I don’t think that type of work expects a tip. But it would be a nice gesture to offer them some bottled water, lemonade or iced tea. Although we don’t have a gardener, my neighbor down the street does. One day, this poor guy was mowing the lawn and it was about 90 degrees outside. I ran inside and made a jug of lemonade and took it along with some cups down to him and his partner. They were thrilled (and a little bit shocked). I just told them to leave the pitcher at Mrs. H’s house and I’d come pick it up later. They ended up bringing it back up to my house after they were done.