Strictly speaking,are certain organic chemicals formed in plant tissues. This term is however also used to cover growth-regulating substances or synthetic chemicals used at very low concentrations to change the growth and development of plants. Plant growth substance is an alternative name. The most important application of these substances is as selective weedkillers. Farmers use these weedkillers to destroy yellow charlock, corn buttercup, fat hen (goosefoot) and other pernicious weeds of cereals. Gardeners use the same materials (usually MCPA or 2, 4-D) for tackling plantains, creeping buttercup and similar weeds on lawns. Selective weedkillers, as their name indicates, leave the crop, e.g. corn or grass, unharmed, provided they are always applied at the correct concentrations. Another use is for rooting which are normally difficult to ‘strike’. June drop and pre-harvest drop — see JUNE DROP — of apples and pears can also be prevented, thereby extending the picking period. The first truss of tomato fruits frequently fails to set properly. Hormone sprays can be used to assist natural — see TOMATO.
Maleic hydrazide is a fairly new compound which retards the development of cells at the growing point. It is used to spray grass verges to cut down frequency of mowing.