Island and Median-Strip Planting

15 Pages·2006·0.92 MB·English

Island and Median-Strip William Flemer III
Planting
A generation ago the use of plantings to di- More and more local planning boards there-
vide highways or regulate the flow of traffic fore are requiring islands of trees to provide
on roads was rare, confined to wealthy resi- shade and add visual appeal to these other-
dential areas in such cities as Philadelphia, wise unsightly spaces. Merchants, too, find
Richmond, and Boston, to cite a few well- that although the islands reduce parking
known examples. The parkways of space somewhat, they are more effective
Westchester County, New York, and Con- than painted lines on the pavement in keep-
necticut, as well as the Shirley Parkway ing automobiles aligned. Thus, improved
along the Potomac River, were among the utilization of space compensates for the loss.
first divided roadways with planted strips In selecting trees and shrubs for islands or
between them. The plantings were meant to median strips, one must be aware of the spe-
create restful driving conditions and to cial difficulties that such sites impose. Nar-
screen out headlight glare at night. After row median strips are especially difficult for
World War II, highway planting began on a trees and shrubs because of wind whip from
nationwide scale when the huge network of speeding traffic. On highways especially,
the so-called national defense highway sys- with cars and trucks speeding by at 60 miles
tem was installed. Almost all of the per hour, the wind damage to foliage can be
superhighways had separated roadways for severe, both in the spring when the leaves
opposing lanes of traffic, and in all but the are soft and tender and in the summer when
most congested urban areas the strips be- weather is hot and dry. For this reason plants
tween the roadways were planted with trees with tough, thick leaves are most successful.
and shrubs. Carefully kept accident-rate In the colder parts of the United States and
statistics for old-fashioned and divided-lane Canada (zone 6 and below), road salting in
highways proved beyond a doubt that the winter presents additional difficulties.
benefits for median-strip planting far ex- Speeding traffic can create a salt spray as
ceeded the costs of installation and mainte- concentrated as ocean spray. The salt settles
nance. on plants and soil, and prevailing winds de-
Now the use of island plantings has begun posit large concentrations on the downwind
to spread to other sites. Among these are the side of roadways. It is essential, therefore,
parking areas surrounding modern suburban that trees and shrubs chosen for these areas
shopping centers. These expanses of pave- are salt tolerant. It is no coincidence that
ment become unbearably hot in the summer tree species that thrive at the seashore are
months, and locked cars quickly reach oven also successful in island plantings. Thus
temperatures during the daylight hours. sugar maple, which is one of the choice
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species for residential surburban planting in and shade tolerant when young. Climax
its native range, is a poor choice for island species ultimately comprise the entire forest
or median-strip planting, while sycamore except on very exposed sites.
maple does well. Canada hemlock, The ecological conditions of island plant-
which is also exceptionally susceptible to ings on highways are extremely harsh for
salt spray, should be avoided, while green tree growth, and only pioneer species or
ash and Japanese black pine (where it is species from dry, inhospitable climates can
wmter hardy) are sound choices. Indeed the be expected to grow well. Island sites are ex-
sugar maple decline m New England, about posed to full sun and wind, as well as the
which so much was written 20 years ago, has turbulence caused by vehicular traffic. They
since been attributed to salt kill. Mamte- are also narrow, so that natural penetration
nance crews tend to use salt with a lavish of rain to the root zone is inhibited. It is es-
hand, a practice that ought to be vigorously sential therefore to plant only those species
curbed. Meanwhile, salt-sensitive species that are tolerant of dry soil. Among the
must be avoided m island plantmgs for cold many species of small maples, the Japanese
areas. maple (Acer palmatum Thunb.), which is
Salt injury to trees in island plantings in strictly an understory tree in the woodlands
parking lots can be very serious even though of Japan, qmckly succumbs in island plant-
traffic is too slow to create salt spray. Here mgs. The Amur maple (Acer glnnala
salt is spread on the pavement, and often be- Maxim.), in contrast, thrives under condi-
fore the snow is melted the salt and snow tions that are lethal to the Japanese tree. Not
mixture is scooped up by front-end loaders surprismgly, the Amur maple comes from
and disposed of "out of the way" on the is- the harsh climate of the Amur river valley of
lands, to the detriment of the vegetation China, which is bitter cold in winter and hot
planted on them. and dry m summer. Our native eastern flow-
ering dogwood (Comus flonda L.) grows
poorly and is subject to drought stress and
Ecological Requirements
severe borer infestations in island plantings,
The forested areas of the north temperate whereas the native species of hawthorns are
zone contain a wide variety of tree and shrub excellent for such locations. The cornelian
species. The greatest number occurs in cherry (Cornus mas L.), which grows out to
areas where the old Tertiary forest was not the edge of the steppes of Russia, is another
extirpated during the last Ice Age, particu- tree of choice.
