Some people here use mesquite trees. Some use elm trees. Both will be the last to come back from winter’s hibernation. Once these trees bud, you can be sure the frost is over and its safe to plant your spring garden. This happened very late this year. The plant nurseries even held off ordering plants for 3 extra weeks. Everything has gone in the ground late and its been a race against time to grow it to maturity before the triple digit temperatures scorch and kill all the spring plants. but as usual, the elm trees bud was right. As soon as the last frost ended the tree put out buds. So did the mesquite trees at the horse ranch i trail ride at. Somehow, they know.