A bud scale is a structure found on some plants which protects buds while they are in dormancy or while they are forming. Plants which form buds without scales are said to have “naked” buds, reflecting the fact that the buds are not protected. When buds break dormancy or open up, the bud scales drop away or enlarge and droop to the side, allowing the bud to emerge fully. Bud scales which drop leave behind distinctive markings known as bud scars.
Bud scales are modified leaves. They may be hairy or sticky, depending on the species of plant. A common location for a bud scale is on a terminal bud, the bud at the end of a branch which controls dormancy for other buds on the same branch. Using bud scars, people can sometimes determine how old a given branch is, as the number of scars will indicate how many times the terminal bud has broken dormancy.
It can be important to determine whether or not a plant has bud scales when using a plant identification key, as scales can be used as an identifying feature to distinguish between different types of plants. Identifying bud scales can also be important when examining plants to see how well they are weathering over the winter, and in identifying early signs of disease. One problem with the protection provided by a bud scale is that it also provides an ideal shelter for fungi, which may overwinter in the bud and then emerge in the spring, infecting the rest of the plant.
When handling plants which use bud scales to provide overwintering protection to buds, it is important to avoid disturbing the bud scales. Knocking a scale off or pinching it may interfere with the health of the underlying bud, which could cause problems for the plant in the following spring. Conversely, pruning and manipulation of buds can be done during the dormant winter months by experienced gardeners to control plant growth to achieve a desired objective.
People may also use the term “bud scale” in reference to spruce bud scale disease, a type of parasitic infection which can be very damaging to trees. The disease is characterized by the appearance of small scales on the tree which house insects and their eggs. Insects can overwinter, emerging in the spring to cause considerable damage, and trees with bud scale disease are vulnerable to fungal infection.