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Roundup (active ingredient: glyphosate) is a translocated herbicide, meaning that it relies on the plant's own circulatory system to move the glyphosate from the foliage to other parts of the plant, including the roots. The action of glyphosate is this: it monopolizes all the bonding sites on a particular enzyme that plants need for protein synthesis. When the herbicide effectively makes the enzyme unavailable, protein synthesis is interrupted and the plant eventually dies. [Animals don't have the enzyme involved, which is why Roundup is essentially nontoxic to animals when used at the concentrations specified by the label.]
These two facts make it clear that Roundup can only work when it is applied to a plant that is physiologically active. Plants that are green during the winter are, to some extent, but at low temperatures their metabolic rate will be extremely slow, and you can expect Roundup to work very, very slowly if at all. Better to wait for spring and apply it when plants are actively growing.
Roundup, like every herbicide, must be applied at the correct point in the plant's life cycle for maximum effectiveness. On perennial weeds, it should be applied at times when the plant has minimum energy reserves in its roots (such as at the end of the spring growth flush or in early fall when it is exhausted by flowering and seeding) and when it is not sending all its energy upward for reproduction. You want it to get to the roots.
Don't mix it at concentrations higher than the label specifies. It's not only illegal, it's ineffective. Immediate browning of the above-ground growth is NOT a good thing (television ads to the contrary notwithstanding). When the foliage is blasted by high concentrations of herbicide, then it can't participate in moving the herbicide to the plant's roots. The top growth may be gone, but the roots will respond by sending up new shoots.
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