Don’t Grow These 11 Plants Next To Your Tomatoes

Tomatoes are among the most popular and versatile garden crops, but they don’t get along with everybody. You can interplant these savory fruits with marigolds, lettuce, basil, and many other vegetables, but there are a few plants that should never grow next to tomatoes. 

Let’s dig into some tomato growth antagonists to keep in separate beds.

What Plants Should I Avoid Growing Next to Tomatoes?

Potatoes, corn, fennel, brassicas, and walnuts are a few of the plants that should be kept away from tomatoes. Nightshade cousins can share many of the same pests and diseases as your tomato plants. Corn grows too tall and can cast shade over sun-loving crops.

Brassicas may compete for water and nutrients, but they can grow in the same bed as long as there is extra fertilizer. However, the biggest antagonists for tomato growth are fennel plants and walnut trees, both of which produce allelopathic (growth-harming) compounds from their roots.

11 Worst Plants to Grow With Tomatoes

Companion planting involves growing symbiotic or complementary crops in the same garden bed. There’s an abundance of great companions for tomatoes. However, some plants can predispose them to more pest and disease issues. Others can steal sunlight, water, or fertilizer from your crops, hindering their growth and yields.

It’s best to keep these plants in a different bed or area of the garden:

Potatoes

This nightshade-family crop is closely related to tomatoes. Generally, potatoes and tomatoes make poor companions because they compete for nutrients and space. Both plants have deep root systems. The tuberous roots of potatoes can easily infringe on the deep, winding tomato roots. 

Both of these plants also share the same pests and diseases. Hornworms, early blight, late blight, and tobacco mosaic virus are major issues for both crops. If you grow them in close proximity, the similar leaf smell can attract pests. 

The vulnerability to pathogens also increases with a lack of airflow or proper spacing. Overcrowding can be a recipe for disaster because it allows fungal spores to easily blow from one plant to another, proliferating in the moist environment of both crops’ leafy growth.

Exception

The only exception to this rule is the unique “Ketchup ‘N Fries.” This grafted plant is made by grafting (splicing) a tomato plant on top of a potato rootstock. Small-space gardeners can harvest tomatoes and potatoes from the same plant!

Rest assured, this plant is not a GMO. It’s more like a mix-mash of spliced plants. You may have heard of grafting heirloom tomatoes onto disease-resistant rootstock. A similar method is used for fruit trees to enhance their overall vigor and resilience. Since these crops are both part of the Solanum genus, they are botanically related enough to grow together as one plant.

Corn

This grass-family crop is typically a bad companion because it casts a shadow over them. Corn grows up to 10 feet tall! This crop must be planted in a block to ensure proper cross-pollination from the wind. 

Tomatoes can also grow tall, but they require a lot of sunshine. If corn is growing nearby, it can shade out your tomato patch. The result could be pale foliage and poor fruit yields. These plants are best grown in separate areas.

Fennel

Research shows that fennel has allelopathic tendencies. Allelopathic means that this plant can release compounds in its root zone that hinders the growth of other plants. Contrary to popular belief, fennel may not be super allelopathic toward mature plants. It is more likely to inhibit the growth of weed seeds or lettuces germinating in its vicinity.

Ironically enough, fennel (and its cousin dill) can actually be excellent plants to grow in a neighboring bed. When these plants bolt, their flowers produce an abundance of beneficial insects like tachinid flies and parasitic wasps to help control hornworms. The flowers also attract lots of pollinators to ensure fertilization of nightshade blossoms.

So fennel won’t outright harm your tomato plants, but it is better to grow it on the margins of the garden in its own grow bag or raised bed. The frilly foliage could also cause too much stagnant air if grown too close.

Cabbage

The giant low-growing stature of a cabbage plant is not ideal for nightshade beds. The main reason why this combo fails is due to seasonality and nutrients. Cabbage is a cool-season brassica that doesn’t do very well in summer’s heat. In contrast, tomatoes obviously love the warmth and thrive in midsummer. Planting them together could cause one to suffer from overheating or the other to struggle in the chill.

