Most of the people I know never think about this question. The reason would have to be that don’t need to know how the chemical works, and most of the time they don’t really care why or how weeds grow. That is perfectly understandable, so I am going to give those people a very simple answer to the question, “When can I kill weeds?” The simple answer would be, “Whenever they are actively growing.” That is something that even the most busy people could figure out…is that plant growing right now?!
Now, for the rest of us that want to know some more about weed killers, here is a little more definition. All weed killers are foliage killers. What that means is the chemical has to touch the foliage of the plant in order for the plant to absorb it. If you spray a weed killer or a everything killer like roundup straight onto dirt, you have just wasted your time and money. Yes, there are products out there that say they prevent weeds, that is a whole other blog. A preventer can be sprayed on dirt to prevent new plants from arriving.
Here is something else to think of when asking when it is too late to kill weeds. Are you going to be seeding within the next 6 weeks, or have you seeded within the last 6 weeks. The weed killer will leave a residual that will destroy the germination rate of grass seed. I like to say 6 weeks is the rule of thumb for seeding on either side of applying weed killer.
Typically, once you get your first hard frost of the fall, the ‘normal’ weeds will disappear pretty quickly on their own. They are seasonal, annual weeds. The actual plant will die for the winter, but its seeds will grow again in the spring.