Radishes and Nasturtium
Beneficial Pair — Good Match
Nasturtium's aphid-trapping ability is genuinely documented, which is why we don't rate this traditional lore outright - but that documented effect doesn't map cleanly onto flea beetles, radishes' more common problem, so the pairing's usefulness for radishes specifically is less certain than nasturtium's reputation might suggest.
Mechanism: Trap Cropping
Nasturtium's documented trap-crop strength is against aphids, which prefer its foliage over most garden vegetables, but radishes' main pest problem is usually flea beetles, which chew small shot-holes in radish leaves and aren't the same insect group nasturtium is known to draw. This pairing borrows nasturtium's aphid-trap reputation and applies it to a crop whose actual biggest threat is a different pest entirely.
This pairing shows up often in companion-planting lists, but it's worth separating what nasturtium is actually good at from what radishes actually need protection from, because the two don't line up as neatly as the pairing's popularity suggests.
Right Trap Crop, Wrong Pest
Flea beetles - the insect most likely to damage radish leaves - aren't the pest nasturtium is documented to draw; that's aphids. If flea beetles are your actual concern, a floating row cover over young radish seedlings is a more directly effective fix than nasturtium, which is better reserved for beds where aphids, not flea beetles, are the recurring problem.
Spacing
Radishes' tight 2-3 inch spacing and nasturtium's 8-12 inch spread mean nasturtium works better as a border along the radish row's edge than interplanted directly among the radishes, where it would quickly shade out the fast-growing roots.
Watch For
Radishes mature in as little as 22-30 days, often finishing before a nasturtium planted at the same time has grown large enough to offer any trap-crop effect at all - if you want the timing to actually line up, start the nasturtium a couple of weeks ahead of the radish sowing.
Frequently Asked Questions
Is there any real benefit to growing radishes and nasturtium together at all?
A modest one - both are fast, low-maintenance edge plants that ask little of a bed, so pairing them wastes nothing even if the specific pest-trap claim doesn't hold up for radishes. Treat it as a low-cost, low-expectation combination rather than a targeted pest-control strategy.