Many growth regulators are used on ornamental plants. These substances do not fit into the five classes of hormones. For example, utility companies in some areas may apply growth retardants, chemicals that prevent plant growth, to trees to prevent the trees from interfering with utility lines. It is sometimes less expensive to apply these chemicals than to repeatedly prune the trees. Scientists are still searching for a hormone to retard the growth of lawn grass so that it does not have to be mowed so often.
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