larly the eastern United States, Japan, and Island plantings of shade trees are par-
parts of China. When cleared land is aban- ticularly exposed to wind damage. Al-
doned in naturally forested areas, a gradual though the Bradford callery pear (Pyrus
process of forest regeneration begins. The calleryana Decne.) is otherwise suitable for
first trees are "pioneer" species that can islands, it is susceptible to breakage when it
stand exposure to full sun and drymg winds. matures. It has been dropped by many state
After these have colonized the open field and highway departments because of this but is
matured, they are slowly replaced by so- still a favorite for sheltered locations in the
called climax species, which are long-lived downtown areas of cities. The tough-wooded
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bur oak (Quercus macrocarpa Michx.) can clones are much improved, however, and
serve as an alternative to Bradford pear. This should rate high on the list of trees for
tree comes from the Plams States, where difficult sites. The Norway maple will grow
violent thunderstorms routinely occur each well in island plantings and in polluted con-
summer. ditions in cities, where the sugar maple and
red maple will not thrive. It is one of the few
Trees and Shrubs Recommended for Island species with attractive flowers, which are a
clear chartreuse color and abundantly borne.
Planting
The fall color is a fine yellow.
The following is a list of trees and shrubs
that have proved adaptable over a wide range
Acer pseudoplatanus L. Height 15-
of soils and climates in the East and Mid-
18 m~. Hardy to -30°F. Sycamore maple.
west. For the subtropical climate of Florida
and the desert conditions of the Southwest, Although it is not a particularly distin-
of course, entirely different lists are needed. guished tree, the sycamore maple ranks high
wherever salt spray or deicing salts are a
problem. After the hurricane in the summer
Trees
of 1948, it was the only deciduous tree with
Acer gmnala Maxim. Height 5-6 m. Hardy green leaves (no browning whatsoever) along
to -50°F. Amur maple. the coasts of Rhode Island and Massachu-
This small tree has the unique characteristic setts. Trees from seeds are often mediocre,
of tolerance of extremes of heat, cold, and but the best clones of the variety purpureum
drought. It can be grown with several trunks are vigorous and shapely, and the purple un-
or pruned to a single stem. The Amur dersurface of the leaves is particularly at-
maple’s beauty lies in its leaves, scarlet in tractive. The tree tolerates dry soil, pave-
fall and a glossy dark green in summer. A ment glare, and alkaline or saline soils.
similar species, Acer tatancum, merely
turns yellow in fall and is much less cold Celtis occidentalis L. Height 12-15 m.
hardy. The Amur maple is very tolerant of Hardy to -50°F. Hackberry.
salts and alkaline soil and can substitute The hackberry is one of the last trees to dis-
well for the Japanese maple (A. palmatum)/
appear from the landscape as one journeys
in the Midwest, where the latter is not
west across the prairies. It also grows in the
winter hardy.
thin soil on basalt and granite hills m New
England. It is not surprising therefore that it
Acer platanoides L. Height 15-18 m. endures the stressful environments of island
Hardy to -30°F. Norway maple.
plantings. Plants from seedlings are variable,
The common Norway maple has been much and many are subject to unsightly twig and
maligned in recent years because trees of foliar diseases. Grafted clones are available,
seedling origin vary greatly, and many are however, which are both shapely and disease
distinctly inferior in growth habit, growth free. Clones and seedling trees are very
rate, and quality of foliage. The best grafted tolerant of dry, alkaline soil and exposure.
