What soil and watering does rosemary need and why are my plants dying?

The single most common reason rosemary dies is overwatering and poor drainage, which causes root rot. Rosemary is a Mediterranean herb that wants to be on the dry side: light, well drained, even sandy or gravelly soil, pH around 6 to 7.5, and full sun. It prefers warm, mild and dry conditions and hates heavy clay, waterlogging and constant wet feet. If your soil holds water or you irrigate too often, the roots suffocate and the plant suddenly browns and dies. Fixes: plant on raised beds or ridges for drainage, water deeply but only when the topsoil has dried, mix sand or compost into heavy soil, and avoid wetting it during the rainy season. It also dislikes very high humidity, which brings powdery mildew, so keep airflow good. Once established it is drought hardy and needs little input. If you are in a high rainfall or heavy soil area, growing rosemary in raised beds or large pots with a free draining mix gives far better survival than planting straight into wet ground.