Diva

Basil goes closest to Diva. The volatile oils are real, and there's a plausible case they confuse aphids and thrips at close range — but the more practical reason is that basil wants the same consistent 1–2 inches of water per week and you're already making the same pass through that 12-inch radius at harvest. French marigolds (Tagetes patula) are worth planting as a bed border specifically because their roots produce thiophene compounds that suppress soil nematodes over time. Given that Meloidogyne root-knot nematodes are a documented problem on cucumbers, that's not a trivial benefit — just don't plant a single decorative row and expect results; you need enough root mass spread across the border to matter. Nasturtiums work well between hills as a trap crop, pulling aphids off the cucumber vines and onto themselves without competing for vertical trellis space.

Beans and corn make structural sense. Beans fix nitrogen, which feeds heavy-feeding vines without extra side-dressing inputs. Corn provides some afternoon shade buffer in peak July heat without cutting into the 6+ hours of full sun cucumbers need, and both crops run on similar watering schedules so you're not managing two different moisture regimes in one bed.

Keep potatoes out of this planting entirely — they share several of the same soil-borne pathogens, including overlapping nematode pressure, and co-locating them just keeps that inoculum level high going into next season. Fennel is a different kind of problem: it produces allelopathic root exudates that suppress germination and root development in most vegetables. Give it a container or a far corner, not a spot anywhere near the cucumber bed.