Hydrangeas (Hydrangea spp.) are beautiful flowering plants that can generally be grown without too much trouble. There are, however, a few common mistakes everyone makes when planting hydrangeas. If you fail to avoid these mistakes, or if you've inherited hydrangeas from a previous homeowner who did, a few problems can crop up. Even if you do everything right, issues can still occur, like the appearance of the insidious Japanese beetle or hungry garden slugs.