Tuesday, July 20, 2010

Acer negudo (Box-Elder), an Overlooked Native Tree

Although great strides have been made in gardening with native plants, a surprisingly large number of the approximately 2,800 plants native to Florida are overlooked or under-utilized by the gardening public, including native plant gardeners. One such plant is Acer negundo, commonly known as box-elder and, more rarely, as the ashleaf maple.



Growth habit in the spring after box-elder has flowered but
before the foliage has grown to full size (courtesy of Wikimedia Commons).

If you live within the natural range of box-elder, I hope that you will give this easily-grown and adaptable native tree a place in your garden, and I have summarized below the information on box-elder that is of most interest to gardeners.

Growth habit of a large, mature, multi-stemmed tree
with fully-grown foliage (courtesy of Wikimedia Commons).

Scientific NameAcer negundo Linnaeus

Family: Sapindaceae

Common Names: Box-Elder or Ashleaf Maple

Habit: Short-lived, fast-growing, often suckering, deciduous tree with a low, dense, much-branched crown and principal branches that tend to hang down as they mature.
Size: Up to 75 feet tall with a trunk diameter of 4 feet, but commonly 45–50 feet tall with a crown that is about as wide as the plant is tall.

Season: Small, greenish or reddish flowers and pale green, new shoots borne April–May; autumn foliage usually unremarkable; female trees with persistent fruits maturing in the autumn.

Male flowers consist of prominent, pendent stamens (courtesy of urtica at Flickr).

Motility: Wind-borne fruits easily spreading to, and the plant sometimes becoming weedy in, open ground.

North American Range: Acer negundo has the widest range of any maple native to North America and can be found throughout a wide area that encompasses most of the eastern and central United States. Further west, it occurs in pockets of cooler, wetter areas in mountainous regions and, after skipping hot, dry, desert regions, it reappears in California. It's north-south range is equally impressive and it extends north to Canada and south to Mexico and Guatemala.

Florida Range: Occasional in the Panhandle and in northern and central peninsular Florida, where it occurs along streams and in wet hammocks and alluvial forests.

Wildlife Value: Box-elder is often disparaged as being short-lived and having weak wood but it has outstanding ecological and wildlife value. As a pioneer plant of open, moist ground, it helps to stabilize wet soils and protects them from erosion. The foliage and twigs are eaten by a variety of insects, and the persistent fruits are eaten by a wide variety of birds and mammals, especially in winter when other food is in short supply. Its lifespan averages only 60 years (with rare trees reaching 100 years of age) and this is a liability where a long-lived tree is sought. However, its early death provides a source of weak wood in which cavity-nesting birds and mammals can easily make a cozy home. As the wood decays, it provides food or shelter for innumerable life forms, including wood-boring insects and wood-decaying fungi, which, in turn, serve as food for other animals.

Propagation: Seeds (remove seeds from the tough, winged fruits and expose to cold, moist conditions for 60–90 days); bud grafts; layering; cuttings.

Maintenance: Do not situate it where falling flowers, fruits, leaves, twigs, and branches will pose a problem, nor where its roots will infiltrate drain fields, pipes, septic tanks, or sewers; can be pruned, if desired, to nearly any shape or height; if planted along streets, the drooping older branches will need to be pruned so as not to interfere with traffic or pedestrians.

Winged fruits are abundantly produced by female trees. The fruits mature in the autumn, persist
into the winter, and serve as wildlife food when few other foods are available (courtesy of bambolia at Flickr).

Comments: Although occurring on wet soils in nature, box-elder is a drought, heat, and cold tolerant maple that is able to withstand harsh climatic conditions. Thus, it is recommended for use in difficult situations, such as wind breaks and compacted urban soils, where other trees would not be expected to do well. It is distinct from other native maples in its compound leaves, which are divided into 3–5 prominently-toothed leaflets, and its fruits, which mature in the autumn. The fruits persist well into winter and may be borne in great numbers by female trees, resulting in an unkempt appearance; however, the seeds are a valuable source of food for wildlife during winter. Male trees obviously lack fruits and are often recommended for their neater appearance, but the planting of exclusively male trees deprives wildlife of an important food resource.

Like most fast-growing, short-lived trees, box-elder produces weak wood and its branches are prone to damage from strong winds and heavy loads of snow or ice. As a consequence, care should be taken not to place it where broken or fallen limbs could pose a hazard to property or passersby. While usually unbothered by pests or diseases, box-elders are the preferred host of the box-elder bug, Leptocoris trivittatus, a beautiful black bug with red-lined wings. These attractive insects rarely cause serious damage, although they can build up very large populations and are a nuisance when they enter nearby houses for the winter. Box-elders are also one of the food plants for caterpillars of the beautiful and spectacular cecropia moth, Hyalophora cecropia, the largest moth native to North America.

Foliage of a plant with leaves divided into 3 leaflets (other box-elder plants may have leaves divided
into 5 leaflets). The leaves are one of the foods of cecropia moth caterpillars (courtesy of Wikimedia Commons).

Garden cultivars of box-elder are mostly males or somewhat to fully sterile females with a slower growth rate and a mature height about half that of the wild species. Such cultivars have diminished wildlife value and they should not be a component of wild gardens or restorations; however, they may be useful as unusual shade, specimen, or street trees, especially in formal gardens, small residential properties, and under utility lines. These are situations where non-native plants tend to be used and, when faced between using a non-native plant or a cultivar of a native plant, the latter would certainly be the lesser of two evils. Some of the cultivars that have been offered include:

'Auratum' with intense golden-yellow leaves fading to chartreuse in summer.

'Aureo-maculatum' with leaves mottled with gold.

'Aureo-variegatum' bears green leaflets with a gold margin.

'Flamingo' is a male clone with bright pink spring foliage that is beautifully variegated green and white in the summer.

'Kelly's Gold' is a short, 15-foot plant with yellow leaves; it needs protection from full sun.

'Sensation' is a male clone with coppery-red spring foliage that matures dark green in summer and turns bright red in autumn.

'Variegatum' is a sterile female essentially identical to 'Flamingo' but without the hot pink spring foliage.

 'Violaceum' is otherwise like the species but is generally not as tall and has maroon flowers, maroon spring shoots, and smooth, glaucous branches.

 'White Lightning' has bluish-white twigs and branches that add interest to winter landscapes.

'Winter Lightning' has improved yellow autumnal foliage and its clean yellow twigs and branches have ornamental appeal in the winter.

Foliage of one of the many cultivars of box-elder
(this one is Acer negundo 'Aureo-variegatum') (courtesy of Wikimedia Commons).

Rufino Osorio 
Author of A Gardener's Guide to Florida's Native Plants

2 comments:

leslie pierpont said...

I'd love to sell these in my native plant nursery here in Jacksonville, but who is growing these? where would I get them? Thanks, Leslie, Nativeandcuncommonplants.com

Stephanie said...

Thank you so much for writing a nice review about the Ash-leaf Maple! I have ordered two of these trees for a wildlife woodland garden I am working on near the edge of my property and there is so much negativity around this poor tree on the internet, mostly focused on box elder bugs, it makes you feel scared to plant one. I live in the Piedmont region of GA though, and the area I'm working with already has some lovely mature oaks, hickories and tupelos. I don't know if you mentioned this, but Box-elder is also supposed to be a shade tolerant tree, so, from what I can gather, it should be useful for the spot I'm looking to fill. I love wildlife, and I want to pant something to benefit birds and insects, not one more useless Norway Maple!