COLD HARDINESS

As a general indication of hardiness we use here the hardiness codes defined in the recently completed European Garden Flora (Cambridge University Press) with a slight modification in that we have given temperatures to G1 and G2. We also give the approximate equivalents of the USDA hardiness zones since these also may be familiar to some readers. These codes need to be interpreted with a certain amount of caution as mentioned below.

Plant hardiness zones
The zones should be interpreted as a tentative indication of the minimum temperatures the plants can withstand.

G2 (USDA zone 11) G2/Z11	heated greenhouse even in south Europe; needs + 5 ºC and above
G1 (USDA zone 10) G1/Z10	cool greenhouse even in south Europe; + 5 to 0 ºC, i.e. frost free
H5 (USDA zone 9) H5/Z9	hardy in favourable areas; withstands 0 to - 5 ºC
H4 (USDA zone 8) H4/Z8	hardy in mild areas; withstands - 5 to - 10 ºC
H3 (USDA zone 7) H3/Z7	hardy in cool areas; withstands - 10 to - 15 ºC
H2 (USDA zone 6) H2/Z6	hardy almost everywhere; withstands - 15 to - 20 ºC
H1 (USDA zone 5) H1/Z5	hardy everywhere; withstands - 20 ºC and below

On this classification, most of the UK falls into the zone H4/Z8 with the western fringes in zone H5/Z9. Most UK gardens are in these two zones. Upland areas of the UK are in zones H3/Z7 to H1/Z5. We have used these zones to construct a zone bar or hardiness indicator for each plant as follows:

Tender	G2/Z11	G1/Z10	H5/Z9	H4/Z8	H3/Z7	H2/Z6	H1/Z5	Hardy
			UK gardens

Each zone defines a temperature range but for most plants we indicate a range of zones in which it can be grown in a permanent position. The tolerance of plants to cold temperatures is influenced by many factors. In the UK for example plants often succumb in the winter to damp-induced rotting rather than to cold per se and keeping plants dry can greatly improve their chances of survival. The effects of microclimate, such as protection by a west or south wall can easily lift small areas of a garden by one zone. It is of course possible to "cheat" the zones. Containerised plants can be moved outdoors in a H4/Z8 garden in summer and returned to the protection of a cool (G1/Z10) or heated greenhouse (G2/Z11) for the winter. Alternatively, heavy mulching or wrapping in fleece, straw and sacking can create microclimates to protect individual plants in situ. Another factor to bear in mind especially when extrapolating the USDA zones is that they take no account of summer conditions. In a continental climate summers are consistently warmer and for a longer period than in the oceanic climate of the UK. For this reason, plants may succeed best in the UK treated as G1/Z10 plants when they may succeed H3/Z7 in the USA. The key to success with these plants, many of which are very new to cultivation in the UK is experimentation. One of the crafts of the renewed interest in tender plants for tropical effect in the garden is learning how to grow them successfully out of their recognised hardiness zones.

The RHS issued a hardiness rating system for the British Isles in association with the 2002 revision of the Award of Garden Merit plants. Rather than try to integrate the AGM hardiness ratings with the system we are currently using they are described here.

2002 AGM plant hardiness rating

H1 *	Requires heated glass
H1+3 *	Requiring heated glass; may be grown outside in summer
H1-2 *	Intermediate between H1 and H2
H2	Requires unheated glass
H2-3	Intermediate between H2 and H3
H3	Hardy outside in some regions or particular situations or which, while usually grown outside in summer, needs frost-free protection in winter (e.g. dahlias).
H3-4	Intermediate between H3 and H4
H4	Hardy throughout the British Isles

* may be further qualified with a suggested minimum temperature.