Winter is a time when much of the garden closes up
shop. Apart from late (and early)
perennials and bulbs, such as hellebores and crocus, and a few winter-blooming
shrubs, the garden is quiet. We
call this winter dormancy, but just what does that mean? Is there something going
stems and underground, while we dream of spring?
For the most part, the plants have
metabolism, almost to a standstill.
They have hunkered down to wait out the months where there is little
light for photosynthesis (and therefore, growing), little chance that any
flowers would be pollinated (and so, reducing the chance to further the
species) and a big possibility that tender plant parts would be damaged by the
Deciduous trees and shrubs go dormant because the
temperature and the day length trigger certain physiological responses. The first
thing we notice is that they
drop their leaves.
The departure of the hormone auxin from the leaf
abscission zone - the place where leaf joins stem - causes a breakdown in the
vascular pathways that were open all spring and summer. These pathways had transferred
products from photosynthesis down to the roots (by way of the phloem) and water
from the roots to the leaves (by way of the xylem). The cells break down at the precise point, the base of the
petiole (the leaf stem); the leaf receives no nutrients or water, and so they
die and drop.
Winter is a good time to observe the twigs, stems,
branches and trunks of deciduous trees and shrubs. We can check out the health of the plant, look for broken
stems that need to be pruned or inspect stems for those grayish, Styrofoam-like
egg casings of tent caterpillars.
Apart from giving trees and shrubs a checkup, we can
learn a thing or two about their anatomy.
In winter, it's easy to see how much growth a plant has put on, because
the newest growth, at the end of the stem, will have a different color from the
older bark. Coral bark maple (Acer
palmatum 'Sangu-kaku') is an easy one to spot, as its newest growth is also the
Other observations may require closer examination. A hand lens is a useful tool to carry
about the garden or your walks in the woods, because it will give you the
opportunity to see small structures more clearly. A 10x lens will give you plenty of magnification to study
such parts as a leaf scar.
The leaf scar shows you the point of abscission; often,
you'll be able to see dots that were the phloem and xylem pathways before leaf
fall. You also can take a good
look at leaf buds, those structures that, like us, are waiting for spring. The terminal
leaf bud is the apical
meristem, the point at which grow begins in spring. During dormancy, the apical bud is well protected from the
elements by layers of leaf scales.
The apical meristem holds the key to a plant's bursting
forth in spring. When the time is
right, this terminal bud breaks first, because it produces hormones that
inhibit buds further down the stem from breaking. Once the apical bud begins to grow, the hormone production
weakens, and successive buds also break.
Spring growth depends on a healthy plant, and
occasionally winter weather can cause damage that will weaken a tree or
shrub. It all begins with the
hardening off process. As those
leaves fall, the plant slowly settles in for winter. But we have all seen what happens when our mild, often warm,
autumn turns suddenly cold. An
early freeze or a rare dump of snow in November can harm deciduous plants that
not have quite shut down for the season.
Even during winter, when the plants have slowed
extensively, certain events can create problems. In the midst of typically cold weather, when morning sun
hits the trunk of a tree, the warmth can cause the sap to begin to flow while
the trunk is still in its slightly constricted winter phase. This causes vertical
frost cracks (sometimes called sun scald) on the trunk.
An early warming can interrupt
dormancy; a bout of warm
weather can cause the star magnolia, for example, to start blooming. Then, when a
late freeze hits, early
flowers can be damaged. This
occurrence may destroy one year's flower show, but a healthy plant can
withstand that. It's only if the
plant is declining for other reasons or if there are repeated blows that it can
do lasting damage. A weakened
plant is a susceptible plant, and disease pathogens can enter the frost cracks
or the mushy remains of previously frozen buds.
But we hope for a fairly normal winter,
when buds stay
put until they are well and good out of the woods. Many plants begin their leafing out and bloom cycle on
predictable cues. Apples, for
example, need from 1,000 to 1,400 hours at 45 degree (temperatures in
Fahrenheit) before they break.
Although days of 50-degree air temperature might not make a difference,
when the bud temperature goes up to 50 degrees the resting period is
overcome. Other plants break
dormancy with a combination of a period of cold followed by a period of
Before bud break, before spring, while all is fairly
quiet above, below ground the root system continues - although in a much
reduced fashion - to take up water and nutrients. When a plant is dormant, it is a good time to apply
fertilizers. The roots take the
nutrients up and hold them until spring, when everything goes into high gear.
conditions, with colder temperatures and lack of
sun, make it necessary for plants to hunker down and wait out the season. Dormancy
is a way to conserve resources
until conditions are right to spend precious assets in order to grow leaves,
and to flower - in order to continue the species. It's also a good time for us to slow down, and take pleasure
in branches and stems and the occasional winter flower.