Washington Palm
Washingtonia robusta
INTRODUCTION
Commonly seen at 40 to 50 feet but capable of soaring to 80 feet
in height, Washington Palm is quickly recognized as the much-used,
straight, singletrunked street palm of years past . The lower leaves
persist on the tree after they die, forming a dense, brown, shaggy
covering below the living, bright green, broad, fan-shaped leaves,
giving it the common name of petticoat palm. These dead fronds
are known to be a fire hazard and a popular bedding roost for rodents
and, because of this, must be removed by law
in some areas. The sharply barbed leaf petioles and tall, thin
trunks make frond removal a rather unpleasant task, but some people
think the rapid growth rate and statuesque appearance more than
make up for this trouble.
GENERAL INFORMATION
Scientific name: Washingtonia robusta
Pronunciation: wosh-ing-TOE-nee-uh roe-BUS-tuh
Common name(s): Washington Palm, Mexican
Washington Palm
Family: Arecaceae
USDA hardiness zones: 9 through 11
Origin: not native to North America
Uses: wide tree lawns (>6 feet wide); medium-sized tree lawns
(4-6 feet wide); narrow tree lawns (3-4 feet wide); specimen; sidewalk
cutout (tree pit); residential
street tree; no proven urban tolerance
Availability: generally available in many areas within its hardiness
range
DESCRIPTION
Height: 60 to 90 feet
Spread: 10 to 15 feet
Crown uniformity:symmetrical canopy with a regular (or smooth)
outline, and individuals have more or less identical crown forms
Crown shape: palm; upright
Crown shape: palm; upright
Crown density: open
Growth rate: medium
Texture: coarse
Foliage
Leaf arrangement: alternate; spiral
Leaf type: costapalmate
Leaf margin: entire
Leaf shape: star-shaped
Leaf venation: palmate
Leaf type and persistence: broadleaf evergreen; evergreen
Leaf blade length: >36 inches
Leaf color: green
Fall color: no fall color change
Fall characteristic: not showy
Flower
Flower color: white
Flower characteristics: showy; summer flowering
Fruit
Fruit shape: oval; round
Fruit length: < .5 inch
Fruit covering: fleshy
Fruit color: black
Fruit characteristics: does not attract wildlife; inconspicuous
and not showy; no significant litter problem
Trunk and Branches
Trunk/bark/branches: grow mostly upright and will not droop; not
particularly showy; should be grown with a single leader; no
thorns
Pruning requirement: needs little pruning to develop a strong structure
Breakage: resistant
Crown shaft: no
Culture
Light requirement: tree grows in part shade/part sun; tree grows
in full sun
Soil tolerances: clay; loam; sand; acidic; occasionally wet; alkaline;
well-drained
Drought tolerance: high
Aerosol salt tolerance: moderate
Other
Roots: surface roots are usually not a problem Winter interest:
no special winter interest
Outstanding tree: not particularly outstanding Invasive potential:
little, if any, potential at this time
Verticillium wilt susceptibility: not known to be susceptible
Pest resistance: long-term health usually not affected by pests
USE AND MANAGEMENT
Washington Palm makes a dramatic statement in the large landscape
and creates a striking accent for multi-storied homes but often
grows out of scale in most landscapes with one-story buildings
because all of the fronds are at the top of the palm. It looks
like a telephone pole with a green hat. Washingtonia filifera
is a much better choice in unirrigated landscapes, since it grows
more slowly, is shorter, and the trunk is thicker. Washington
Palm needs full sun for best growth but will endure some shade
while young. It will tolerate poor soil and drought, and is hardy
to about 20-degrees F. Transplant with a large root ball to ensure
survival. Washingtonia filifera is shorter, has a thicker trunk,
and is better suited for planting in dry urban landscapes, such
as in Texas. They reportedly suffer and often die from root rot
when irrigated. Select
Washingtonia robusta in an irrigated landscape and for the eastern
U.S. Propagation is by seed.
Pests
Coconut mealybug, palm leaf skeletonizer, palm platid planthopper
and a variety of scales infest this palm.
Diseases
Root rot can occur if this palm is planted on a wet site.
by Edward F. Gilman and Dennis G. Watson
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