Nasturtiums (Tropaeolum majus) earn a permanent spot in vegetable and flower gardens alike. Their orange, yellow, and red blooms brighten borders, their leaves and petals are edible with a peppery bite, and their work as a trap crop makes them a working part of pest control. The catch: they do not get along with every plant in the bed. Companion planting works in both directions, so knowing which neighbors to keep apart matters as much as knowing which pairings help. This article covers the worst companions for nasturtiums, the reasons those combinations fail, and the plants that grow happily beside them. For the full rundown on site selection, sowing, and care, start with this guide to growing nasturtiums for landscape beautification and garden health.
Why Nasturtiums Clash With Broccoli and Other Brassicas
The most reliable rule in companion planting is to keep nasturtiums out of the brassica bed. Broccoli, kale, cabbage, cauliflower, and Brussels sprouts all share the same weakness: aphids. Nasturtiums are famous for pulling aphids onto themselves, which makes them a good decoy when placed near crops that aphids rarely touch. Positioned right next to a brassica, the decoy becomes a delivery system. As the aphid population on the nasturtiums builds, winged forms move to the nearest food source, and a broccoli head or kale plant is exactly that. Garden experts who grow both say the pairing simply does not work. One longtime nursery grower puts it bluntly: she avoids planting broccoli, kale, and cabbage near nasturtiums because they do not get along, and the aphids drawn to the nasturtiums move into the brassicas, where they cause severe damage.
How the Aphid Trap Crop Backfires
Trap cropping works when the sacrificial plant sits far enough away, or when you are willing to sacrifice it. Nasturtiums lure aphids with soft, succulent growth and a mild mustard-like scent. In a mixed bed, that same lure concentrates thousands of aphids within inches of your harvest. Research on trap cropping in vegetable systems shows the trap crop must be more attractive than the crop it protects, or positioned so pests encounter it first. When the two are interplanted, aphids colonize both.
Reading the Damage Pattern
Watch for the signature signs: distorted new growth, curling leaf edges, sticky honeydew on the foliage, and a black sooty mold that grows on the honeydew. Ants patrolling the stems usually mean aphids are being farmed for honeydew. Catch the infestation before it moves from the nasturtiums to the broccoli by checking the undersides of leaves twice a week in warm weather.
| Plant | Why the pairing fails | Better placement |
|---|---|---|
| Broccoli | Aphids spill from nasturtiums onto heads | Separate bed or row |
| Kale | Heavy feeder; competes for nitrogen | Herb garden near basil |
| Cabbage | Same aphid transfer risk as broccoli | Distant row, monitor weekly |
| Fennel | Releases growth-inhibiting chemicals | Own container or corner |
| Cucumbers | None: pairing works well | Plant together |
| Tomatoes | None: aphids diverted from fruit | Border the tomato bed |
Spacing the two groups apart is the simplest fix. Put brassicas in one bed and nasturtiums at least two rows away, or use the nasturtiums as edging on the sunny side of the garden where they do not shade the brassicas. Keeping the two apart protects plant health across the whole bed and keeps aphid numbers low enough for ladybugs and lacewings to manage.
Kale: A Heavy Feeder That Competes for Nutrients
Kale sits in the same brassica family as broccoli, so it carries the same aphid risk. It adds a second problem: appetite. Kale needs a steady supply of nitrogen, potassium, and phosphorus to build its large, bushy leaves, and it keeps drawing on the soil all season as you harvest the outer leaves. Nasturtiums prefer the opposite conditions. Given rich, heavily amended soil, they produce lush foliage and very few flowers, which defeats the point of growing them.
Nutrient Competition Between Heavy Feeders and Lean-Soil Flowers
The two plants want different soil. Heavy feeders like kale, cabbage, and Swiss chard respond to compost and balanced fertilizer; nasturtiums flower best in lean, well-drained soil with moderate moisture. When they share a bed, the fertilizer you give the kale pushes the nasturtiums into leaf, and the kale roots outcompete them for whatever is left. The result is a yellowing, leggy nasturtium plant next to a kale plant that still looks hungry.
Where to Put Kale Instead
Kale does better in its own bed with other brassicas, or in a cool-season block that gets morning sun and afternoon shade. If the only open spot sits in partial shade, prepare the soil the same way you would for shaded plant beds, working in compost before transplanting and keeping the surface mulched.
Fennel and Other Allelopathic Companions
Fennel is the plant that gets along with almost nobody in the garden. It releases compounds from its roots and decomposing foliage that inhibit germination and growth in neighboring plants, a process called allelopathy. Fennel also attracts beneficial insects that gardeners want near their vegetables, which makes excluding it feel counterintuitive, but the growth suppression it causes is measurable and persistent.
