How many years does a rosemary plant keep producing and how should I prune it?

A healthy rosemary plant can keep producing useful sprigs for several years, often many years, which is its big economic advantage: plant once and harvest repeatedly with low input. To keep it productive you must prune. Left unpruned, rosemary turns woody and leggy with few tender sprigs, which is what chefs reject. Prune regularly by harvesting and trimming the soft green tips, which encourages the plant to branch and stay bushy with fresh growth. Avoid cutting hard into the old bare woody stems, because rosemary is slow to regrow from old wood; always leave some green leaf on each stem. A light shaping after the main flush keeps it compact. Harvest sprigs in the morning, take the tender top growth, and never strip a plant bare. Keep it in full sun and well drained soil and it will reward you for years. The main mistakes that shorten its productive life are overwatering (root rot), heavy soil, and neglecting pruning so it goes woody. Treat it like a small shrub you keep tidy, not an annual crop, and it stays a reliable backbone in your herb basket.