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

Camellia
Shares part shade and moderate water; blooms in midwinter, 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 7 to 8.

Serviceberry
Takes part sun to part shade and moderate water; blooms in mid spring, before this one opens; both are strong pollinator plants, and they feed in different months; its white flowers settle the green.
Taking over as Lady's Mantle finishes

Summersweet
Shares part shade and moderate to high water; overlaps, then carries the display into late summer; both are strong pollinator plants; taller at 6 ft, for the layer behind it.

Northern Sea Oats
Shares part shade and moderate water; overlaps, then carries the display into early fall; its seed heads stand through winter, for structure and the birds; taller at 3.5 ft, for the layer behind it; this one spreads, so it will crowd its neighbor in time.
Blooming alongside

Azalea
Shares part shade and moderate water; opens in mid spring and overlaps into this one's season; both are strong pollinator plants; evergreen, so the bed keeps its shape through winter.

Periwinkle
Shares part shade and low to moderate water; opens in mid spring and overlaps into this one's season; both are strong pollinator plants; evergreen, so the bed keeps its shape through winter.

Pachysandra
Takes part shade to full 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.

Bugleweed
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 6 in, for the front of the border.