Companion plants for Petunia
Petunia blooms from May through October. It wants full sun and moderate water. The 8 companion pairings below each state why the pairing works.
Blooming before Petunia opens

Windflower
Takes full sun to part sun and moderate water; blooms in early spring, before this one opens; both offer pollinator forage, in different months; lower at 6 in, for the front of the border.

Hyacinth
Shares full sun and moderate water; blooms in early spring, before this one opens; lower at 10 in, for the front of the border; both offer pollinator forage, in different months.
Blooming alongside

Joe Pye Weed
Shares full sun and moderate to high water; its seed heads stand through winter, for structure and the birds; both offer pollinator forage; taller at 6 ft, for the layer behind it.

Sneezeweed
Shares full sun and moderate water; its seed heads stand through winter, for structure and the birds; both offer pollinator forage; taller at 3.5 ft, for the layer behind it.

Perennial Sunflower
Shares full sun and moderate water; its seed heads stand through winter, for structure and the birds; both offer pollinator forage; taller at 5 ft, for the layer behind it.

Rosemary
Shares full sun and low to moderate water; opens in early spring and overlaps into this one's season; evergreen, so the bed keeps its shape through winter; both offer pollinator forage.

Winterberry
Shares full sun and moderate to high water; overlaps, then carries the display into early winter; both offer pollinator forage; stands well above at 6.5 ft, a canopy to plant beneath.

Lilac
Shares full sun and moderate water; opens in mid spring and overlaps into this one's season; stands well above at 10 ft, a canopy to plant beneath; echoes the purple.