How Allelopathy Works
The active compounds, including anethole and related terpenoids, leach into the soil and can slow growth for weeks after the fennel itself has been removed. Sensitive crops show stunted stems, pale foliage, and delayed flowering when planted in soil that previously held fennel. Because the effect lingers, rotating a crop into an old fennel patch can fail even in a year when the fennel is gone.
The fix is placement, not removal. Grow fennel in its own corner, in a raised bed, or in a large container where the roots cannot reach neighboring soil. Dill and fennel should also stay apart from each other, since they cross-pollinate and produce seed that grows into plants with mixed, often bland, flavor. Containers give you full control, so you can enjoy fennel foliage and anise-flavored bulbs without letting the compounds slow plant growth in the beds that matter.
When Trap Crops Turn Into Pest Magnets
Nasturtiums are usually described as a cure-all for aphids, but the truth is more conditional. They are outstanding at concentrating pests in one place, which is only useful if that place is expendable. Placed at the edge of the garden, they draw aphids and whiteflies away from tomatoes, peppers, and beans. Placed in the middle of the vegetable rows, they guarantee a pest explosion at the center of your harvest.
Building a Sacrificial Border
The standard strategy is a dedicated border or a few container plants set 10 to 20 feet upwind of the crops you want to protect. Aphids find the nasturtiums first, colonize them, and leave the vegetables alone. You then manage the infestation on the trap plants: hose them off with a sharp stream of water, prune the most infested shoots, and attract ladybugs and lacewings to clean up the rest. Leaving the border unmanaged is what turns a trap crop into a reservoir.
- A position on the windward edge of the garden
- At least 10 feet of separation from vulnerable crops
- A weekly inspection and wash-down routine
- Sacrificial plants you are willing to lose
Aphid feeding adds plant stress at the exact moment a pepper or tomato plant needs its energy for fruit set, and heavy infestations can transmit viruses that yellow and distort entire plants. Treat the border as part of the crop plan, not an afterthought.
Better Neighbors: What Thrives Next to Nasturtiums
The list of good companions is long. Cucumbers, squash, zucchini, and melons benefit from nasturtiums as a trap crop for cucumber beetles and squash bugs. Tomatoes, peppers, and eggplants sit far enough from the nasturtium pest profile that the border strategy works cleanly. Radishes mature quickly and fill gaps between slower crops. Beans fix nitrogen in the soil, which the light-feeding nasturtiums enjoy. Among herbs, basil is the classic pairing, and dill, parsley, oregano, and chives all grow well alongside.
The Cucurbit Connection
Cucurbits are where nasturtiums do their most valuable work. Cucumber beetles and squash bugs are drawn to the bright flowers and soft foliage, and a ring of nasturtiums around a squash mound intercepts them before they reach the vines. Many organic growers interplant nasturtiums directly in the squash hill, accepting that the nasturtiums will take pest pressure the squash would otherwise carry.
- Cucumbers, squash, zucchini, and melons
- Tomatoes, peppers, and eggplant
- Radishes, beans, and peas
- Basil, dill, parsley, oregano, and chives
Give the bed a clean start before planting. Work in compost only where the heavy feeders need it, keep the nasturtium side of the bed lean, and top the soil with a 2-inch layer of organic mulch, since mulching plant beds helps hold moisture, suppress weeds, and keep roots cool through the hottest weeks.
Designing a Nasturtium-Friendly Garden Layout
A layout that respects the incompatibilities above is mostly a matter of grouping. Put brassicas in one block, heavy feeders in another, and use nasturtiums as edging, border, or container plants rather than scattering them through every row. Three layout rules cover most gardens.
- Give nasturtiums an edge position with full sun and room to trail.
- Keep brassicas at least two rows away from any nasturtium planting.
- Reserve fennel for containers or a dedicated corner.
- Rotate the nasturtium border to a new bed each season so aphids do not build up in one spot.
Spacing and Rotation Rules
Space bush-type nasturtiums 10 to 12 inches apart and trailing types 18 to 24 inches apart so air moves through the foliage. In raised beds, use them as a living edge that spills over the sides. Rotate them away from last year’s brassica bed, since aphid eggs and soilborne problems can persist where either crop grew. A two-year rotation between brassicas and nasturtiums keeps both beds healthier.
The same discipline applies to every crop in the plan: give each plant room, keep feeders apart from lean-soil flowers, and place pest magnets deliberately. The spacing rules that guide tomato plant spacing work everywhere, because crowded plants stress each other and invite the exact pests nasturtiums are supposed to deter. Plan the distances first, and the nasturtiums will do their job without costing you the broccoli.