Mistletoe may symbolize love and prompt a kiss this time of year, but what if it's killing your trees? Nancy Miller of Canyon Country wrote to the SoCal Garden Clinic about her problem:
Ten years ago we planted two elms from 36-inch boxes. About five years ago, after the leaves fell, we could see mistletoe sprouting in spots. Online resources said not to pull them out. Instead, we sprayed a product, Florel, that inhibits flowering and has stickiness to stay on the leaves.
That worked for that year. However, each winter more and more mistletoe sprouts. I spray Florel, year after year. The mistletoe dies and drops off, but each winter we can see it again. This year I could see mistletoe even through the green leaves, so it is thriving.
Is there an in-ground application that would help kill the parasite and not the tree? Our home has western exposure, and we really need the shade.
For an answer we turned to Alan Uchida, a third-generation nurseryman at Bellefontaine Nursery in Pasadena, with an assist from A. James Downer, acting director for UC Cooperative Extension in Ventura County. Uchida said:
Mistletoe is a semi-parasitic plant that depends on a host for moisture and minerals. Mistletoe roots can penetrate a tree's water vessels, sometimes causing the branch to swell, and after many years the tree will die.
There seems to be no way to kill mistletoe systemically — that is, by applying treatment to the ground, so it's absorbed through roots. You mentioned that you have had success with Florel, which contains ethephon, a growth regulator. If you would like to reapply it, this would be the best time, during the elms' dormancy.
The doses needed to control mistletoe in your trees, however, might endanger the surrounding plants. Instead of paying for the chemicals and hiring someone to apply them, might you consider some safe pruning? Infected branches can be removed in the winter, when trees are dormant. Mistletoe spreads only when seeds land on new host branches; prune it out and you won't have any seeds.
As for that snippet of mistletoe readers may have hung in their homes: Gardeners may wonder what kind of threat that mistletoe — leaf or seed — might pose if tossed in the home composting bin or city green waste bin. The answer: little, if the material is composted properly. The seeds that are mistletoe's only means of spreading should die in the composting process.
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