Rosemary does not require extensive pruning to retain its health and shape, but a little extra pruning and attention can help keep rosemary thriving and growing year after year. Pruning rosemary invigorates the plants and encourages abundant foliage and flowering. Timing and technique are important to avoid damaging the plant or limiting foliage production. Pruning rosemary also is part of harvesting the fragrant, edible leaves.
Rosemary originates in Mediterranean climates, where it grows year-round. The needle-like leaves have an aromatic scent and fresh flavor, and are commonly used in cooking. Rosemary is a small, woody shrub that grows 3 to 5 feet (about 91 to 152 cm) tall. It grows well in rocky soil with good drainage and a spot in the sun.
The best time to prune rosemary bushes is in spring, around late March or early April. At this stage in the growing season, the plants are producing new green growth but have not yet begun to flower. Pruning can be done anytime during the year, but some flowers will be lost. Rosemary can be pruned in the fall as well, as long as the plants have at least one month to recover before the first frosts.
Pruning rosemary for shape keeps the plants from becoming leggy and ungainly. The outer new growth should be removed with a pair of sharp shears to create a rounded shrub. Cutting back the top of the plant encourages growth laterally, creating a low-growing, bushy shrub.
Hard pruning should be avoided when pruning rosemary, as it can result in bare spots. The old wood close to the stem does not produce new growth, so if the plant is pruned all the way back to the old wood, it is unlikely to recover. One can trim up rosemary by removing new growth from the tips of the branches. Pruning rosemary back to the trunk only is suitable when creating a tree-like shape; the branches can be removed at the bottom of the shrub and left at the top.
As a culinary herb, rosemary clippings can go straight to the kitchen at pruning time. Harvesting rosemary as needed keeps the plants consistently trimmed up and provides a supply of fresh rosemary for cooking. The best way to harvest rosemary is to clip the top 4 inches (about 10 cm) from the tip of each branch. On small plants, remove only 20 percent of the length of the branch to avoid cutting back to old, unproductive wood.