This concept is of most significance in regions with cold winters. The lowest temperature that a plant can survive regulates where it can be grown successfully. Tender plants cannot survive a frost and therefore can only be grown in regions or seasons that are frost free. Most bedding plants (Pelargonium, Impatiens etc.) fall into this category.

Hardy plants can survive a frost. The extent to which they can tolerate cold weather depends upon a mix of species physiology, duration and magnitude of sub-zero temperatures, soil type/drainage and numerous other factors.

Hardiness zones have been defined for Europe and the USA based on isotherm maps – joining areas of equal low winter temperatures. These are useful as guidelines but local conditions can vary a great deal, making the maps of limited practical use.

US zone map follow this link - http://www.usna.usda.gov/Hardzone/ushzmap.html

European zone map, follow this link - http://www.gardenweb.com/zones/europe/

In this website, plants are classified according to the abbreviations and definitions given below.

Half hardy plants = plants requiring protection during winter.

Hardy plants = capable of passing their whole life without protection (not normally applied to bulbs).

Tender or greenhouse plants = not capable of withstanding frost conditions.

Abbreviations for plant hardiness:

HA = hardy annual
HHA = half hardy annual
GA = greenhouse annual
HP = hardy perennial
HHP = half hardy perennial
GP = greenhouse perennial
HB = hardy biennial
HHB = half hardy biennial
HS = hardy shrub
HHS = half hardy shrub
GS = greenhouse shrub
HC = hardy climber
HHC = half hardy climber
GC = greenhouse climber

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