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Mistletoe

Mistletoe

Mistletoe is the common name for hundreds of species of hemiparasitic plants. It sends down root-like structures, called haustoria, that penetrate through the bark into the vascular tissue of the host. Mistletoe draws nutrients and water from the host tree. Commonly infected trees include hackberry, bois d’arc, silver maple, red oak, and others.

Is Mistletoe Cause for Concern?

Mistletoe is somewhat unique because it is a hemiparasitic plant. Although they still draw nutrients and water from their host, hemiparasitic plants also perform photosynthesis and are consequently less dependent on their host for survival. Accordingly, they are less of a concern than holoparasitic plants, which survive only using resources drawn from the host.

However, mistletoe can be a problem for trees that are already stressed. Trees maintain a “feedback loop” in order to achieve homeostasis. For example, in response to drought stress, a tree may close the tiny pores (called stomata) in its leaves to prevent water loss. Mistletoe, being a parasite, is not part of this feedback loop. It will continue to draw nutrients and water even while the host is trying to conserve its resources. When compounded by mistletoe, an already stressed tree may decline further. But, for the most part, mistletoe poses little threat to a healthy tree.

Sam Hill Tree Care recommends removing mistletoe found on high-value trees or trees that are close to your home. For other trees, as long as the host is not otherwise stressed, consider leaving some mistletoe. Its white berries are an important food source for birds.

Treating Mistletoe

Because the haustoria penetrate into the branch, removing mistletoe is not as straightforward as pruning it out. The embedded haustoria will simply cause the mistletoe to grow back. To prune mistletoe, we remove infected branches that are 2 inches in diameter and smaller.

For larger branches that should not be removed and for the trunk, mistletoe is a recurring problem that can take several years to completely eradicate. In these cases, we cut the mistletoe flush with the branch. As noted earlier, this will result in the mistletoe coming back in a few years to be pruned again. Repeatedly pruning out the new growth will offer control and gradually kill the mistletoe by starving it.

Some alternate methods of controlling mistletoe have also been proposed. John Lichter and Dr. Alison Berry from UC Davis found that cutting mistletoe flush and covering the infected area with black plastic wrappings or pruning paint can control mistletoe by depriving it of light. However, this method is not always pragmatic because it is labor-intensive and the wrappings and paint may wear off.

Another option is applying Florel, a growth regulator which essentially aborts the mistletoe. However, applications do not offer complete control and the spray must be carefully controlled to avoid spray drift to other landscape plants.

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