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

Tall Oregon Grape
Shares part shade and moderate water; blooms in early spring, before this one opens; evergreen, so the bed keeps its shape through winter; taller at 4.5 ft, for the layer behind it.

Winter Daphne
Shares part shade and moderate water; blooms in early spring, before this one opens; evergreen, so the bed keeps its shape through winter; taller at 4.5 ft, for the layer behind it.

Japanese Aucuba
Shares part shade and moderate water; blooms in early spring, before this one opens; evergreen, so the bed keeps its shape through winter; stands well above at 8 ft, a canopy to plant beneath.

Hellebore
Shares part shade and moderate water; blooms in late winter, before this one opens; evergreen, so the bed keeps its shape through winter; echoes the pink.
Blooming alongside

Western Wild Ginger
Shares part shade and moderate water; opens in mid spring and overlaps into this one's season; evergreen, so the bed keeps its shape through winter; lower at 8 in, for the front of the border.

Woodland Stonecrop
Shares part shade and moderate water; opens in mid spring and overlaps into this one's season; evergreen, so the bed keeps its shape through winter; lower at 8 in, for the front of the border.

Tufted Hairgrass
Shares part shade and moderate water; its seed heads stand through winter, for structure and the birds.

California Fescue
Shares part shade and low to moderate water; opens in mid spring and overlaps into this one's season; its seed heads stand through winter, for structure and the birds; green against pink, for contrast.