Trees add height,
visual impact, form and definition to the landscape
while shrubs and ground covers bring texture and
seasonal color to the ground plane. A category of plant
generally described as large shrubs or small trees
occupy the middle level of the landscape. They provide a
backdrop for flowering under-story plants, a visual
transition from the ground plain to the trees and are
useful in screening machinery (air conditioners, utility
boxes), for low walls and in defining patio areas. A
number of desert landscape trees fall into this medium
stature group. Two under utilized Acacia species in this
category are Black Bush (A. rigidula) and Catclaw Acacia (A.
greggii).
Catclaw Acacia (also called Devil's Claw Acacia) Acacia
greggii, takes it name from the curved thorns that are
found along the smaller branches and new growth. It
grows as a small tree or large shrub throughout the
southwest US and northern Mexico to a height and width
of 15 to 25feet. Catclaw Acacia is found on canyon slopes,
along arroyos and desert flats at elevations below 5000
feet. It readily tolerates desert heat and winter cold
to 0 degrees F and is among the most drought tolerant
trees in the Sonoran desert.
Like many desert natives, established trees can survive
without supplemental irrigation but do best with monthly
deep soaking during the hottest months (May through
September). It grows at a moderate rate, thrives in full
sun and is semi to fully deciduous in winter. Two inch
long spikes of mildly fragrant, cream to pale yellow
flowers are produced from spring to fall. Unpruned, Catclaw
Acacia grows as a large shrub with limbs reaching the
ground. In native settings it forms thickets that
provide shelter and food for desert wildlife. Trees left unpruned in the landscape make excellent security
planting with thorns that snag and hold clothing or to
create habitat to attract desert wildlife. Conversely,
this is not a tree well suited for high traffic areas,
patios or near play equipment.
Pruning lower branches from mature specimens reveals the
graceful, twisted and gnarled trunks. Nursery grown
specimens are typically trained into multiple trunk and
standard trunk tree forms. This form is easily retained
with regular pruning and shaping. They will revert to
their natural form if left unpruned. Its rustic trunks,
natural form, summer flowers and hardiness make Catclaw
Acacia an ideal accent or perimeter tree in high and low
desert landscapes. Trees may also attract bird and other
wildlife in landscape that border native desert.
Black Bush Acacia is native to western and southern
Texas and a several northeastern Mexican states. In
natural setting it is typically found on rocky hillsides
and along canyon floors at elevations from 1100 to 1800
feet above sea level. It occurs as scattered individual
plant and in clumps or thickets. Under natural growing
conditions the tree matures to about 10 to 15 feet tall
and as wide but will likely grow larger in landscape
settings. Arizona boasts the co-national champion Black
Bush at 26 feet tall, 29 feet wide and a trunk diameter
of 16 inches as designated by the Arizona Chapter of the
National Registry of Big Trees.
Yellow to cream colored flowers arranged in 2 to 3 inch
long, slender spikes appear from March to mid-June.
Slender pods about 3 inches long develop from these
flowers. The tree is deciduous and the branches are
armed with numerous thorns. Black Bush thrives in full
sun and a variety of soil types and conditions including
very poor calcareous (caliche) soils. Trees grow at a
moderate to slow rate. Regular irrigation is needed for
establishment but, in the absence of severe drought, can
be naturalized to survive on desert rainfall. Monthly
summer irrigation of mature, established trees optimizes
growth and appearance. Immature trees are sparse and
rangy, becoming increasingly dense as they reach
maturity.
Part of the unique charm of this tree, besides its
modest statue, is its dark green, glossy, closely spaced
foliage, borne on stiff, densely packed, gray barked
branches. In summer Black Bush is easily mistaken for
Texas Ebony (Pithecellobium flexicaule). These qualities
combine to make Black Bush an excellent screen, barrier
or foundation plant, as a backdrop for other flowering
plants in the landscape or for erosion control.
Specimens can be left as a large mounding shrub or
pruned to a sculptural tree form that accents the
contrast between green leaves and gray bark. As a
deciduous tree it is best used in combination with
evergreen trees or shrubs.
