HORTICULTURE HORTICULTURE
Dormancy and Cold Hardiness in
Fruit Crops
Mark Longstroth
Dormancy, Rest and Winter Cold Hardiness
Because most plants do not grow during the winter, we say they are dormant. There
are actually two types of dormancy during the winter.
One is called rest (endodormancy) and the plant will not grow even under
good warm conditions. The other is called
dormancy (ectodormancy) and the plant is ready to grow but the conditions are not right,
usually too cold. Rest occurs first. During rest, the plant accumulates chilling hours. The chilling hours required varies for different
plants from 700 to 1300 hours or more. If
warm weather occurs before the completion of rest no growth occurs. This allows the plant to track the winter and know
when it is Spring. Chilling and rest normally
prevent plants from beginning growth during warm spells in the middle of the winter. This would cause the trees to lose cold hardiness. Once growth begins the plant cannot increase its
ability to withstand cold and can be injured by very cold weather. Once the chilling requirement is meet, the plants
have completed rest and are merely dormant because the weather is too cold for rapid
growth. Chilling requirements are usually completed by midwinter. With the return of warm weather the plant begins
to grow.
The Onset of Dormancy or Rest.
Plants can track the hours of daylight during the day. In late summer the
days begin to shorten. By the time of the first day of fall when the day and night are of
equal length, perennial plants are getting ready for winter. Cooler temperatures are also
a good indication that winter is on the way. Frost and freezing temperatures are the
final clue that winter is coming. Many woody perennials lose their leaves as they
become dormant, such as maples and apples. Evergreen plants keep their leaves such as pine
trees, strawberries and cranberries (for evergreens water loss during the winter is a BIG
problem). As the temperatures get colder the plants enter Rest or Deep Dormancy when they
will not grow even if the conditions a good for growth. Horticulturists call this Endodormancy
(Endo is Greek for inside) meaning that something inside the plant is causing it to remain
dormant.
Winter Cold Hardiness
The perennial fruits grown in Michigan can withstand most of the conditions of a
Michigan winter. Extreme winter cold will cause little or no damage if the plants
(trees, vines, bushes) are hardy and the cold snap is not preceded by warm weather.
Most fruit plants can become acclimated to cold temperatures and can stand temperatures 10
to 20 degrees colder than the temperatures they are experiencing at the time. Sudden
drops in temperature of more than 20 degrees may cause damage or winter
injury. If warm weather with temperatures above freezing occur the plants will
lose the cold hardiness it has accumulated. The plants revert to what I call the
base cold hardiness. This is the minimum hardiness that a plant has and may be 10 F
to zero degrees (some wine grapes fall into this range). Renewed cold will cause the
plant to reacquire its cold hardiness. There is a maximum cold hardiness that the
plant can accumulate. There is a maximum cold hardiness which the plant can attain
and temperatures below this are damaging or fatal to the plant. (For example, peach flower
buds will begin to die at -13 to -15 F and most are gone by the time the temperature is
below -16 F, but the tree is not damaged until the temperature drops below -20 F.) This
number also falls over a wide range and varies for each plant species. During the
winter a plant in Michigan will gain and lose cold hardiness. The temperatures before a freeze are important in determining if a given
plant will be damaged. Plants that can not survive a Michigan winter are seldom
grown here.
Winter Dormancy
Another process which occurs in the winter is the accumulation of chilling units.
This is how a plant keeps track of winter and knows when it is Spring and time to
begin growth. This process prevents the plant from beginning growth during a warm
spell in the middle of the winter. This would cause the trees to lose cold hardiness
and once growth begins the plant cannot increase its ability to withstand cold.
Chilling is the name used to describe the process whereby the plant measures time during
the winter. The plant tracks the amount of time above 32 F and below 50 F. The plant ignores temperatures below freezing or
warm temperatures above 50. Chilling is
measured in hours. Actually
temperatures between 40 and 45 seem to be better than those that are much colder or
warmer. The chilling units required may vary for 700 to 1400 hours or more. If
warm weather occurs before the completion of rest no growth will occur because of internal
restraints. These chilling requirements are usually completed by mid to late
January. In 2006-2007, we had a warm winter and the plants
probably completed rest early. There is one very easy way to know if rest has been
completed. You take cuttings from the plants you are interested in and bring them inside,
placing them in a vase or glass of water, to force them.
If the buds begin to swell and grow within a week to ten days then you know that
rest is completed and the plants are appear dormant, only because of cool weather and that
warm weather will cause them to grow. Horticulturists call this Ectodormancy
(ecto is Greek for outside) meaning that conditions outside the plant are causing it to
remain dormant.
Spring Growth and Spring Frosts
There is usually a slow progression of development as the plant begins to grow.
This first becomes apparent buds swell and then green tissue emerges from the bud.
As the buds develop they become more and more
susceptible to cold temperatures and frost or freeze damage. I have several
other pages of information on spring frosts, reducing frost damage
and assessing spring frost damage.
