The hardiness zones follow the U.S.D.A. classification, commonly used in USA and constantly gaining popularity in Europe.
This categorization is based on the average minimum temperatures in particular regions. The lower the number of the zone, the more hardy a plant is.
Cold hardiness maps have been developed for "Katalog Roślin: drzewa, krzewy, byliny polecane przez Związek Szkółkarzy Polskich" on the basis of W. Heinze and D. Schreiber's work "Eine neue Kartierung der Winterhärtezonen für Gehölze in Europa". We publish them with the consent of Agencja Promocji Zieleni.
Hardiness depends on a variety of factors, and therefore the division into zones should be treated only as a guide and not looked upon as a limiting factor. Within every zone there can be several regions with a milder or more severe microclimate than the average. Within a garden as well, plants growing in a sheltered position will withstand lower temperatures than those facing east, exposed to the morning sun and chilling gusts of wind.
Another hardiness factor relates to a particular plant's ability to adapt to fluctuations in temperature. Plants gradually acclimate and become more cold hardy during the fall and early winter season and possess maximum cold hardiness in mid-winter (December, early January). Then, with the arrival of spring cold hardiness decreases. Acclimated plants can deacclimate very rapidly when exposed to warmer temperatures and after the vegetation period has started even the hardiest plants may be considerably damaged by slight ground frosts (e.g. actinidia). The greatest peril of frost damage occurs in February and March, especially in sunny areas, when plants can't withstand rapid temperature changes from day to night. This is particularly dangerous for evergreen plants. Covering those plants with burlap or coniferous branches should offer sufficient protection.
Young plants are always more susceptible to cold than older, deeply rooted ones. Therefore special protection should be given to plants during 2-4 years after planting. This can be achieved through e.g. wrapping them in straw.
Considerable differences in hardiness levels occur also between different parts of a plant. Roots are substantially less hardy than above-ground, thick and woody stems, therefore good soil protection is essential. In regions where severe frosts without snow cover may occur it's good to cover the ground with e.g. bark mulch thus creating an isolation layer that will help protect the roots. It's advisable to put down 10-15 cm winter mulch at the base of a plant so as to help survive the buds that will enable plant regrowth even if the whole above-ground part freezes. Mulch layer also proves helpful in summer as it helps retain soil moisture and slows weed growth.