What Not to Plant Together: The Few Pairings That Really Conflict
GardenDraft Team · May 23, 2026 · 6 min read
Part of: Garden Planning Guides · Crop Rotation & Companion Planting Guides
Most "enemies" lists in companion-planting charts are folklore, but a handful of plants really do work against each other, and the reasons are concrete: competition, shared diseases, allelopathy, and shade. Knowing what not to plant together — the few combinations that genuinely cause trouble — is more useful than memorizing a grid of supposed rivalries.
What not to plant together: conflicts with real reasons
Onions and the legumes. The onion family (onions, garlic, leeks, chives) sits awkwardly next to beans and peas. Gardeners have reported stunted legumes near alliums for generations, and while the mechanism is debated, it's a low-stakes pairing to simply keep apart.
Fennel next to almost anything. Fennel is mildly allelopathic: it releases compounds that inhibit the growth of neighbors, including tomatoes and beans. Give it its own corner or a separate container.
Potatoes and tomatoes. Both are in the nightshade family, so they share the same devastating diseases: early blight, late blight, verticillium. Planting potatoes beside tomatoes hands those pathogens a bridge from one crop to the other and makes an outbreak far harder to contain.
The bigger rule: don't crowd the same family together
The most reliable "what not to plant together" guidance isn't about specific grudges; it's about families. Two heavy feeders from the same family planted side by side compete for identical nutrients and, worse, swap the same pests and soil diseases freely. A block of tomatoes, peppers, eggplant, and potatoes, all nightshades, is a buffet for the pests and pathogens that specialize in them.
| Avoid clustering | Because |
|---|---|
| Tomato + potato + pepper + eggplant | Same nightshade pests and blights |
| Onion/garlic + beans/peas | Reported stunting of the legumes |
| Fennel + most vegetables | Allelopathic; suppresses neighbors |
| Two tall crops shading a short one | Light competition, not chemistry |
Competition for light counts too
Plenty of "incompatibilities" are simply one plant shading out another. A row of corn or staked tomatoes on the south side of lettuce or spinach will starve them of light by midseason. No chemistry involved, just geometry, and it's solved by putting tall crops on the north side, as in any sound garden layout.
Spacing and rotation do the real work
Two tools handle nearly every conflict on this page. Plan generous spacing so neighbors don't compete (use the square-foot spacing guide) and move families to a new spot each year so same-family diseases never settle in; see crop rotation made simple. For the pairings that genuinely help, see companion planting that works.
Frequently asked questions
- What vegetables should not be planted together?
- A few pairings with real reasons: onions, garlic, and leeks next to beans and peas (reported stunting of the legumes); fennel beside most vegetables (it's allelopathic and suppresses neighbors); and potatoes beside tomatoes (both nightshades, so they share blights and pass them back and forth).
- What's the most reliable rule for what not to plant together?
- Don't cluster the same plant family. Two heavy feeders from one family — say tomatoes, peppers, eggplant, and potatoes, all nightshades — compete for identical nutrients and trade the same pests and soil diseases freely. Spreading families out matters more than any single 'enemy' pairing.
- Is it bad to plant tall crops next to short ones?
- It can be — but that's geometry, not chemistry. A row of corn or staked tomatoes on the south side of lettuce or spinach will shade them out by midseason. The fix is simply putting tall crops on the north side of the bed.