Nutrient competition is also a concern since both plants are heavy feeders. Cabbage generally requires a lot of fertilizer to grow those giant broad leaves and dense central heads. Similarly, tomato plants use loads of nutrients to fuel their leafy growth and large fruit sets. It’s best to keep them in separate beds at different times of the year.

Kale

This leafy brassica is not necessarily a villain, but it has a few attributes that make it a poor companion. Like its cabbage cousin, kale prefers cooler spring or fall weather, while nightshades prefer summer warmth. 

Kale is also very shrubby and can grow quite tall, potentially causing overcrowding issues near the base of tomato plants. A lack of airflow from leaves brushing up against each other could cause unwanted disease problems like powdery mildew.

The nutrient needs of kale are not as high as cabbage, but they may still compete below the surface. Both plants require significant nitrogen. Ultimately, kale has far better companions to consider. 

Broccoli

It’s rare to see a giant broccoli plant growing pleasantly alongside a tomato plant. While this combo could work, it probably isn’t worth the added effort. The main issue is plant growth habits and spacing. Tomatoes need a lot of pruning to keep oxygen flowing through their lower leaves, especially if they are trellised with closer spacing. The big leaves of broccoli could create problems with diseases.

A broccoli plant grows wide and bushy, often reaching one to three feet tall and wide. They need ample space to spread out and produce a decently-sized head of florets. As you can imagine, these large plants suck up lots of soil nutrients, which could hinder the growth of their neighbors.

If you must grow broccoli in the same bed, provide at least three to four feet of space between the crops. Irrigate regularly, as both plants are water-demanding. Add lots of extra fertilizer to ensure there is no nutrient competition. 

Turnips

This bulbous brassica isn’t usually combined with tomatoes in the kitchen or the garden. Firstly, turnips are ideal for spring and fall growing. They aren’t usually a main summer crop. Secondly, turnips can bolt in hot weather or drought conditions. Tomato plants thrive in the heat and tend to suck up lots of water from the soil. These plants don’t line up with seasonality or moisture needs. 

Spinach

Cool-season spinach rarely overlaps well with warm-season nightshades. The seasonality of these crops is misaligned. Spinach seeds will not germinate in soils hotter than 75°F (24°C). The germination rates become dismal once the weather heats up. In contrast, tomatoes prefer soils ranging from 65 to 85°F (18-29°C). These temperature windows don’t overlap enough to make this a worthwhile combination.

If you want to grow greens at the base of your tomato patch, I recommend lettuce or malabar spinach. Summer lettuce appreciates the dappled shade of tomato leaves. This will prevent bolting and keep the low-growing lettuce plants cool. Malabar spinach is heat-tolerant and can climb a trellis next to indeterminate plants.

Eggplant

While eggplant is a nightshade family member, it is not a great companion due to size and shared pests. Eggplant grows quite stout and could easily get shaded out by a tall tomato plant.

The Solanaceae-family crops can both be attacked by hornworms, aphids, flea beetles, and potato beetles. Growing them in a closely spaced area may attract even more insects with the nightshade smell of their leaves.

Carrots

At first glance, this seems like a good symbiosis: Carrots are small and tomatoes are tall. But their growth habits are not actually complementary. Carrots require full sunlight and consistent moisture. Their long taproots dig deep in the soil to yield big, delicious carrot snacks. If you plant carrots nearby, the competition for water could be a major issue. 

Tomato plants could cause carrot taproots to grow weird and spindly. Moreover, shaded carrots usually fail to produce strong roots because their foliage is weak, and they don’t receive enough sunlight to properly photosynthesize.

Black Walnut

Walnut trees are widely known for their allelopathy. They produce a compound called juglone, which suppresses or kills surrounding plants. This explains why walnuts tend to grow in concentrated groves. They prefer to keep company with their own kind. While it is unlikely to plant a tomato near a walnut tree, you must also consider the leaves, roots, and bark of this plant. 

Mulching your garden with walnut leaves can cause problems. Do not build raised beds anywhere near a walnut grove, as the roots can still reach far beyond the tree’s canopy, releasing allelopathic compounds into the garden. Lastly, avoid mulching walnut bark or wood chips near vegetable beds.