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The hackberry tree (Celtis occidentahs ~. Photo-
graphs with this article from the Archives of the steppes of Russia. It is not as showy as the
Arnold Arboretum. large-flowered species (Cornus flonda), but
it becomes a haze of yellow in early April.
Cornus mas L. Height 5-6 m. Hardy to The dark green leaves are thick and leathery
-30°F. Cornelian cherry.
and do not scorch in summer droughts. It is
The toughest of the tree-sized dogwoods, the difficult to grow Cornus mas in tree form; it
cornelian cherry grows wild on the bleak is best grown as a large clump. This tree is
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ing spring. Unfortunately, no thornless
clone of this species is available, but its
thorns are much shorter and less dangerous
than those of C. crus-galli.
Fraxinus pennsylvanica Marsh. Height 15-
18 m. Hardy to -40°F. Green ash.
While the white ash (Fraxinus americana L.)[
is preferable for its superior autumn color, it
is not as suitable for stressful environments
as the green ash, which will tolerate
drought, heat, cold, and saline and alkaline
Cornelian cherry (Cornus mas).). soils with impunity. Seedling trees are vari-
able, and many female trees set large crops of
free of the borers diseases that plague seed and defoliate very early in the fall. As is
or
Cornus flonda. common to trees with a broad latitudinal
range, green ash has a number of geograph-
Crataegus crus-galh L. var. mermis. Height ical races. Trees grown from Florida seed are
6-8 m. Hardy to -30°F. Thornless cockspur as hardy as orange trees in North Dakota,
hawthorn. while trees from North Dakota provenance
grow as well as balsam fir would in Florida!
The common cockspur hawthorn is one of Several fine male clones are available, all
our toughest small trees, but because of its from the North Central States, where green
long, needle-sharp thorns, it constitutes a ash is an important shade tree. ’Marshall’s
danger in areas where pedestrian traffic is Seedless’ is unsmtable, however, because
present. The thornless form has the favor- nurserymen have found that it has begun to
able qualities of the species without the seed overabundantly.
dangers: tolerance of drought and exposure,
fine glossy foliage, and long-lasting red Gleditsla triacanthos L. var. inermis Willd.
fruits. Height 15-18 m. Hardy to -30°F. Thornless
honeylocust.
Crataegus phaenopyrum (L.f.) Medic. Height
The selection and introduction of number
a
6-9 m. Hardy to -30°F. Washington haw-
thorn. of thornless clones with shapely crowns
have transformed the honeylocust from
an
One of the finest small-flowered trees for is- unattractive weed tree mto an important
land planting. In the mini parks of street tree. The thornless honeylocust is
downtown New York City, it thrives under rapid growing, easy to transplant, and toler-
the most adverse conditions. It is attractive, ant of very difficult urban environments.
although not spectacular, in bloom. The Like green ash, it is among the trees that
glossy foliage turns red in the fall, and the persist longest as one crosses the northern
brilliant red berries hang on until the follow- prairie states with their harsh extremes of
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climate and alkaline soil. Such tolerance for tree is young to prevent interference with
adversity is an mdication of the honey- passing traffic. The thick, corky bark of ma-
locust’s suitability for island planting. It is ture trees is an attractive feature. The
particularly desirable for parking lots be- foliage, which is free of pests and diseases,
cause its tiny leaflets blow away after drop- turns a clear yellow in the fall. Staminate
ping and do not have to be removed. Though (male) trees are preferable for urban settings,
cold hardy, it does not thrive in the very acid as pistillate (female) trees produce large
soils of parts of Maine and Nova Scotia. crops of fruits.
Malus baccata (L.) Borkh. Height 9- Platanus x acerifolia (Ait.) Willd. Height
12 m. Hardy to -50°F. Siberian crab apple. 21-27 m. Hardy to -30°F. London plane
tree.
