Cold hardy

Cold resistance [ Botany - Agronomy - Meteorology - Physiology ]	Dictionary of botanic terminology - index of names
Synonym: Frost Tolerance, Cold hardiness, Freezing resistance, Hardiness value, Degree of hardening, Winter resistance, Freezing thresholds.

Opuntia hystricina hybr. 'Hanau'
(Outdoor under a coating of ice)
Photo Yannick Gregorn - Slovenia

The freezing resistance of most plants changes with the season in step with changes in environmental temperature. In summer when temperatures are normally high many plants are more susceptible to a fall in temperature than they are in winter when it is colder. This process, which is known as acclimatization, is well illustrated in alpine plants.

The key to frost-hardiness is the avoidance of intracellular freezing and some plants are remarkably effective in achieving this. One of the ways in which the freezing point of plant sap may be depressed is by the accumulation of solutes (cf. the use of 'anti-freeze' in water-cooled automobile radiators

An alternative explanation of frost tolerance is that a higher proportion of the water in hardy plants is bound to cell constituents and so does not freeze as readily as that in tender plants where it is mainly in a free state

Some plants are able either to suppress the formation of ice crystals (supercooling), or at least keep them in the cell walls and prevent them from entering the living part of the cell (the cytoplasm and nucleus). It is the growing ice crystals, which like miniature needles, pierce through the structures of the cells and prevent them from functioning when temperatures warm up again, that kills

Cold resistant (Adjective) [ Botany - Horticulture ]
Synonym: Winter hardy, Frost resistant or Frost hardy

Plants that are able to survive winter frosts without damage to their leaves (ie Evergreens), stems (ie succulents) or damage to perennating organs like dormant buds or roots (ie Deciduous plants).

The term 'frost hardy' (Cold resistant) is often misleading because of the degrees of frost (i.e. light frost vs. hard killing frost) in fact there is not an universal standard to define frost resistance of plants according to their ability to survive degrees of frost. Frost hardines is very much relative to geographic areas.

Hardy plants can tolerate frost but vary in the severity and duration they can survive.
Here a common classification of plant hardiness:

Frost hardy plants can stay alive even when temperatures go down to -5°C
Fully hardy plants can cope with short spells as low as -15°C.
Half-hardy plants cannot tolerate frost can survive when temperatures go down to 0C.
Tender plants maybe damaged by low temperatures below 5°C.

These are general guidelines because if a plant is in a protected or sheltered spot it might well survive lower temperatures.

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Holdfast roots [ Botany ]
Dictionary of botanic terminology - index of names

Some species of climbing plants develop holdfast roots which help to support the vines on trees, walls, and rocks. By forcing their way into minute pores and crevices, they hold the plant firmly in place.

Climbing plants, like the poison ivy (Toxicodendron radicans), Boston ivy (Parthenocissus tricuspidata), and trumpet creeper (Campsis radicans), develop holdfast roots which help to support the vines on trees, walls, and rocks. By forcing their way into minute pores and crevices, they hold the plant firmly in place. Usually the Holdfast roots die at the end of the first season, but in some species they are perennial. In the tropics some of the large climbing plants have hold-fast roots by which they attach themselves, and long, cord-like roots that extend downward through the air and may lengthen and branch for several years until they strike the soil and become absorbent roots.

Major references and further lectures:
1) E. N. Transeau “General Botany” Discovery Publishing House, 1994