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John P. Begeman: Cutting out mistletoe should limit tree damage

If you're new to the Tucson area, you've probably wondered what those stringy clumps of growth are in the native palo verde and mesquite trees. It's mistletoe - a shrubby parasitic plant of yellowish-green stems - that can be deadly to the host tree. The type of mistletoe that inhabits the Tucson area is desert mistletoe. Although it appears as a leafless mass of cascading green stems, it does have tiny, scalelike leaves. Other types of mistletoe, common to higher elevations of Arizona, have small, flattened leaves. Desert mistletoe is classified as a semi-parasitic plant. Though it has green chlorophyll and can make the sugars and starches needed for growth, it needs a host plant for water and nutrients. Although mistletoe may appear to be of epidemic proportions, it really infests only a small number of trees. The population of mistletoe stays relatively constant - so there is no danger of our native palo verde and mesquite being wiped out or even reduced in numbers. New trees sprouting in the desert from seed are constantly replacing those lost to mistletoe. It's only by chance that the mistletoe is able to grow in the first place. Birds eat the berries and transplant the seeds by wiping their bills or leaving their droppings on twigs and branches. If seeds stay attached to the tree, and if conditions are just right, they will germinate and penetrate the branches. Rootlike structures then penetrate into the branches, through the water and nutrient-conducting cambium layer. Water and nutrients are absorbed, depriving infested branches of their ability to support normal growth. Eventually, these branches die, and if many branches are infested, the whole tree can be lost. On valuable trees, it's important to control the mistletoe. First - and most important - any mistletoe should be removed. Cut it out with sharp pruning shears or a pruning saw. If the mistletoe is growing on a small branch, remove the entire branch 2 to 3 feet below the mistletoe. If the mistletoe is growing on a branch too large to remove, then cut it off flush with the branch. Unfortunately, this will leave the rooting structure in place. Mistletoe can resprout from its rooting structure at the point where the cut was made. If it does, the new sprout can be easily broken off while it is still young and tender. If you keep the mistletoe cut out, this will prolong the life of the tree for many years. Widespread control of mistletoe is not warranted since it is a natural part of our desert ecology. But control is warranted on valuable landscape trees. The key is persistence. Keeping mistletoe cut out of trees is the best and only way to minimize its damage.