Thinning plum trees is important to prevent branches from breaking under the weight of the fruit. If branches do break, prune them back to undamaged wood, ideally cutting back to a natural fork to avoid leaving stubs. Be sure to water the young trees heavily every week during the first growing season to help promote growth. Then, water regularly. It’s best to water the plant deeply at the soil line, then let the soil dry out (though not completely) and water again.