Like people, plants have strong mechanisms to prevent the spread of microorganisms within their tissues. Trees are capable of "walling off" wounds to prevent infections from spreading throughout the tree. This healing process also slows, and eventually stops, the flow of sap from tapholes. By using good tapping sanitation practices such as replacing spouts each year, periodically changing portions of the tubing system, using good tubing cleaning procedures, or using check-valve spouts, maple producers are able to achieve significantly higher sap yields from trees by delaying the response to wounding. The response from wounding does eventually take place generally at the end of the maple season when the leaves start to come out and the tap hole closes off for good.