Mistletoe is an insidious plant parasite. It roots into your tree, sucks out the water and raw nutrients headed up toward the leaves of the tree. Once a cluster of Mistletoe becomes about eighteen inches in diameter it starts to flower and fruit. Birds eat the fruits, and expel the seeds onto other branches in your tree and other trees. More Mistletoe will begin to grow and suck out the raw materials the trees need to survive, making them more susceptible to disease, borers and other attackers. Mistletoe can be trimmed out by removing the branch that it has attached itself to a foot or so below the attachment.