Acclimation – complex process during which plants develop cold tolerance. It begins during late summer when shoots stop growing and become brown and woody or “harden off”. Tissues acquire increased cold hardiness through a number of factors and mechanisms. Most fruit trees acclimate in response to both short days and low temperatures.

Advective freeze – is characterized by a massive passage of cold air during which little stratification of air temperature occurs with elevation changes.

Cold Hardiness – is the ability of plant tissue to survive during exposure to low temperatures.

Cold/Winter Injury – is the killing of some part of the tree by low temperatures.  This can be damage to the fruiting or vegetative bud or damage to the trunk and limbs in the form of phloem damage.  Xylem damage rarely occurs.

Cold Tolerance – see cold hardiness.

Cold Avoidance – is a strategy to survive cold injury by not being exposed to it. These include proper site selection as well as freeze protection strategies such as the use of wind machines.

Deacclimation - is the process when tender fruit trees loose hardiness and are ready to resume growth. It is the transition from a cold hardy to a cold tender state and is the period from maximum cold hardiness to bud break.

Dew (frost) Point – is the temperature at which water vapour in the air condenses from a gas to a liquid. It is an important concept in the sense that when the dew point is below critical damaging temperatures, plant tissues can be more susceptible to cold injury.

Differential Thermal Analysis (DTA) – is a technique to conduct research on the mechanisms of freeze tolerance and also to predict lethal freezing temperatures of tender fruit fruiting buds. Temperature differences are recorded between the fruiting buds and a reference over time under identical thermal cycles. Differential temperature changes over time provide data about when critical freezing events have occurred.

Dormancy – time between the end of leaf drop and bud break with absence of visible growth.


Fruiting Bud – a bud that produces a flower and, if fertilized, fruit.

High Temperature Exotherm (HTE) – heat released when supercooled water freezes extracellularly (in the intercellular spaces); extracellular freezeing is considered non-lethal.

Lignification – is the process where tender fruit tree organs accumulate lignin which allows more resistance to cold and water loss. Lignin is incorporated into a complex tissue called periderm on the surface of tender fruit shoots in the fall.

Low Temperature Exotherm (LTE) – heat released when supercooled water freezes intracellularly (within the cytoplast & vacuole); Intracellular freezing is lethal.
LTE 10 – is the temperature at which 10% of the fruiting buds will be killed
LTE 50 – is the temperature at which 50% of the fruiting buds will be killed
LTE 90 – is the temperature at which 90% of the fruiting buds will be killed

Periderm – secondary covering (bark) which is composed of cork (phellem), the cork cambium (phellogen), and the phelloderm in the fall. The periderm forms from the phellogen which serves as a lateral meristem. The periderm replaces the epidermis, and acts as a protective covering like the epidermis.

Phloem – vascular tissue that transports sugars and other solutes throughout the plant.

Radiational Freeze – occurs when freezing temperatures develop during calm and clear sky conditions. Radiant heat is lost from the earth because there is no cloud cover to trap radiant heat or wind to mix the air. This results in very cold conditions at the surface.

Fruiting Spur – a short stout twig that bears the fruit buds in apple and pear trees.  Not present in tender fruit trees as these trees produce fruiting buds on shoots rather than spurs.

Supercooling – the ability to withstand very low temperatures where the contents of the cell can remain liquid during subfreezing temperatures. Buds supercool to avoid freezing injury.

Temperature Inversion – is the reversal of the normal behaviour of air temperature in which a layer of cooler air at the earth’s surface is overlain by a layer of warmer air. Under calm, winter conditions, ambient temperature is warmer above the ground than at the surface.

Vascular cambium – tissue of older shoots, limbs and trunks that generates new phloem and xylem annually.


Vegetative Bud – a bud that develops into a leafy shoot and does not produce flowers.

Xylem – vascular tissue that primarily transports water and minerals.