A native of one of the world’s harshest cli-
mates, the Siberian crab apple is a first This vigorous hybrid is the city tree par ex-
choice among flowering trees for island cellence, a standard agamst which others
plantmgs. It survives drought and a wide must be judged. Tolerant of drought, poor
range of soil conditions. The flowers are red soil, reflected heat, and atmospheric pollu-
to pink in bud and pale pink to pure white on tion, it is easy to transplant and grows rap-
opening. Bloom is heavy on alternate years. idly. The original clone, now often called the
The tree is virtually immune to apple scab "Bloodgood strain" is resistant to anthrac-
disease and mildew and resistant to fire nose leaf disease, which defoliates our native
blight. In fact, in the Pennsylvania crab- sycamores during wet springs. The London
apple trials, which have been conducted for plane tree has gone through several cycles of
many years, the only clones to receive rec- popularity and disapproval. Many years ago a
ommendations have been Malus baccata few nurserymen grew the trees from seed
seedlings or hybrids derived primarily from that produced great variation m habit of
baccata. A very desirable feature of this growth and disease resistance, and this may
species is its disease-free foliage, a trait that be one cause for the disapproval. Another
is shared by M. xatrosanguinea (F.L. Spath) may be the plane tree’s vulnerability to
C. K. Schneid. and M. flombunda Siebold ex canker stam disease, a serious condition
Van Houtte. Crab-apple trees chosen for is- spread by pruning tools or other mechanical
land planting where pedestrian traffic is pres- means. The severity of the disease once led
ent should be trees that bear tiny fruits or the city of Philadelphia to enact ordinances
few fruits. that prohibited planting the tree. Still, where
a large tree is needed for island planting, it is
Phellodendron amurense Rupr. Height 9- a first choice.
12 m. Hardy to -40°F. Amur cork tree.
Having originated in the fierce climatic ex-
Pyrus calleryana Decne. Height 12-
tremes of the Amur River valley, this small,
spreading tree easily endures poor, dry soil, 15 m. Hardy to -30°F. Callery pear.
reflected heat, and atmospheric pollution. The cultivation of the vigorous, thornless
Its lower branches must be pruned when the ’Bradford’ pear by the U.S. Department of
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Agriculture Plant Introduction Station at Sophora japonica L. Height 15-18 m.
Beltsville, Maryland, transformed an un- Hardy to -30°F. Japanese pagoda tree.
known, thorny scrub tree mto one of the
This unusual summer-flowering tree has
most popular street trees. This tree has ev-
been in the nursery trade since the 19th cen-
erything to recommend it: rapid growth,
beautiful pure white flowers, and richly col- tury but only recently has become a popular
ored fall foliage. Like all pears, it grows well street tree. It is one of the most variable of
in compacted, poorly oxygenated soil. Brittle all species; a single seedlot can produce both
wood is its only weakness, and mature trees dwarf weepers and tall, full-headed trees.
Now improved clones with first-rate shade
can literally collapse in a violent summer
tree form are readily available. This species
wind storm, as the parent tree did at
tolerates compacted soils (including the
Beltsville. The Pennsylvania highway de-
"brick yard" soils of Washmgton, D.C.), high
partment and others have removed it from
their planting lists for that reason. However, pH, salt, drought, and polluted air. It is a
conspicuous bloomer in August and retains
for street or island plantings in more shel-
tered urban locations it is still an excellent its dark green leaves later in the fall than
other deciduous trees. Like honeylocust it
choice. Several cultivars that are more wind
firm and/or more cold hardy than ’Bradford’ will not grow in highly acidic soils, however.
are now available in the nursery trade.
Syringa reticulata (Blume) Hara. Height 6-
9 m. Hardy to -30°F. Japanese tree lilac.
Quercus macrocarpa Michx. Height 18- This tough, hardy small tree is covered with
24 m. Hardy to -50°F. Bur oak. huge pamcles of white flowers in June, after
Many species of oaks make excellent shade the blooming season of most other flowering
trees under ordinary street conditions, but trees has passed. It withstands exposure and
few thrive constricted island plantmgs, alkaline soils and is not troubled by mildew
m
especially where soil pH is high. Members of on the leaves or stem borers as are other
lilacs. The lower branches must be pruned
the black oak division of the family (pin, red,
scarlet, willow, and black oaks) turn yellow when young so that they will not interfere
with pedestrian traffic. Several clones
from chlorosis under these circumstances are
and gradually die out. The bur oak is unique now available, including one from Canada.
