Watermelon and Nasturtium
Beneficial Pair — Good Match
The aphid-trap mechanism has real documentation behind it in general, even without a watermelon-specific trial. The pollinator-draw claim is plausible - nasturtium flowers genuinely attract bees - but we aren't aware of evidence that a few nasturtium plants meaningfully increase bee visitation to watermelon flowers specifically over what's already happening in a garden with other blooming plants nearby.
Mechanism: Trap Cropping
This pairing carries two distinct claims worth separating: nasturtium's documented aphid-trapping ability, applied to watermelon's vine tips the same way it's cited for other cucurbits, and a second, different benefit - nasturtium's bright, nectar-rich flowers drawing bees to the general area, which matters more for watermelon than for most crops since watermelon fruit set depends entirely on successful bee pollination of separate male and female flowers on the same vine.
Watermelon is one of the more pollination-dependent crops in this reference, since each vine produces separate male and female flowers and needs a bee to physically carry pollen between them for any fruit to set at all - which makes the pollinator-draw angle of this pairing worth taking seriously alongside the more commonly cited pest-trap claim.
Two Claims, Different Levels of Relevance
The aphid-trapping mechanism is the one usually cited for nasturtium and cucurbits generally, but for watermelon specifically, the pollinator-support angle may matter more in practice, given how directly fruit set depends on bee activity. Neither claim has a controlled trial behind it for this exact pairing, but the pollination angle is at least addressing watermelon's single biggest yield-limiting factor rather than a secondary pest concern.
Spacing
Watermelon vines sprawl extensively, 36-72 inches depending on variety, so nasturtium works best as a perimeter planting around the watermelon patch's outer edge rather than something interplanted directly among the vines, where it would quickly be overtaken.
Watch For
Watermelon's fruit set depends on bees moving pollen from male to female flowers, and a garden with poor pollinator activity overall will see reduced fruit set regardless of what's planted nearby - if pollination seems to be the limiting factor, addressing broader pollinator habitat in the garden matters more than a single nasturtium border.
Frequently Asked Questions
Will nasturtium actually improve my watermelon's fruit set?
Possibly as one small part of overall pollinator support, though we can't confirm a measurable effect from nasturtium specifically. If poor fruit set is a recurring problem, broader pollinator-friendly plantings and avoiding pesticide use during bloom will likely matter more than any single companion plant.