Time the application of herbicides to achieve maximum effectiveness. The right time to apply systemic poisons is when water and sugars are being rapidly moved (translocated) around the plant (usually spring and summer). Herbicides are likely to be less effective on plants which are not growing due to the season (winter) or which are stressed by drought, cold, frost and waterlogging (Buchanan 1989). Plants which sucker easily from the roots, and plants with underground reproductive organs must be treated just after flowering when sugars (and hence herbicide) are moving down into the bulb or tuber. Treatment when sugars are moving upward from the underground organs to form the new season’s shoot will be useless (Buchanan 1989).
While this is the general rule there are always exceptions, so check the CRC for Australian Weed Management website at www.weeds.crc.org.au for more details.