Dec 12, 2016

By Linda Lewis Griffith  UCCE Master Gardener

Mistletoe may be charming at the holidays,  but it's considered a pest in landscape trees in California.



Broadleaf mistletoe (Phoradendron macrophyllum) is a parasitic seed plant that pierces the surface of host plants, stealing their water and nutrients for itself.  Healthy trees can tolerate a few mistletoe infections.  But heavily parasitized plants or branches lose vigor and eventually die.

Hosts for broadleaf mistletoe include alder, ash, birch, box elder, cottonwood, locust and walnut.  A separate species infests only oaks.

Broadleaf mistletoes have green stems with thick nearly oval-shaped leaves. Plants often develop in clusters that can be two feet or more in diameter.  Small, sticky, white berries are produced in October and December.  Mistletoe is most visible on deciduous trees in winter when host trees have dropped their leaves.

Mistletoe plants are either female (producing berries) or male (producing pollen).  The berries are attractive to birds, especially cedar waxwings and robins.  The birds eat, digest and excrete the living seeds that adhere tightly to any branches on which they land.

Initial infestations are most common in large, well-established trees because birds are more likely to perch on them.  Seeds may fall from the upper branches onto lower ones, creating new clumps in the tree.

After seeds germinate, the plant grows through the bark and sends rootlike structures called haustoria up and down the branch. Growth is slow at first; it may take years for the plant to bloom and produce more seeds.

The best way to control mistletoe and prevent its spread is to prune out infected branches as soon as they appear.  Cut at least one foot below the point of mistletoe attachment to completely remove imbedded haustoria. If cutting back isn't possible, remove the mistletoe, then wrap the affected area with several layers of wide, black polyethylene to exclude light.

Source: ucanr.edu