoms of walnut wilt in tomato plantSymptoms of walnut wilt in tomato leaves
Brown or streak stemsBrowning leaves
Stunted growth that may be flacid
(those found a short distance away)
Leaf drop
Wilting or dead tomatoes
(those near the black walnut)
Yellowing weeds

Avoid planting tomatoes close to a walnut family tree. Besides, it would help if you waited for up to 2 years to plant your tomatoes on a piece of land where these trees have grown before, mainly because juglone takes a long time to dissipate from the soil.

3. Corn are a definite no no as a companion plant for tomatoes

When considering tomato companion plants, pest control is among the top consideration in every gardener’s companion guide. However, priorities remain to find a companion plant that can lure away pests from the tomatoes or disrupt the pest’s lifecycle.

These plants work by act as sacrificial crops that you plant weeks earlier before your tomato seedlings. Such plants include:

  • Thyme
  • Cowpeas
  • Radish
  • Collards
  • Basil

Unfortunately, corn and tomato attract the same pest. The most prevalent is the earworms which are common in corn. These earworms are also referred to as the fruitworm (Larva Helicoverpa Zea) in tomatoes.

Planting these two crops in the same garden will broaden the egg-laying grounds for adult earworms and fruit warms.

Increasing the worm population within your garden can really increase your tomato yields, In this article I show you how to do just that. You will lean how to increase the earth worm population in your very own garden.

The result of both corn and tomato attracting the same class of pests includes damage to these plants’ tissues and quick spread or multiplication of pests.

Such pests will have more food instigating tomato parts and tissue damage besides leading to a decimated garden. Therefore, this scenario makes corn a bad companion plant for tomatoes.

4. Don’t use potatoes as a companion plant for tomatoes

Plants such as potatoes and tomatoes that belong to the same family cannot be grown together. What makes them unsuitable companions is that they are both susceptible to the same pests and diseases.

Like other tuberous plants, potatoes and tomatoes are prone to common diseases such as blight fungus. In addition, early bright, common in tomato and potato plants, causes adverse effects such as reduced yield and wilting tomato foliage.

Common symptoms of early blight include a solani that can cause the following:

  • Spots on the plant foliage
  • Yellow leaves
  • Collar rot
  • Concentric rings in the tomatoes
  • A frequent drop of the infected fruits

Potatoes often remain the most common harbor of this disease-causing fungus. Besides, when the fungus drops on the ground, it survives in either the debris in the ground soil, tomato seed, or plant. It can also stay in other solanaceous hosts, such as potatoes.

Farming advice and separation of potato planting, instead of it being a companion plant to tomatoes

It is advisable to eradicate volunteer tomato plants from fields intended for potatoes to minimize the risk of blight fungus for the best farming practice.

Generally, avoid planting tomatoes near potatoes due to visible incompatibilities such as common diseases and pests.

With this, there are other ways to plant potatoes! Learning the best ways to plant potatoes can really help, and I wrote an article on getting the best potatoes and doubling your harvests for them.

5. Brassica family should not be paired as a companion plant for tomatoes

Brassica family crops and tomatoes are unfriendly pairs for companion plants. When planted together with tomatoes, these plants often occupy a bigger ground space since they grow close to the ground.

They also compete for root space which leaves the tomatoes, weaker plant disadvantage. The tomatoes, in turn, exhibit stunted growth, smaller fruits, and weak plant stem when grouped with these plants.

Most common brassica family crops you should not plant with your tomato seedlings include the following:

CabbageCollards
Brussel SproutCauliflower
TurnipKohlrabi
BroccoliKale

While most companion plant guides concentrate on the effects of brassicas on tomatoes, it is also essential to consider how tomatoes can affect brassicas such as cabbage.

For instance, when you plant cabbage and tomatoes, they may block sunlight from penetrating the cabbage. This will hinder optimal growth and production.

Remember, cabbages grow best closest to the ground, and so, they require companion plants that do not hinder sunlight from them all day.

6. The Nightshade family would be an unfair companion plant for tomatoes

Planting tomatoes and any plant from the nightshade family would be incompatible because the latter plants are susceptible to diseases similar to tomatoes.

The nightshade family crops include but are not limited to pepper, potato, and eggplant. Effects of planting this incompatible duo include withering of plant leaves and death of plant tissues.

A disease such as the early and late blight, which affects both tomatoes and nightshade family crops, has a high probability of building up in the soil or spreading in the entire garden.

The most common nightshade family plant is the eggplant that cannot be a suitable companion plant for tomatoes. Like the other nightshade plants, the eggplant is also an unfriendly pair to the tomatoes.

Plants That Can Be Beneficial but also Harmful Companion plants for Tomatoes

Some plants are both good and bad companion plants to tomatoes. Therefore, gardeners cannot totally avoid planting them with tomatoes due to the beneficial influence on tomato productivity.

However, you must be aware of the stages when these plants impact your tomatoes negatively to mitigate the risk of affecting productivity. These plants include the following:

7. Dill may have positives but still be a threat as a companion plant for tomatoes

While young dill has positive effects, such as improved tomato plant health, it may cause stunted growth if gardeners introduce tomato seedlings to a garden comprising mature dill.

Then again, mature drill’s tiny flowers attract beneficial insects such as parasitic wasps that eliminate spider mites and aphids from the farm.

Dill also poses a challenge to gardeners by attracting tomato fruit worms and tomato hornworms.

The best decision for planting drills is to separate them with tomatoes by growing them some distance away to prevent them from inhibiting tomato growth.

8. Pepper can’t be a companion plant to tomatoes

Pepper is also another beneficial but still harmful companion plant for tomatoes. Although this plant adds value to the tomato plant by enhancing its flavor, the two plants remain susceptible to blight diseases.

If planted together, they can easily accelerate the spread of disease, which could prove detrimental to the overall tomato productivity. Thus, the best practice here remains to grow the two separately despite the few benefits.

9. Don’t put in Mint as a companion plant for tomatoes

Although mint improves tomato flavor, its adverse effects include causing stunted growth for tomatoes. If you must plant it with your tomatoes, it is best to put it in a container away from the garden to prevent it from spreading.

But in general, it still does not qualify as an ideal companion plant for tomatoes.

FAQ’s on growing tomatoes

Conclusion on companion plants to avoid when growing tomatoes

Sometimes you may be tempted to plant more than one plant in your garden to save resources, time, and space. However, as you concentrate on the latter, remember that not all plants are ideal companions for each other.

For instance, when you are planting tomatoes, consider companions that would not cause adverse effects on the growth and productivity of this crop.

While this article may lead to caution and fear on what to group with tomatoes, we have an excellent piece to combat this cautionary tale. If you would like to know more about tomatoes, I have written an ebook about the subject. It is a free resource, so why not download it today.

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Tony O'Neill

I am Tony O'Neill, A full-time firefighter, and professional gardener. I have spent most of my life gardening. From the age of 7 until the present day at 46. My goal is to use my love and knowledge of gardening to support you and to simplify the gardening process so you are more productive

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