These have been selected for their superior
in that it grows well in alkaline soils, stands
foliage, growth habit, and larger blossoms.
drought, heat, and cold and is tolerant of
deicing salts. It is not so easy to transplant as
the pin oak but is comparable in this regard Tilia cordata Mill. Height 15-20 m. Hardy
to -30°F. Small-leaved European linden.
to red and scarlet oaks. It grows slowly but
becomes a tough, long-lived tree. Its native Lindens in general grow well under city con-
range extends farther west than that of any ditions, and the best clones of this species
of the other eastern oaks, which means that are especially reliable. Like those of the
it is naturally adapted to ecological condi- pagoda tree (Sophora japomca populations
tions similar to those of island plantings. of small-leaved lindens grown from seed are
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extremely variable. Growth may be either
rapid or slow; wood is sometimes weak; and
leaves may be small and leathery or large
and easily scorched. Grafted trees do not ex-
hibit graft mcompatibility and reproduce
exactly the very best forms. Easy to trans-
plant, fairly rapid growth, and tolerant of
m
many soils and climatic conditions, they are
excellent subjects for urban islands. The
flowers are not conspicuous, but their rich
fragrance is a great rarity among shade trees.
Tiha x euchlora C. Koch ’Redmond’. Height
12-15 m. Hardy to -30°F. ’Redmond’ lin-
Flowers of the Japanese tree lilac (Synnga re- den.
ticulata).~.
Much controversy exists as to whether this
tree is a Crimean or an American linden cul-
The small-leaved European linden (Tiha cordata)./. tivar. It has the large leaves and resistance to
spider-mite attacks characteristic of Amer-
ican lindens. A compact, upright tree that is
native to Nebraska, it is mured to climatic
extremes of heat, cold, and drought and
thrives m urban locations.
Ulmus x hollandica Mill. hybrids. Height
15-18 m. Hardy to -30°F. Dutch elm.
The Dutch elm disease, which was first
identified in Holland, has destroyed a major
portion of the splendid elm populations of
Europe and North America during this cen-
tury. In response to the crisis, the Dutch
government selected and bred elms to pro-
duce forms that would be immune to the
disease. Seven clones were distributed, some
of which have demonstrated remarkable re-
sistance to even the most virulent strains of
the fungus. These have the upright, rectan-
gular crowns of European elms rather than
the wine-glass shape of American elms. Al-
though vulnerable to elm-leaf beetle, they
are exceptionally well adapted to urban
22
street conditions and grow well in narrow these sites. They must be tolerant of poor
planting pits and poorly oxygenated soil. soil, drought, and exposure to wind and heat.
Salt spray is just as damaging to shrubs as to
trees m island plantings, and it is not sur-
Zelkova serrata (Thunb.) Mak. Height 15- prising that several of the species listed
18 m. Hardy to -20°F. Japanese zelkova. below are also first choices for seashore
Not many decades ago this species was planting. In addition, shrubs, like trees, are
rarely encountered outside arboreta and bo- vulnerable to the problem of vandalism. The
New York City Public Housing Authority
tanical gardens. Seedling trees are very vari-
able, and most have irregular, zig-zag habits has found that it is best to avoid shrubs with
of growth and small yellowish foliage. The conspicuous flowers except for extremely
original introductions from the vigorous growers such as forsythia. All of the
came
warmer regions of Japan and were not too shrubs listed below are on the approved list
for the authority and have withstood the
cold hardy. It was not until the Dutch elm
disease destroyed the American elm and the severest tests of time in very unfavorable
search began for replacement species, that inner-city conditions. They are relatively
zelkova began to receive serious attention. free from diseases and insect pests and can
truly be termed low-maintenance shrubs.
