I put Feed in the Weed on my lawn earlier this year. Can I use my grass clippings now to mulch around my tomatoes in the garden or will this weed chemical that I put on the lawn a month and a half ago still be active and cause problems in the garden?
No you should not use the grass clippings from the lawn that were treated with a weed and feed treatment.
The herbicide may or can carry over the garden soil and plants.
This also goes for compost, do not compost treated lawn clippings.