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River birch - Betula nigra
General information: River
birch can grow 50 to 90 feet tall but is often seen 40 to 50
feet. It normally grows with a central leader and small-diameter,
dark-colored lateral branches. It has a narrow, oval to pyramidal
crown when young, spreading wider with age as several branches
become dominant. It lacks the white trunk bark associated with
other birches but is distinguished by reddish, brown bark peeling
off in film-like papery curls providing interest all year round.
River birch can be easily trained with one central leader or
as a multi-stemmed tree. Some nurseries plant two or three trees
together to form a clump, but these trunks will not fuse into
one strong trunk. Should be grown more as a single-trunked specimen.
Branches droop particularly when they are wet, so regular pruning
in the early years will be required to remove lower branches
when they are located close to areas where clearance is needed.
The river birch occurs along stream beds
from Massachusetts to Florida, zones 4 through 9A. It prefers
moisture, but will tolerate dry soils.
Birches are much loved for bonsai because
of their elegant beauty, but they do not take easily to pot culture.
Their short lives are also seen as a disadvantage; however, some
birches will live as long as 300 years.
Family:
Betulaceae
Lighting:
Unlike most birches, the river birch prefers partial shade, and
may suffer leaf burn if kept in full summer sun.
Temperature:
River birch thrives in zones 5-8. It may need protection from
cold winds to prevent twig die-back.
Watering:
The river birch thrives in wet areas, and appreciates lots of
water.
Feeding:
Wait one month after leaves open, then feed every two weeks till
the end of summer.
Pruning and wiring:
River birch leaves will reduce significantly, to 1 1/4 inches,
perhaps even smaller. Unlike many birch species, river birch
does not seem to share the common problem of excessive "bleeding"
after pruning, which makes this species easier to work with as
a bonsai. Pruning upward branches at the second internode will
encourage foliage to extend away from the trunk, opening the
branch structure and keeping foliage from massing on branches
parallel to the trunk.
Pests and diseases:
Pests: No pests are of major concern. Resistant to bronze
birch borer.
Diseases: Leaf
spots; chlorosis on soils with a high pH.
Repotting:
Birches younger than age 10 will need repotting every two-three
years. Older trees need repotting as necessary. Repot in spring,
before bud burst, using rich, acidic soil mix. Care should be
taken to protect the bark when wiring. The tree may sucker from
the roots; these should be removed quickly to keep from sapping
the vigor of the main trunk.
Bibliography:
Tomlinson's "Complete Book of Bonsai"
Resnick's "Bonsai"
"The Creative Art of Bonsai" by Isabelle and Remy Samson
"Simon and Schuster's Guide to Bonsai"
Species information from Mitchell's "American Nature Guides:
Trees," and "The Hearst Garden Guide to Trees and Shrubs."
USDA Fact Sheet ST-94
- Compiled by Sabrina Caine
Edited by Thomas L. Zane
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