Timing depends on the plant, and getting it wrong is why people lose a season of flowers. Spring bloomers — lilac, forsythia, viburnum, azalea — set next year's buds on old wood, so they get pruned right after they finish flowering. Cut them in fall or early spring and you've removed next spring's blooms. Summer bloomers like spirea and panicle hydrangea are pruned in late winter while dormant. Evergreens such as yew, boxwood, and arborvitae are best shaped in late spring once new growth has hardened off. One rule that applies to nearly everything in a cold climate: avoid hard pruning in late summer or early fall, since it pushes tender growth that won't survive the winter. Deadwood can come out any time of year.