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This Information Was Obtained From The
University Of Arizona Cooperative Extension:
Article by: Ursula K. Schuch
Pruning
Shrubs in the Low and Mid-Elevation Deserts in Arizona
Pruning is the intentional removal of parts of a plant.
Pruning needs of shrubs commonly planted in the low and
mid-elevation deserts in Arizona vary from no pruning to regular
seasonal pruning. Requirements vary by plant species, design
intent, and placement in a landscape. Fast growing shrubs
generally need frequent pruning from the time of establishment
until maturity, while slow growing shrubs require little to
none. Pruning should only be done when necessary and at the
right time of year. Using the natural growth form of a shrub is
a good guide for pruning. Shearing shrubs should be avoided
except for maintenance of formal hedges or plant sculptures. All
pruning should be done with sharp hand pruners or, for thicker
stems, loppers.
Why Prune?
Reasons for pruning shrubs include maintenance of plant
health, controlling plant size (for preventing obstruction of a
view, sidewalk, or driveway), and rejuvenating old plants.
Maintaining plant health includes the removal of diseased,
dying, injured and dead branches. Stems that rub against each
other should be removed. Control of shrub size for visibility
and safety concerns is sometimes necessary. These can be
minimized by allowing sufficient space for the plant to reach
its mature size in the landscape. Renovating or rejuvenating old
or overgrown shrubs through pruning generally improves the
structure and quality of the plant, and results in improved
displays for flowering shrubs. Some shrubs are grown as formal
hedges and require continuous pruning to maintain their size and
shape.
How To Prune?
Selective thinning refers to removing
branches back to the point of attachment to another branch, or
to the ground. This type of pruning opens the plant canopy,
increasing light and air movement (Figure 1). Thinning cuts do
not stimulate excessive new growth. They serve to maintain the
natural growth habit of the shrub. When light can penetrate the
canopy, entire branches can maintain leaves whereas in a dense
canopy branches have leaves near the tip but are bare further
back. Selective thinning is suitable for all plants and is
generally the most desirable type of pruning cut.
Heading cuts remove parts of a stem or branch
resulting in multiple new shoots just below the cut (Figure 2).
This can create a bushy plant and is sometimes done when plants
are very young to stimulate more branches. However, repeated
heading is similar to shearing and eventually results in a dense
canopy with branches having leaves at the tip and no leaves
further back. Heading cuts should only be used for formal
hedges, for rejuvenation, or when a cluster of branches is
desired. Stubs left by heading cuts will usually die back,
unless cut just above a bud.
No pruning or little pruning is required of some slow growing
shrubs (Table 1). Such plants are ideal for low maintenance
landscapes and include creosote, hop bush, Texas mountain
laurel, Arizona rosewood, pomegranate, jojoba, and juniper.
These shrubs should be planted where they can reach their
natural size without interfering with other plants, structures,
or lines of visibility. A yearly inspection can determine
whether any corrective pruning is necessary, but generally these
species will grow for many years with minimal maintenance.
Renovating or rejuvenating older or overly
large shrubs extends their life and improves their aesthetic
value. One method is to cut all stems about 12-18 inches above
the ground. This is a severe measure and changes the appearance
drastically (Figure 3). However, when done in spring before bud
burst, a great proliferation of stems will grow just below each
cut by mid-summer. At that time, about half of the stems should
be removed and the remaining ones should be cut back to
different heights. Varying the height and cutting just above an
outward pointing bud will stimulate growth of new branches out
of the canopy. This procedure works well for larger fast growing
shrubs like Texas ranger, xylosma, and oleander and for slower
growing shrubs like hop bush and creosote. Some shrubs such as
cassias, arborvitae and junipers do not respond favorably to
this treatment and might die. Rosemary is also not a candidate
for this procedure. Thicker stems of older rosemary shrubs
without needles should not be cut because they do not initiate
new shoots behind the cut. The voids created by cutting woody
stems of creeping rosemary will not be filled in by new growth.
Several smaller shrubs such as brittle bush, autumn sage, and
Mexican honeysuckle benefit from being cut to six inches above
the ground annually. They don’t require any follow-up thinning
of the new growth. Cutting frost-damaged plants back close to
the ground after the danger of frost has passed in spring
rejuvenates lantana and red bird of paradise.
A less severe approach to rejuvenating shrubs is to remove half
or more of the older unproductive branches at the base of the
plant or those growing into the canopy. This thins out the plant
to a much greater extent than regular maintenance pruning.
Follow-up care requires removing a portion of the new shoots a
couple of weeks later, which might be too numerous and result in
an overly dense canopy. The third method of rejuvenation spans
three to four years and is less noticeable. About one third to a
quarter of the oldest unproductive branches are removed each
year. This method requires thinning excess branches and cutting
back the remaining new branches as described before.
Shearing shrubs entails cutting back branches
to a uniform surface. This should not be done other than for
formal hedges or special topiaries (Table 2). Shearing shrubs
cuts the branches to stubs which results in a proliferation of
new dense growth just behind the cut. There are several reasons
why shearing is not recommended other than for formal hedges
(Figure 4, 5). Shearing is labor intensive and requires repeat
shearing to maintain the shape. It destroys the natural growth
habit and gives shrubs an unnatural look. It is difficult to
control the plant height since the new dense growth shades the
inside of the canopy which can defoliate for lack of light.
