Lammas growth, also called Lammas leaves, Lammas flush, second shoots, or summer shoots, is a season of renewed growth in some trees in temperate regions put on in July and August (if in the northern hemisphere, January and February if in the southern), that is around Lammas day, August 1, which is the Celtic harvest festival.
It can occur in both hardwoods and softwoods. Examples of common trees which exhibit regrowth are oak, ash, beech, sycamore, yew, scots pine, sitka spruce and hawthorn. This secondary growth may be an evolutionary strategy to compensate for leaf damage caused by insects during the spring. It is not present in poplar, birch or willow.
Lammas growth declines with the age of the tree, being most vigorous and noticeable in young trees. It differs in nature from spring growth which is fixed when leaves and shoots are laid down in the bud the previous year. The lammas flush is free growth of newly made leaves/needles throughout the tree. One or more of the buds set in the Spring on the ends of terminal and lateral stems will break, and begin to grow, producing a new shoot.