An apparatus (10) comprises a water storage tank (14), a heater (16), a pump (38) and an outlet duct (70) to convey heated water to a plurality of lances (24) through hoses (22). The heater (16) is arranged to heat the water in at least two successive stages (66, 67), to an intermediate and then to a final temperature, at least the second stage (67) of the heater being activated in pulses, the frequency and/or duration of the pulses being adjustable to control the resultant temperature. Foaming agent is introduced (18, 40) into the water. Upstream of each hose (22) is a diversion valve (85) connecting to a water return circuit (54) leading back to the water storage tank (14) and the water return circuit (54) includes a heat exchanger (55, 56). The heater (16) may be liquid fuelled. The apparatus can be mobile, for example on a motor-vehicle (12), and it enables a number of people to control weeds separately, using separate lances (24), from the same apparatus (10).
