Question
In the spring, my globe willow, 3 years old, comes out so green and pretty.  During the hot desert summer, it turns yellow then brown on the ends of stems.  Underwatering or overwatering?

Answer
Willows need more water than most tree species so I would guess under watering is the problem, I would water with 1 inch if water --place a pan under the tree and turn the sprinkler on and when the pan has 1 inch of water in it stop. Do this every third day unless it rains. Also if you have not done so I would mulch around the tree with not more than 3 inches deep of organic mulch not piled up on the trunk. This will help hold moisture around the roots.