Trimming shrubs stimulates their ﬂowering and controls their development. If you want to encourage flowering or reduce the size of an overgrown shrub, you must trim at the time that's most propitious for encouraging the next season's flowering.
End of May to mid-June, after flowering:
- Spring-flowering shrubs, because this will give them enough time after flowering to form the floral buds for next spring. They flower on the previous year's wood.
- Plants with decorative leaves such as dogwood or ninebark, and those that bloom on new wood such as hydrangea and potentilla, should be pruned in early spring.
October, or in April and May before the buds open:
- Shrubs that flower in mid-summer or early autumn.
- Shrubs that bloom on old wood such as lilac, weigela and spirea, should be pruned after the flowering period.
- Decorative-foliage shrubs.
- Shrubs that need severe cutting back.
To promote better growth, remove any damaged branches and dead wood as soon as you notice it.
Never remove more than 30% of old growth during any given year, as this will weaken your shrub.