Many growth regulators are widely used on ornamental plants. These substances do not fit into any of the five classes of hormones. For example, utility companies all over the country often apply growth retardants, chemicals that prevent plant growth, to trees in order to prevent them from interfering with overhead utility lines. It is less expensive to apply these chemicals than to prune the trees, not to mention safer for the utility workers. Also, azalea growers sometimes apply a chemical to the terminal buds rather than hand-pruning them. Scientists are still searching for a hormone to slow the growth of lawn grass so that it doesn’t have to be mowed so often.