The Golden Rule of Pruning
The most important factor in deciding whether to prune a flowering shrub in late summer is understanding if it blooms on "old wood" or "new wood." This isn't jargon; it's the simple secret to successful pruning. Plants that bloom on old wood produce their
flowers on stems that grew in the previous year. If you prune these in August, you risk cutting off next spring's flowers. Conversely, plants that flower on new wood produce blooms on the growth from the current season. Pruning these in late summer can often encourage more flowers and vigorous growth. Knowing this difference is the main check every gardener needs to make.
Green Light: Plants to Prune in August
For many summer-flowering shrubs, August is the perfect time for a trim. This encourages them to produce another flush of flowers, maintains their shape, and improves air circulation to ward off diseases. Roses, for instance, benefit greatly from deadheading spent blooms and a light shaping this month, which signals the plant to produce new buds instead of seeds. The same principle applies to hardy hibiscus; trimming them now can stimulate more of their showy blossoms before the season ends. Wisteria also requires a summer prune in August to control its vigorous growth and encourage the formation of flower spurs for next year. Other candidates for an August trim include lavender (after it has flowered to prevent it from becoming woody) and summer-flowering clematis.
Red Light: Plants to Leave Alone
For some plants, an August pruning session would be a mistake. The primary group to avoid are shrubs that flower on old wood and have already set their buds for next year. This includes many popular garden staples like azaleas, rhododendrons, and lilacs. Pruning these now means you will be cutting away all the potential flowers for the following spring. Similarly, certain types of hydrangeas, particularly Bigleaf (macrophylla) and Oakleaf varieties, bloom on last year's stems. The best time to prune these is right after they finish flowering in mid-summer, not late in the season. If you trim them in August or later, you sacrifice the floral display for which they are so beloved. If you are ever unsure, the safest bet is to wait.
Pruning for Health and Tidiness
Beyond encouraging flowers, August pruning serves a vital health purpose. This is an excellent time to inspect all your plants—even those you shouldn't be hard-pruning—for any dead, damaged, or diseased wood. Removing these branches is always a good idea, regardless of the season. It helps prevent the spread of fungal issues and pests, improves the plant's overall structure, and allows sunlight and air to penetrate the center. Use clean, sharp shears to make precise cuts. This type of maintenance tidies up the garden after the peak of summer growth and ensures your plants are in strong condition as they head into the next season. This is also a good time to lightly shape hedges and remove suckers from the base of shrubs and trees.
