Pruning a shrub rose contains its size, keeps it healthy, and encourages it to produce more flowers. And contrary to rumor, it's not all that difficult. In fact, the hardest thing about pruning a shrub rose is “being brave,” says Kit Ganshaw, gardener and chief rose pruner for the Arnold Arboretum in Jamaica Plain, Massachusetts. “Sometimes, you just have to cut a branch that's alive in order to improve its shape, or get the best flowers.”
Shrub roses are best pruned in the late winter or early spring, just as their buds are swelling, but well after the threat of frost has past.
1. Cut out the dead wood. Using sharp pruners. Ganshaw removes any dead, browned branches at their bases. Avoid anvil pruners, which will crush the stem. At this point, don't worry about leaving “holes” in the shrub-further cuts will let you shape it.
2. Trim out the dying or diseased wood. Next, Ganshaw removes the dying or diseased wood, cutting back to healthy wood, which has a white core. “If you leave on the sick wood, the plant tries to heal itself, when it could be putting that energy into new growth,” she explains. At this point, Ganshaw also cuts out any branches that are crossing, since their rubbing may lead to wounds that let in pests and diseases.
3. Remove the hips and trim back protruding branches. Ganshaw removes the hips (fruit), then trims the remaining branches to give the shrub a pleasing shape. She takes off no more than a third of any branch, and tries to place her cuts just above outward-facing buds so that the shrub will keep its open shape as it grows.
4. Remove the stubs. Removing entire branches occasionally leaves a stub at the base of the shrub. (This is especially true if you can't quite get your pruners between the branches.) Stubs are both unsightly and can let in disease if they don't heal properly, so Ganshaw cuts them off cleanly using a pruning saw. Sharp loppers will also work.
5. Remove suckers. Shrub roses are often grafted onto hardier rootstocks. To keep the rootstock from taking over, nip off any suckers growing from the roots. Gallica roses have a tendency to sucker, even when on their own roots, and will form a dense thicket over time unless the suckers are removed.
From Horticulture Magazine