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Plant of the Week
Chinese Fringe Tree
Latin: Loropetalum chinense
 The ice has melted and most of the power is back on, so now we can turn our
attention to an issue that will affect our landscapes as spring arrives –
winter hardiness.
I’ve chosen Loropetalum for this discussion because it represents what
gardeners love to do, push the envelope on where plants will grow. Sometimes
we get away with it, sometimes we don’t.
Loropetalum is hardy in zones 7b or 8a, but even the experts aren’t sure
just how hardy the plant is because it’s only been widely planted in the
Eastern landscape for about 10 years. Since then, we’ve had mild winters and
little challenge to its northward creep.
The plant should be reliably hardy south of Little Rock, but areas north of
there, or with higher elevations, run some risk.
The business of guessing about plant hardiness began with a Harvard
scientist named Alfred Rehder. And let me assure you it is a guess, because
many things are involved in determining plant survival. Rehder had a long
career working at Harvard’s Arnold Arboretum. He devised a system in the
late 1920s to predict where plants could be grown in the U.S. His system was
based on minimum winter temperature.
The 10 zones Rehder developed remained in place until 1962, when scientists
at the U.S. Department of Agriculture developed a better hardiness zone map
based on the better temperature data they had. The USDA zone map had 12
zones, and it gradually replaced the Rehder map. It was updated about 1990
with a plethora of reporting stations giving even better zonal gradation
within states and counties.
As good as the map is, it’s still based on averages. Occasionally we have
one of those winters that help clarify our thinking about just where a plant
can be safely grown.
The ability of a plant to survive low winter temperatures relies on a plant
cell’s ability to protect itself from an ice crystal formation inside the
cell. As winter approaches, a plant cell exports water from the cell. It
maintains a bare minimum amount of so-called bound water inside the cell.
If thermocouples are placed in plant tissue and the temperature is slowly
dropped, scientists can tell when a plant cell is killed by freezing based
on when the water freezes. When water changes from the liquid state to the
solid state, a burst of energy is given off and thermocouples can measure
this short-lived spike as an electrical impulse.
As the temperature falls, the first water to freeze is the free water that
is contained between the cells, or in the vascular system of the plant. This
gives off a large spike of energy, called an exotherm, usually somewhere
between 15 and 28 degrees. This large, first exotherm is not the one that
determines plant survival. It’s a second, smaller event.
As the temperature drops lower and lower, eventually the water inside the
cell freezes. When this water freezes, the exotherm spike is smaller, but it
signals the death of the cell. The reason the water does not freeze at 32
degrees is because it’s in an ultra-pure state and lacks an ice-nucleating
agent for ice crystals to form on.
The temperature at which this second exotherm occurs marks the minimum
temperature that a plant will survive. For Loropetalum, this second exotherm
probably occurs at around 5 degrees. The absolute minimum temperature that
any plant cell can forestall this freezing is minus-40 degrees, which marks
the boundary of the timberline on mountain tops.
Different parts of a plant will tolerate different temperature extremes.
Roots are the least tolerant of freezing, followed by evergreen leaves,
flowers, the phloem tissue and, finally, the xylem. The top of a plant may
freeze back, but the area close to the ground may not sustain as cold a
temperature as the exposed branch tips.
Gardeners will probably start observing signs of winter kill on many
marginally hardy shrubs as warm weather returns. The best approach is to
take a wait-and-see attitude and not be in a rush to prune. The extent of
pruning needed can be easily determined as March arrives.
By: Gerald Klingaman, retired
Extension Horticulturist
- Ornamentals
Extension News -
January 12, 2001
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