Question
IS WATERING GRASS WITH DISHWATER OR BATHWATER CONTAINING LAVENDER OIL HARMFUL TO THE GRASS?

Answer
Dear Jeff:

I can see no harmful effect from putting dishwater on a lawn.  It probably contains stearates, phosphorus (maybe), and oils.  The pH is alkaline and will help most lawns except centipede, unless you have very alkaline soil already.

There may be a good effect from adding lavendar beyond a pleasant aroma.  It is considered a moth repellant by some, which, if successful, would repel the parents of grubs, etc.

Best wishes.