Plant of the Week
Dwarf Alberta Spruce
Latin: Picea glauca 'Conica'
Winter has finally arrived. Two days ago, I watched a barefoot boy play
football with his pals on a balmy January morning; within a few hours, the
temperature had plummeted almost 70 degrees with the mercury hovering near zero.
Such swings can't be good people or their
plants.
Plants have adapted mechanisms to cope with the cold of winter so I thought
it might be interesting to consider how some of the most cold hardy plants adapt
to their chilly haunts. One of the most cold-tolerant plants we commonly see in
our landscapes is the Dwarf Alberta Spruce (Picea glauca 'Conica').
This dwarf tree can reach 15 feet tall in a perfect location in the northland
where conifers grow best, but most of us are more familiar with it around the
holiday season when mass merchants sell thousands of the foot tall, perfectly
conical living Christmas trees as tabletop decorations. The trees maintain that
perfect conical form as they slowly increase in size over the years.
The needles are light green, about a half-inch long and radiate at all angles
around the stem, giving a bottlebrush effect to the small branches. It's a
genetic mutant that doesn't produce cones as would be expected with most spruce.
Dwarf Alberta Spruce was discovered in 1904 in the Northern Rockies near Lake
Laggan, Alberta as two Arnold Arboretum botanists awaited the arrival of their
train to take them back to Boston. Today, they would meet with serious
government red tape and social castigation, but at the time they spotted the
stunted seedling and recognized it as something new and different. Without a
second thought, they dug it up and took it home with them.
Back in Boston, the dwarf spruce was found to be easy to propagate and was
released to the nursery trade a few years later.
The most famous of the pair was Alfred Rehder, who became known to thousands
of students for his Manual of Cultivated Trees and Shrubs. In the1940
edition, Rehder was the first to use a plant hardiness zone map based on minimum
winter temperatures. His first hardiness map had only seven zones but served as
the precursor to the 12-zone USDA map we use today.
Using the modern version of the hardiness zone map, the Dwarf Alberta Spruce
will grow from zone 2 to 7. The zone 2 rating means that this diminutive tree
will tolerate temperatures as low as -40 degrees Fahrenheit.
Ironically, at this cold temperature the centigrade and Fahrenheit temperature
scales cross, and quite by accident, it represents an important biological
juncture as well.
Minus 40 degrees is the lowest temperature that pure water can remain in a
liquid state. Common experience tells us that water freezes at 32 degrees
Fahrenheit,
but it was the German inventor of the mercury thermometer, Daniel Fahrenheit
(1686-1736) who first demonstrated that pure water could go to colder
temperatures before freezing.
To freeze, water needs what is called an "ice
nucleation agent" - something to begin the
crystallization process. Plants that tolerate very cold temperatures use
membranes to separate the liquid water in their cells from these nucleation
agents, thus they can grow in very cold environments.
The timberline at the top of a mountain, usually between 13,000 and 14,000
feet, is a natural marker for this -40 degree temperature
zone. All hardy woody plants, to a lesser or greater degree, have the ability to
create these reservoirs of super pure water in their cells, but to do so takes
time.
Rapid temperature drops can freeze the water in the cells before it has a
chance to undergo purification. The ice crystals puncture membranes and the
cells die.
The Dwarf Alberta Spruce will grow in Arkansas and is occasionally seen as
far south as Little Rock.
Summer heat and humidity are not to its liking, so try to locate it where it
gets some afternoon shade but otherwise good light. The north or east side of
the house is probably the best location. Make sure it's planted in a fertile,
well drained soil not allowed to get too dry during the summer. In our heat,
spider mites can be a serious problem, so inspect the plants in June and July
and spray if needed.
By: Gerald Klingaman, retired
Extension Horticulturist
- Ornamentals
Extension News -
January 9, 2004
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