Subsequent cuts into the new growth will shear close to the
surface, but over time the dimensions of the canopy will
increase. Cutting into the older, bare wood by shearing results
in a leafless shrub, limits the plants ability to produce their
own food, and depletes their reserves to grow new leaves. This
stresses the plant and can result in decline or death. Regular
shearing of shrubs removes flower buds, flowers, and reduces
their beauty.
When to prune?
The general guideline for pruning is after flowering shrubs have
completed flowering (Table 3). Therefore, spring flowering
shrubs like cassia or rosemary should be pruned in late spring.
Summer or fall flowering shrubs like oleander, dalea, or Texas
ranger should be pruned after flowering or in late winter to
early spring. Spring flowering plants complete flower bud
formation the previous year, therefore pruning in early spring
will remove the current season’s flowers. Summer and fall
flowering shrubs usually generate flowers on the current
season’s new growth. They should be pruned either after
flowering or before new growth resumes in spring. Selective
pruning of a few branches can be done on most plants throughout
the year. Some very vigorous growing shrubs like bougainvillea
or firethorn need light pruning throughout the growing season,
unless they are given ample space to grow naturally.
Frost-damaged plants such as bougainvillea and yellow trumpet
flower should be cut back to living wood after all danger of
frost has passed or when regrowth resumes. Red bird of paradise
is generally cut back to six to ten inches above the ground.
Lantana is cut back to just above the ground after frost danger
has passed.
Hedges
Hedges are used as a screen or in place of a fence. A formal
hedge is sheared into a geometric shape by cutting the sides and
top. Informal hedges use shrubs in their natural growth habit
and maintain the general shape through selective thinning or
heading back of individual branches. Plants with small
internodes (short distance between leaves) are most suited for
hedges which should have a dense canopy (Table 2). Myrtle,
Japanese or waxleaf privet and xylosma tolerate frequent
shearing and are good candidates for formal hedges (Figure 6).
Oleander, hop bush, heavenly bamboo, pomegranate, and arbovitae
cultivars are examples of plants suitable for informal hedges
(Table 2, Figure 7). Keeping the mature plant size in mind is an
important consideration when choosing a species for an informal
hedge. Some oleander cultivars can grow as tall as 20 feet while
the petite varieties will grow only two to three feet tall.
Formal hedges are trained soon after planting. At this time they
require cutting off the top third of the plant to induce more
branches. The next year half of the new growth is sheared to
induce more branching and a full canopy. By the third year
hedges are often shaped into the desired form, however this
depends on the final size and growth rate of the shrubs used.
This might be earlier for low hedges or later for taller ones.
The base of a hedge should be slightly wider than the top to
allow light to reach the lower branches. Hedges that are kept
incorrectly narrower at the bottom than the top will often lose
their leaves on the lower branches (Figure 8). The top half of
hedges can be rounded, peaked or flat. New shoots that grow out
of a formal hedge require follow-up trimming (Figure 9). Formal
hedges should be cut only a few inches at a time,
otherwise leafless branches may become visible. Maintenance of
established hedges usually starts after spring growth is
completed. Follow up depends on the species and the level of
formality desired. Old, overgrown hedges can be rejuvenated, as
discussed before, provided the shrubs respond well to severe
cutback. Formal hedges are time consuming to maintain compared
to informal hedges, which are allowed to grow in their natural
form and need very little maintenance.
Training large shrubs into small trees
Some shrubs can be trained into small trees. Shrubs that
naturally grow to a larger size are suitable for this
manipulation. They include Arizona rosewood, Texas olive, Texas
mountain laurel, oleander, waxleaf privet, and xylosma (Figure
10, 11). This process should be initiated when shrubs are small
and at this time they are trained as either a single trunk or a
multi-stem tree. Multi-stem trees are best to accommodate the
natural form of large shrubs. For a single trunk tree a straight
central leader needs to be selected early. Training is complete
when the plant reaches mature size. Regular follow-up
maintenance is usually required to keep the trunks free of new
adventitious shoots and the base free of suckers. Plants vary in
their requirements for selective maintenance. Single stem
oleanders demand very frequent removal of sucker growth from the
base.
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Shrub
Pruning Summary
▪ Don’t
prune unless necessary.
▪ Use the appropriate, sharp tool for the
job.
▪ Prune at the right time of year.
▪ Use the natural growth form of the shrub
as a guide for natural pruning.
▪ Shear only formal hedges.
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Literature
Brenzel, K.N. 2001. Sunset Western Garden Book. Sunset
Publishing Corp., Menlo Park, CA.
Duffield, M.R. and W. Jones. 2001. Plants for Dry Climates.
Perseus Publishing, Cambridge, MA.
Johnson, E.A. 1997. Pruning, Planting, and Care. Ironwood Press,
Tucson, AZ.
Acknowledgements
Photos for Figure 3 were taken by Jack Kelly.
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