Deweeding large areas

I recall seeing a reference to a easier method of deleting all of the weeds/trees in an area. (Easier than doing it one at a time.) Could somebody enlighten me on this, please?

 

I've tried that, but it doesn't seem to work. It only deletes any objects I have made.

 

Set the terrain type as water when you are first creating your map, then right click and press: Fill with tile type.

 

That worked! Thank you very much...