I am clearing an area of small trees & dense brush.There are holes I want to fill. I don't know what made the holes. Can I use this to fill the holes?

Hard to say without seeing or knowing what size and kind of holes they are. You might be better off using a slurry type mix. That will fill in the gravity of holes in ground better. Its very wet liquid plastic cement sand and water. Lowe's has it in bags that can easily be mixed in a wheelbarrow and dump into the hole. The mix also in the bag has no rock in it. It's a very good high quality material with good strength. Good luck hope this helps some. :-)

Using this product can damage the remaining plants as it can wick off materials to other root structures, effect drainage and air penetration.

Yes, QUIKRETE® Fast Setting Concrete Mix can be used to fill the holes provided that the holes are at least 2 inches deep. Note that the QUIKRETE® Fast Setting Concrete Mix will not bond to the surrounding dirt. Unless the the holes are large, the concrete may come out of the holes. This suggestion presumes that the filling of the holes is not being performed for structural purposes. If it is, please check local building codes for any specific requirements. Please contact the QUIKRETE® Technical Center directly at 1-800-282-5828 if you would like to discuss further.