Spraying Mistletoe with Roundup

Similarities Shared by All Mistletoes

• Mistletoe grows in nearly every state and province in North America. Some grow at high altitudes; some prefer pine trees and evergreens, while others prefer deciduous trees. Many are host-specific. All mistletoe possess similar growth habits that makes them difficult to control. Their root-like structures grow into the tree's tissues making it possible for this parasitic plant to absorb water and nutrients from the host without returning any benefits to the tree. Growth is slow at first while getting established.

Sticky Seed Situations

• The genus Phoradendron has large fleshy leaves and forms clumps visible from distances once established -- its seeds tend to be spread primarily by birds. This type is more familiar because of Christmas traditions. More than 235 species of Phoradendron are known. The genus Arceuthobium, the group of dwarf mistletoes, is leafless and very difficult to detect at first. There are 34 known species of Arceuthobium. Seeds are spread by forceful ejection from a parent plant. Both genera have sticky seeds well adapted to adhering to, and germinating in, the tree’s bark. Healthy, younger trees are less susceptible to mistletoe infection.

Roundup, the Systemic Herbicide

• Glyphosate, the active ingredient in the popular systemic weed killer Roundup, is a non-selective herbicide that kills all living plant material. It moves, translocates, through the leaves and into the plant’s vascular system and down into the roots, killing the plant. If the plant spreads through roots or stolons, the glyphosate will also move from plant to plant, killing each.

Problems with Using Non-Selective Herbicides

• Spraying Roundup on mistletoe has not been effective for several reasons. It presents a risk to the host tree because of the relationship between the mistletoe’s root-like structures (haustoria) and the tree's water and nutrient vascular system. Killing individual plants with Roundup will eventually kill the tree as it passes into the vascular system through the haustoria.

  Sun-loving mistletoe prefers growing higher in tree tops to benefit from maximum sunlight and most homeowners are ill-equipped to spray an herbicide at this distance. Attempting to spray mistletoe in a tree will be accompanied by herbicide drift damaging not only the host tree but surrounding plants. Containment of the drift is impossible when spraying from a distance.

Mistletoe Control Options

• Learn how to recognize signs of mistletoe, especially early signs. These signs vary considerably. Try to remove the mistletoe before it produces seeds. Cut off infected tree branches. Know what mistletoe/tree combination is a local problem and plant trees resistant to mistletoe in general or to the specific mistletoe species attacking local trees. Wrap new mistletoe infection sites on the tree’s branch with black polyethylene not to solarize it, but to exclude light from the newly germinated, sun-loving mistletoe.