Lengthen your garden season by including monkshood in your planting areas. This purple-flowered perennial starts opening buds as frost is knocking at the garden gate—and keeps blooming through light frosts. Deer and rabbit dislike monkshood, which is poisonous to people if consumed. Plants can be slow to establish. Divide only when clumps are crowded, which shouldn’t occur before the three-year mark. Plants are hardy in zones 4 to 8.