When tilling, till only in the weed patch so roots and seeds do not get spread. Always clean equipment and machinery after working in a weed patch to prevent spread. Many biological control agents are available for control of large weed patches. This is a long-term process and not recommended for small patches. Weed management is a long term process and hence a long term commitment to the land.
Drought causes plants to shut down their growth process.
Spraying weeds during dry periods is not recommended because effectiveness diminishes greatly. Treat after rainfall IF the weed is still in the proper stage for effective control. Not all herbicides work equally on all weeds nor can every herbicide be used in every situation. Noxious weeds, in particular, are often not controlled successfully with products available at nurseries, garden shops and other retail markets. Read the label , and consult weed manuals and experts for the most effective chemical to use.