While we may use
it during the holidays for stealing kisses, mistletoe, as a
parasitic plant, is specially adapted to stealing nutrients and
water from host trees and shrubs. It is not surprising that the
genus of the two mistletoes species commonly found in the desert
southwest, Phorodendron, literally translated mean tree
thief. These two mistletoes are P. californicum (a
narrow leafed type) and P. tementosum var. macrophyllum
(a broad-leafed type).
Landscapes that
are immediately adjacent to undisturbed desert (golf courses,
residential or commercial properties) may wish to control
mistletoe in infested trees for cosmetic reasons or in an effort
to limit spread to other trees. It is instructive to think of
mistletoe as a weed and as such control strategies revolve
around removing the target plant, controlling seed sources or
chemical control. Simply removing the stems of the mistletoe
will not effect the "roots" (haustoria) embedded within the
branch. Ultimately new stems will be produced. Stem removal will
slow the growth of individual plants within a tree and have the
added benefit of reduce seed production but will not eradicate
the plant from infested trees. Removing infested branches is
another option but such pruning may only further disfigure the
tree or generate other problem often associated with heavy
pruning. Appreciate that other infested trees in the surrounding
desert will still be a significant source of seeds for new
infestations. Chemical control has never proven effective
experimentally and there are no labeled herbicides for the
control of mistletoe. In short, control is difficult, time
consuming and usually ineffective.
Mistletoes are
seed plants that grow either as a parasite (extract all
nutrients for growth, including sugars, from the host) or a
semi-parasite (capable of photosynthesis but depends on its host
for water and mineral salts). Flowers are petal-less,
inconspicuous and produce large numbers of sticky seeds. The
seeds are coated with a gelatinous material that makes them
stick to the beaks of birds and the feet of animals. Birds are
the primary carriers of seeds from infested to healthy trees.
Seeds can germinate anywhere but can only penetrate young, thin
bark. Once inside the tree, small root-like growths called
haustoria grow in the spaces between the host plant cells. It is
through these haustoria that mistletoe extract needed water and
nutrient from the host plant. The age of a mistletoe plant can
be calculated by counting the tree rings from the region of the
first haustoria to the surface of the bark. Many mistletoe
plants are 60 to 70 years old and one has been reported as
living 419 years.
The aerial
portions of mistletoe are leafy, evergreen clusters of shoots
(stems) that appear to emerge from the branches of host trees.
Stems and leaves contain chlorophyll and are generally green in
color though often with yellowish, brown or olive tints. Plants
have opposite leaves with round, segmented stems. Growth is
initially slow but tufts can grow to 3 feet across in 6 to 8
years. Aerial portions do not live much longer than that but new
clusters can arise from haustoria surviving within the tree.
Phorodendron californicum has small, narrow, nearly
inconspicuous leaves, making the plant look like a cluster of
short, densely packed, green stems. P. californicum
is typically found on desert legumes, especially native and
screwbean mesquites but is also common on cat claw, palo verde
species, and occasionally on creosote and desert ironwood. In
contrast, P. tementosum var. macrophyllum
has broad, thick leaves arranged opposite each other along the
stem. It is typically found on stream-bank trees such as
cottonwoods, willows, sycamores and ashes but also on walnut,
Mesquite, and Hackberries. Both are found throughout the desert
southwest.
Mistletoes are commonly found in native
stands of mature desert trees but rarely, if ever, in cultivated
trees. Obviously it can be introduced into the landscape on
desert salvaged or relocated trees. Mistletoes can damage and
weaken older trees, cause unattractive branch swellings and in
rare cases the weight of the plants can cause branches to break.
The impact of mistletoe on the appearance of trees is highly
subjective. Some people find the infestations unattractive while
others find they add color, density and character to maturing
trees.
The relationship between mistletoes and host
desert trees is ancient. Over the millennia they have learned to
tolerate each other and safely co-exist. Perhaps desert
horticultural professionals should take a lesson from the native
mesquite and just learn to live with mistletoes.