Try This:
Collect a few water or milk plastic gallon jugs. Place a screw on the side of the jug, very near the bottom, and opposite the handle. Fill the jug with water and take to the site, remove the screw and position the jug so the water runs towards the tree. Now that you have done that, what to do with the screw? Just drop it in the jug. You will find it there next time you are going to water. The jug will be empty in about 5 minutes, and no water will be wasted. Pickup your empties, fill them up again and continue watering your trees. If you have a cart or just a little red wagon you can water many trees simultaneously.
Someone asked me what kind of screw to use...It does not matter, self tapping, drywall, wood, pan head, it is immaterial, just don't use a machine screw, you would have a hard time getting it started. In lay mans terms, use one that has one sharp end.