Companion plants for Coreopsis
Coreopsis blooms from June through September. It wants full sun and low water. The 8 companion pairings below each state why the pairing works.
Blooming before Coreopsis opens

Rock Cress
Shares full sun and low water; blooms in mid spring, before this one opens; both offer pollinator forage, in different months; evergreen, so the bed keeps its shape through winter.

Rosemary
Shares full sun and low water; blooms in early spring, before this one opens; both are strong pollinator plants, and they feed in different months; evergreen, so the bed keeps its shape through winter; they only overlap in zones 8 to 9.

Wisteria
Shares full sun and low to moderate water; blooms in mid spring, before this one opens; both are strong pollinator plants, and they feed in different months; purple against yellow, for contrast.

Flowering Crabapple
Shares full sun and low to moderate water; blooms in mid spring, before this one opens; both are strong pollinator plants, and they feed in different months.

Lilac
Shares full sun and low to moderate water; blooms in mid spring, before this one opens; both offer pollinator forage, in different months; taller at 10 ft, for the layer behind it.
Taking over as Coreopsis finishes

Goldenrod
Shares full sun and low water; overlaps, then carries the display into mid fall; both are strong pollinator plants; its seed heads stand through winter, for structure and the birds.

Switchgrass
Shares full sun and low water; overlaps, then carries the display into mid fall; both are strong pollinator plants; its seed heads stand through winter, for structure and the birds.
Blooming alongside

False Indigo
Shares full sun and low water; opens in late spring and overlaps into this one's season; both are strong pollinator plants; its seed heads stand through winter, for structure and the birds.