When Can I Trim My Forsythia Bush Back?

Time Frame

• The ideal time to do major pruning or substantial trimming on forsythia is from late winter to mid-spring. This provides ample time for regrowth to mature and the plant to recover prior to the onset of the first fall frost. Gardeners often withhold pruning until flowering ends, since they hope to enjoy the yellow blossoms before branches are removed. Dead, broken or diseased branches may be pruned and removed any time of year.

Rejuvenation Pruning

• Old forsythia shrubs that have become large and mangy in appearance make good candidates for a severe rejuvenation pruning. Cass Turnbull, of the Plant Amnesty website, conducts major rejuvenation on forsythias once every 10 to 15 years by cutting back all branches to about 6 inches tall with loppers or pruning saw in late winter, sacrificing that year's flowering. Any branch bases with soft, corky bark are further removed so that only younger, smooth-barked stubs remain. New growth sprouts from these healthy stubs over summer, and, by winter, the shrub is tall and comprises many branches to produce flowers the next spring.

Annual Pruning Technique

• On forsythias with well-established roots, about three to four years after being planted, gardeners may opt for annual pruning of one-third of the oldest branches. Rather than a harsh coppicing, the largest diameter branches are cut back to their base and new sprouts regrow to replace them. In following years, the next oldest or largest branches are then cut back. This continually renews the shrub with new growth without destroying overall shrub structure.

Pruning Insight

• Even though you remove branches with flower buds when you prune in late winter, the bare branches allow you to best see the structure and health of the branches and bark. Turnbull said on his website that he doesn't prune forsythia during periods of drought or when subfreezing temperatures dominate, such as from late fall to late winter. Generally speaking, it's never a good idea to prune shrubs after late summer, since any regrowth is tender and immature and likely killed by the onset of the first frosts and freezes. Plus, since forsythia branches already develop flower buds by late summer, late pruning reduces flowering for the next spring.