Should I Prune Weak, Spindly Knock Outs in Spring?

When to Prune

• "Knock Out" roses should be pruned when the buds begin to swell, but before the leaves have emerged. The exact timing of this depends on the local climate and how late winter lingers, but it is generally in the months of February or March. Pruning later in spring after the leaves have emerged will not cause significant harm to the plant, but will set flowering back a few weeks. It is better to prune a little late than to not prune at all.

Pruning Principles

• "Knock Out" roses can grow quite tall and will appear "leggy" if left unpruned. Heavy pruning leads to shorter, thicker stems and a greater portion of the plant will be covered in blooms as a result. Pruning is stimulating to the shrub and actually causes a stronger growth response in the coming season than if it was left unpruned -- especially important for a weak, spindly specimens. Weak plants should have any dead, diseased or damaged wood removed at this time.

Pruning Tools and Techniques

• Use hand pruners to prune stems up to 1/2 inch in diameter and bypass loppers for larger branches. Thin out small, densely spaced branches and any that grow towards the center of the shrub. Then, prune the remaining canes back to one-third to one-half of their length. Cut each one back to about 1/4 inch above an outward-facing bud, with an angled cut facing away from the bud. To help prevent the spread of disease, wipe the pruning equipment with alcohol to sterilize it between cuts and before and after pruning each individual shrub.

Another Factor to Consider

• If a "Knock Out" rose appears weak and spindly, it may need more than just pruning. For example, they often appear this way if they do not receive enough sunlight. "Knock Out" roses are healthiest and flower profusely with six to eight hours of direct sun each day, but can tolerate as little as four hours before showing signs of poor health. If this is the case, prune the shrub and then transplant it to a sunnier position so it can flourish in the coming growing season.