Hardier clones with excellent shade-tree
shapes are now available and are being Shrubs that grow to be too tall for their lo-
widely used on city streets. Tolerant of pol- cation can be safely cut down to the ground
lution, drought, and heat, they have shown and will resprout and grow more densely
remarkable vigor in downtown locations in than ever. On level sites a well-sharpened
Washington, D.C., and Baltimore, Maryland. brush mower can be used at a great savings
They are not as cold hardy as most impor- in labor costs. Cutting back is best done in
tant shade trees but otherwise are excellent November or December so that the cut sur-
choices for island planting. Any tree with faces can dry out and seal themselves before
such a wide native distribution as zelkova the sap begins to run m the spring. Also, cut-
must have hardy races m the colder parts of ting off at this time will result in the most
vigorous regrowth.
its range, and a serious attempt to find them
would increase the uses of this excellent
tree. Although artifical innoculations of
Zelkova serrata with the Dutch elm fungus
have demonstrated that this species is vul- Acanthopanax sieboldianus Mak. Height
nerable to the disease, this factor can be ig- 2-3 m. Hardy to -30°F. Five-leaf aralia.
nored because the insect vectors of the dis-
ease do not feed on this tree. This dense, many-stemmed shrub is espe-
cially tolerant of poor, dry soil and atmo-
spheric pollution. It is thorny and makes
an
excellent barrier planting. Other than its
Shrubs
abundant disease-free foliage, it has no spe-
Shrubs for urban islands should share the cial beauty but is most useful for its vigor
same characteristics as trees chosen for under adverse conditions.
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Berberis thunbergii DC. Height 2 m. but, being thornless, it should not be planted
Hardy to -20°F. Japanese barberry. where it can be trampled by pedestrians.
Japanese barberry plants were once sold by Ssepvreeraadli nfgo rarmes gtrhoatw nar ien l Joawpearn ,a bnudt m uonrfeortu-
the millions for low-growing hedges, but
their use has declined as formal clipped nately these are not available in this coun-
try.
hedges have lost popularity home gardens.
m
It is still useful as a barrier and tall ground
cover, however, and will grow well in condi- Elaeagnus umbellata Thunb. Height
tions of poor soil and neglect. The numer- 3-4 m. Hardy to -40°F. Autumn olive.
ous, small thorns are needle sharp, a real de- The foliage of this species is not so showy as
terrent to trespassers without being danger- the pale silvery leaves of the Russian olive
ous. The brilliant shades of scarlet foliage in (Elaeagnus angustifoha~. Yet is has the ad-
the fall and the persistent red berries are very vantage of being better adapted to the East-
attractive. The redleaf form is colorful
ern States and other areas where summer
throughout the growing season. humidity is normal. The extensive highway
plantmgs of Russian olive m the East in the
1950s all have gradually succumbed to twig
Chaenomeles lagenana (Loisel.) Koidz.
Height 1-2 m. Hardy to -30°F. Flowering blight, while the disease-free autumn olive
quince. has become extensively naturalized in the
same areas. Autumn olive is a superb tall
One of the most colorful early-flowering shrub for roadway or seashore planting,
shrubs, the flowering quince resists van- withstanding salt, poor soil, and drought
dalism because of its numerous prickly without setback. The silvery green foliage
thorns. It is a popular substitute for the and pretty bronze-to-red fruits are decidedly
Kurume azaleas m cold areas where the lat- ornamental. The U.S. Soil Conservation
ter are not winter hardy. There are numer- Service has introduced a strain grown from
ous named clones with flowers ranging in seed called ’Cardinal’ with fruits that are a
color from pure white through various bnghter red.
shades of pink and orange to deep crimson
red. They vary m height and density, and the
low, bushy forms are excellent for ground Forsythia x intermedia Zab. Height 2 m.
Hardy to -20°F. Showy border forsythia.
cover.
Wherever it is wmter hardy, this is one of
the best shrubs for screening purposes. It is
Deutzia gracilis Siebold & Zucc. Height
vigorous, pest free, tolerant of city condi-
I m. Hardy to -30°F. Slender deutzia. tions, and unaffected by deicing salts. Van-
This low, twiggy shrub has greatly in- dals do break off branches m the blooming
creased in popularity m recent years for the season (April), but the plant quickly recovers
purpose of mass plantmgs. It is covered with from the injury.
pure white flowers m May and has abundant Three excellent new clones of the much
pest-free foliage. It is also useful as a hedge hardier Forsythia ovata have been bred for