Fall is the time to spray weeds

    The arrival of cooler weather in the fall is often the best time of year to spray for tough-to-control perennial and biennial weeds.
    Perennials, of course, are plants that live for three or more years — for example, dandelion, bindweed and Canada thistle. Biennial weeds live for two years, spending their first season growing vegetatively and accumulating the energy reserves that help them overwinter and flower the next year. A few examples of biennial weeds are Scotch thistle, burdock, and mullein.
    Over the spring and summer, both types of weeds dedicate most of their available energy to above-ground growth and/or flowering and seed production. During this time, the flow of sap, water and other resources in the plant goes from the roots up. When sprayed during this time, this one-way flow in the plant often prevents herbicides from moving down into the roots in adequate quantity to kill the entire plant. When this happens, above-ground growth will be damaged or burned back, but the plant quickly regrows from the roots in most cases.
    In the fall, however, plants begin to prepare for winter by reversing the direction of flow, in order to stockpile energy in the roots. When sprayed at this time, more herbicide will be pulled into the roots and has a much better chance of killing the entire plant. This reversal of flow direction begins as temperatures lower and days shorten in early fall, but it is the first light frost of the year that really accelerates the process for most weeds.

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