Althea, known botanically as Hibiscus syriacus and more commonly as Rose of Sharon, is a flowering shrub or small tree that thrives in warm and temperate climes. Althea reaches up to 12 feet in height at maturity with a spread of up to 10 feet. According to North Carolina State University it has a late but long bloom season from July through September and into October in warmer climes.
Althea does not require regular trimming or pruning for bloom. It can, however, tolerate significant pruning for shape and size if so desired and will benefit from regular light grooming to remove dead or damaged tissues.
Prune away dead, diseased, damaged or otherwise compromised leaves and branches throughout the year as you spy them. Place the cut back to the point of healthy tissue just above a leaf node of bud.
Trim away spent flower heads as they fade by pinching with your fingers or by cutting with secateurs. This will prevent the plant energy being directed towards seed head formation. Leave the spent flowers in place if you want seed heads to develop and mature on the tree in fall and winter.
Conduct any extensive trimming of your althea in the early spring to control shape, size or encourage larger flowers and a bushy, full canopy. Refrain from late spring and summer pruning as this will disrupt bloom significantly and can expose the tree to heat stress and moisture loss at the wound sites.
Cut back perimeter branch tips to the desired length being careful to remove no more than 1/3 of the tree foliage and branch mass in any one trimming session to prevent stress and shock from occurring.