Tomatoes and Nasturtium
Beneficial Pair — Good Match
Trap cropping for aphids using nasturtium is one of the more genuinely evidence-backed mechanisms in companion planting - the strength of aphids' preference for nasturtium over many other plants has been documented in multiple field studies, not just repeated as folklore. The main caveat is timing and monitoring, covered below, which matter more here than with most other pairings on this site.
Mechanism: Trap Cropping
Aphids, particularly black bean aphid, show a strong feeding preference for nasturtium over most nearby vegetables, and that preference is well documented in trap-cropping literature - it's one of the clearer, better-studied examples of the trap-crop mechanism in home-garden companion planting generally. Planted near a tomato's growing tips, nasturtium can draw aphids away from the tomato and concentrate them on itself, where they're easier to spot, hose off, or simply tolerate on a sacrificial plant rather than on the crop you're trying to harvest.
Trap cropping is one of the few companion-planting mechanisms with real research behind it, and nasturtium next to tomatoes is close to the textbook example - which also means it's one pairing where getting the details right actually matters.
Why This Trap Crop Actually Works
Black bean aphid and several other common aphid species show a measurable, repeated preference for nasturtium's soft new growth over most nearby vegetable crops, and researchers have used exactly this preference to study trap cropping as a pest-management strategy in its own right, not just as garden folklore. Planted where it can intercept aphids moving through the bed, nasturtium genuinely pulls a meaningful share of the local aphid population away from a tomato's tender growing tips, where aphid feeding stunts new growth and can spread viral disease.
The Trap Can Become the Trap
The mechanism that makes nasturtium useful is also its main risk. A trap crop works by concentrating pests, and if that concentration is never managed, the nasturtium eventually becomes a large, thriving aphid colony sitting a foot or two from your tomatoes rather than a sink that removes them from the system. Once nasturtium foliage gets heavily colonized, some aphids will disperse looking for new growth, and a tomato right next door is an obvious next stop. This is different from most companion pairings on this site, where the plant just sits there passively - nasturtium as a trap crop requires you to actually watch it and intervene, whether that's hosing off heavy infestations, cutting back badly colonized growth, or in extreme cases pulling and replacing the plant.
Used with that maintenance in mind, this is one of the more genuinely defensible pairings in companion gardening - real research backs the underlying aphid preference, and the sacrifice-plant logic is sound. Used passively, without ever checking on the nasturtium, it can quietly work against you by the second half of the season.
Spacing
Nasturtium sprawls, so give it real room - 12-18 inches from the tomato stem for a bush type, more if you're growing a trailing variety that will vine along the ground. It works well planted at the outer edge of the tomato bed rather than tight against the stem, since its job is to intercept aphids before they reach the tomato, not to grow underneath it.
Watch For
This is the one pairing on this site where the trap crop can become the problem: if the nasturtium's aphid population is allowed to explode unchecked, it can eventually overflow onto the tomato anyway, especially once the nasturtium is fully colonized and aphids start dispersing to find fresh growth. Check the nasturtium regularly through the season, and be ready to cut back or remove a heavily infested plant rather than letting it sit as a permanent aphid reservoir next to your tomatoes.
Frequently Asked Questions
How do I know if my nasturtium trap crop is working or backfiring?
Check it weekly once aphids appear anywhere in the garden. A working trap crop shows a visible aphid colony on the nasturtium while the tomato stays relatively clean; a failing one shows aphids spreading onto the tomato despite the nasturtium being present, which usually means the nasturtium is overwhelmed and needs cutting back.
Can I use nasturtium as a trap crop for pests other than aphids?
Aphids are the best-documented case, but nasturtium is also a known feeding target for cabbage white caterpillars, which is why it's paired with brassicas elsewhere on this site. Its usefulness against tomato-specific pests like hornworm is much less established.