Summary.In loamy soil with a low humus content (0·8% organic carbon) simazine at a concentration of 2′5 ppm reduced growth of shoots, roots and trunk diameter in 1‐year‐old scions of five apple varieties on M IX rootstock. The most susceptible was Cox's Orange Pippin; 5 ppm was lethal for this variety only. In experiments with vertically divided pots, where one part of the root system developed in soil with simazine (2–5 ppm and 5–0 ppm for apple and 5·0 ppm for plum) and the other part in simazine‐free soil, there was less effect on shoot growth and less leaf damage than where all the soil contained simazine. Growth of that part of the root system in soil containing simazine w