Most ornamental crabapples are grafted trees. The root stock is selected because it is vigorous and adapted to typical growing conditions. Crabapple varieties will have root suckers to varying degrees. You cannot use herbicides to control root suckers. The suckers are connected to the main plant, and chemicals will move from the root suckers to the tree, possibly causing damage. Likewise, diseases such as fire blight can move from the root sucker to the trunk of the tree and cause significant damage. Root suckers should be pruned off at the soil line whenever they appear.