Plant of the Week
Arborvitae
Latin: Thuja orientalis
 Fashion is a fickle friend. One day you know what’s happening, and the next
you discover it’s all changed.
Gardening is no more immune to the ebb and flow of trends than any other
segment of our society, but because plants are more difficult to discard than a
pair of bell bottom pants, we see relics of past trends in contemporary
landscapes. And just as with any good trend, they usually get recycled every 30
to 50 years.
During the first half of the 20th century, needled evergreens such as
arborvitaes and junipers were a hot item in landscapes. A major reason for this
popularity was because the nursery industry was located in the upper Midwest and
New England states, and these were the kinds of evergreens that would survive
their cold winters. One of the plants that achieved considerable popularity
during this period was the Oriental arborvitae, Thuja orientalis.
The Oriental arborvitae is an egg-shaped plant with innumerable branches
radiating from the base to the tip. The needles are held erect in flat,
hand-shaped arrays that give the plant a neat, compact appearance. That is until
an ice storm hits, and then it parts down.
Is it a tree or a shrub? Given enough time the Oriental arborvitae will reach
tree-like stature. But most gardeners plant it in impossibly cramped quarters
where annual shearings keep it somewhat under control and with more bush-like
dimensions.
The American love affair with needled evergreens sputtered and failed,
primarily because of the voracious appetite of an insect called the bagworm.
This switch began after WWII and was pretty much complete by the end of the Viet
Nam war, with the new darlings of the landscape being hollies and azaleas. Of
all the needle leaf evergreens, the arborvitae must taste the best for it seemed
to always be the first attacked, so it was the first to be removed from the
landscape.
The bagworm is a native insect that most of us notice in August when we awake
to bare branches festooned with ugly, 2-inch long bagworm cases. At this late
date, the cases are cigar shaped with a bulge in the middle and attached to the
branch by a silken thread, a sign the female insect inside is pupating and will
soon be eagerly awaiting a visit from an amorous male.
The female moth is flightless and never leaves the protective confines of the
case. The outer surface is camouflaged with bits and pieces of the plant on
which the insect has been feeding.
Most of us miss the insect when it begins its life cycle. The female, after
mating with the male, lays her eggs and then promptly dies. The eggs overwinter
in the bagworm case with hatching occurring in mid to late May. The young larvae
spins a silken thread to serve a parachute and jumps off the bush, trusting that
the winds will carry it to some hospitable and tasty abode. Once it alights, it
begins feeding and builds the case over its abdomen to protect it from birds. At
this stage, the insects are free to move about the shrub to consume anything
green. By mid June the insects are about a quarter-inch long; in another month
they will be reaching maturity.
Control is not especially difficult if it is started early in the
season. The first step is to bribe a grandchild into collecting every bagworm
case on the bush. This reduces the insect pressure and in rare cases (such as an
isolated planting) may be sufficient for control.
But in most situations, spraying with the insecticide Sevin or the biological
control agent Dipel will be needed to ensure complete control. Bagworms become
increasingly difficult to control as they get larger, so many gardeners simply
spray as a matter of course in late May or early June and get the insects when
they first start feeding, well before any evidence of damage can be seen.
By: Gerald Klingaman, retired
Extension Horticulturist
- Ornamentals
Extension News -
June 20, 2003
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