What not to plant with tomatoes gets answered with long lists online. The reasons behind those lists rarely get explained.
Every companion planting guide claims tomatoes and broccoli repel each other. None of them explain why. One frustrated gardener said exactly that, and the complaint is fair.
That gap between advice and evidence is exactly where real answers live. Some warnings trace back to genuine, documented problems. Others turn out to be folklore nobody has actually tested.
This guide covers which plants to keep away from tomatoes for real, sourced reasons. It also covers one popular warning that could not be verified against any citable source.
The short version
Black walnut trees pose the clearest, best documented risk to tomatoes through a toxin called juglone. Corn and tomatoes share a major pest too. It is the same insect known as both the corn earworm and the tomato fruitworm.
Potatoes and tomatoes share susceptibility to late blight, a serious fungal disease. Broccoli and other brassicas are technically susceptible to a different disease, verticillium wilt. The real story there is more nuanced than simple avoidance.
Fennel shows up on almost every companion planting list as a tomato enemy. No university or extension source could be found confirming that claim.
Black walnut: the clearest case
Kansas State University’s horticulture resource center describes what happens plainly. Tomatoes growing next to a walnut tree abruptly wilt and die in early to mid summer.
Plants at greater distances may only stunt and wilt without dying outright. The cause is juglone, a compound the tree produces in its leaves, fruit hulls, inner bark, and roots.
University of Illinois Extension adds specific symptoms to watch for. Affected plants show leaf yellowing, wilting, stunted growth, and eventual death. Tomatoes and other members of the nightshade family are especially sensitive.
A grower known as cetompkins described a real world version of this problem. Two forty foot walnut trees sat within twenty feet of a garden. That garden had produced tomatoes successfully for thirteen years.
Production declined gradually over the following three years. That gardener asked a fair question neither source fully answers. Does soil toxicity build gradually, or does it cross a threshold all at once?
Kansas State’s fact sheet does not specify an exact safe distance. It notes only that severity depends on proximity to the tree. It does offer one concrete number worth remembering, though.
Avoid planting tomatoes for at least two years after a walnut tree is removed. Juglone lingers in root bark left behind, even though the chemical itself breaks down quickly once released into soil.
Corn: a shared pest, not a shared disease
The tomato fruitworm and the corn earworm are not two different insects. Utah State University Extension identifies them as the same species, Helicoverpa zea. That single species simply uses different host plants depending on what is available nearby.
Timing drives the real risk. That same source explains that once corn silks turn brown, fruitworm moths move on. They seek other nearby hosts for egg laying instead.
Tomatoes, peppers, and eggplant growing near corn that has finished silking become the next target. The damage shows up as tunnels in fruit filled with frass. Premature ripening and rot from secondary fungi often follow.
For home gardens, Utah State recommends targeting eggs and newly hatched larvae before they tunnel into fruit. Bacillus thuringiensis, known as Bt, along with spinosad and pyrethrin based sprays are the residential options that source lists.
Potato: a shared disease worth taking seriously
Tomatoes and potatoes belong to the same plant family, and they share a genuine disease risk. University of Maryland Extension identifies the culprit as late blight, caused by the water mold Phytophthora infestans. It can affect all parts of both plants.
The disease favors cool, wet weather and can strike at any point in the growing season. Spores travel short distances on rain and much longer distances on wind. That spread pattern is part of why proximity between susceptible crops matters.
University of Maryland Extension recommends selecting late blight resistant varieties where the disease is a known local concern. Protectant fungicides built around chlorothalonil, mancozeb, or fixed copper can shield healthy foliage if applied before infection takes hold.
Removing weeds in the tomato and potato family matters too, since those weeds can host the pathogen. That step counts just as much as spacing the two crops apart.
Broccoli and brassicas: more nuanced than the folklore suggests
A Houzz forum contributor named adrien_gardener gave a more careful answer when a gardener asked about pairing tomatoes with broccoli. Rather than warning against the pairing outright, that contributor pointed to verticillium wilt and suggested rotation timing mattered more than simple avoidance, calling broccoli followed by tomatoes a good idea. University research backs up that instinct with real numbers.
