The influence of rain and drought before, and air temperature during, weed control with hot water was studied in laboratory experiments on the test weed Sinapis alba (white mustard). The plants were grown in a greenhouse and treated outdoors. There was no difference in weed control effect when S. alba plants at the four-leaf stage were treated at the air temperatures 7°C and 18∘C. The effective energy dose for a 90% fresh weight reduction was 465 kJ m−2 for both air temperatures. Weed control of S. alba at the four- to six-leaf stage in rainfall above the rainwater run-off level increased the required effective energy dose by 20% (i.e. 120 kJ m−2) compared with dry plants. A short period of drought just before treatment on S. alba at the two- to four-leaf stage increased the plant fresh weight reduction, which was 22% at low energy dose (190 kJ m−2) and 44% at high energy dose (360 kJ m−2). Hot water weed control should thus be carried out when the plants are drought stressed and avoided when the plants are wet. The air temperature seems to be of little importance in the range 7–18∘C.