When you're grafting fruit trees, it's ideal to work with scions that are only one season old. Compared to old growth, they're smoother and easier to work with and tend to grow faster. In other words, you're looking at new wood that has grown over the spring and summer months and has accordingly gone dormant. In fact, many nurseries maintain mother blocks of fruit tree varieties, which are pruned in winter, as they yield the healthiest new growth for scion. However, you can't graft it immediately, so you must store it for a much longer duration in this case, risking desiccation.