Thanks for contacting Ask an Expert with your herbicide question.
I expect that you are trying to control weeds such as dandelions, clover, and plantain in your lawn, and I will go on that assumption. Most common lawn weed killers are most effective when these weeds are actively growing. As temperatures sink into the 50s, the weeds slow their growth. As a result, the weed control chemicals are not circulated fully in the plant, and the weeds are not controlled.
Temperatures in Oregon City are in that range, and I would not expect to see much result from an application of either a spray or a weed-and-feed product. In addition, excess spray may be washed off the lawn and into the waterways.
If weeds are flowering, continued mowing could keep more seeds from falling and adding to next season’s problems.
The Oregon Extension Service has two publications that can help get your lawn into shape for the coming spring. The primary lesson of all is that a healthy, well-cared for lawn is the best defense against weed invasion.
All the best,