Depends on what weed killer they used. Today I used a small amount of weed/grass killer around my yard and I noticed on the label that it said it was ”rain safe” in 10 minutes. I assume this means the manufacturer expects the chemical to be absorbed by the weed/grass very very quickly, or it’s not water soluble.

The best way to get answers to your question is to ask the lawn care company. It they are a good company they will answer your question. IF not, I believe it is federal law that they must give you the Material Safety Data Sheets for all chemicals they use on your yard.

You can then check with the manufacture of the chemical if it is not clear from the MSDS

Furthermore, the best time to apply a fall app of herbicide is in mid September after the heat of summer is gone.

This is my biggest problem with the TrueGreen's of the world. They do what they want to do and not what is best for the turf. Keeping the right fertilizer and herbicide (if needed) is not that hard to do. If you'd rather not keep making their boat payments for them, you can ask a few questions on this forum and learn how easy it can be to do it yourself.

When I first came to this forum in 2002, I found out what I thought I knew about lawn care was wrong. I had to unlearn most of what I got from my dad and neighbors over the years. Go ahead and keep the mowing service, but three apps of fertilizer and two apps of herbicide should be all you need. You buy the stuff and pay a kid to apply it at the proper times for you if you really don't have time to DIY. We can help you identify problems and give you some alternatives to fixing them. If you want the greenest lawn on the block, we can help. If all you want is to keep the mud from tracking in the house and don't care about having a perfectly manicured space, we can help with that, too. About all we need to get you going is your location and the type of grass you would like to have. And by location, you may need to be specific. For example Los Angeles is too vague, because that covers the area from the beach to the mountains. But Vermont is usually close enough because they don't have the range of soils and climates that you find in other parts of the country.