University of Wisconsin Horticulture Extension confirms broccoli, cabbage, Brussels sprouts, cauliflower, and kale are all technically susceptible to verticillium wilt. That disease is the same one covered in this site’s verticillium wilt guide. Those crops just show milder symptoms than tomato or potato do.
Simultaneous planting is not really the relevant comparison, though. University of California Agriculture and Natural Resources research found something closer to the opposite effect through rotation.
Two successive broccoli crops grown ahead of a susceptible crop reduced verticillium dahliae populations in infested soil. The reduction ran roughly ninety four percent. Glucosinolates in the decomposing broccoli residue break down into sulfides and other compounds that damage the fungus directly.
Isolates of the fungus taken from tomato plants specifically were among those suppressed in that research. The key is timing. Used as a rotation crop before tomatoes, rather than as a same season neighbor, broccoli can work in a gardener’s favor.
Fennel: the claim that could not be verified
Fennel appears on nearly every companion planting list as a plant tomatoes cannot tolerate nearby. That claim traces back to general folklore about fennel disliking most garden neighbors.
No university extension page, peer reviewed source, or government resource turned up in research for this guide confirmed a specific mechanism or documented case. University of Illinois Extension’s own rundown of allelopathic garden plants names black walnut, tree of heaven, garlic mustard, and certain grasses instead. Fennel does not appear on that list at all.
That gap does not prove the warning is false. It means the claim currently rests on repetition rather than documented evidence. This guide would rather say so than repeat it as settled fact.
Frequently asked questions
Why shouldn’t I plant tomatoes near a black walnut tree?
Black walnut trees produce juglone, a compound that causes tomatoes to wilt and die, sometimes abruptly. Symptoms grow milder with distance from the tree, though extension sources have not published an exact safe distance.
Do tomatoes and corn really attract the same pest?
Yes. The corn earworm and tomato fruitworm are the same insect, and it moves from corn to nearby tomatoes once corn silks turn brown. Timing and spacing both matter as a result.
Can I plant tomatoes where I grew potatoes last season?
It carries real risk. Both crops are susceptible to late blight. Infected debris or nearby wind blown spores can carry the disease from one crop to the other.
Is broccoli actually bad for tomatoes?
Not in the way folklore suggests. Broccoli is a mild host of verticillium wilt. Research shows using broccoli as a rotation crop before tomatoes can sharply reduce that same pathogen in the soil instead.
Is it true that fennel should never be planted near tomatoes?
That claim gets repeated constantly online. No citable university or extension source confirms a specific reason or documented case, though. It may hold some truth, but it currently is not backed by verifiable research.
The bottom line: what not to plant with tomatoes comes down to a short list of documented risks rather than a long list of folklore. Black walnut, post silking corn, and potatoes carry real, sourced reasons to keep some distance. Broccoli deserves a second look rather than automatic avoidance, and fennel deserves honesty about what is not actually known. For the full companion planting picture, see our tomato companion plants guide. For more on the verticillium wilt connection to brassica rotations, see our verticillium wilt tomato guide. For the pest lineup that includes corn’s shared threat, see our tomato pests guide.
SOURCES
Kansas State University, Horticulture Resource Center: Tomato, Walnut Wilt. https://hnr.k-state.edu/extension/horticulture-resource-center/common-pest-problems/documents/Tomato%20-%20Walnut%20Wilt.pdf
University of Illinois Extension, ILRiverHort: Plants That Inhibit Other Plants. https://extension.illinois.edu/blogs/ilriverhort/2018-08-06-plants-inhibit-other-plants
Utah State University Extension: Corn Earworm, Tomato Fruitworm. https://extension.usu.edu/pests/ipm/notes_ag/veg-tomato-fruitworm-corn-earworm
University of Maryland Extension: Late Blight of Tomato and Potato. https://extension.umd.edu/resource/late-blight-tomato-and-potato
University of Wisconsin Horticulture Extension: Verticillium Wilt of Vegetables. https://hort.extension.wisc.edu/articles/verticillium-wilt-of-vegetables/
University of California Agriculture and Natural Resources: Broccoli Rotations Lower Pathogen Populations and Reduce Disease Incidence of Verticillium Wilt. https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=44308