Plants for Vertical Greening Systems

1. Introduction

1.1. Plant hardiness zones

Plant hardiness is the ability of a plant to withstand year-round climatic conditions.
Hardiness zones are geographically defined areas in which plants are capable of growing.
This allows for a quick and simple assessment to be made as to whether the particular plant is site appropriate and what its lifespan is likely to be [1, 2, 3].

Hardiness ratings were developed by various countries to provide a system to classify the zones. There are many different hardiness rating systems, which are in part determined by location and climatic conditions. There is no standard system and this is where it becomes confusing. The USDA (United States Department of Agriculture) system is most widely used and is based on average annual minimum winter temperature. Several other systems are also based on the USDA system [4]. Using a system developed for use in North America can be misleading when applying it to Europe. The weather in Europe is variable and unpredictable due to the relatively close proximity of the Alps and the Atlantic Ocean. The RHS system (Royal Horticultural Society, UK) is based on the UK minimum winter temperatures and temperature swings (especially during spring and autumn) and provides a description [5]. Having knowledge of the systems, their deficiencies and climate classifications, is an additional tool to be used when choosing plants.

Interactive hardiness zone maps for Europe – based on the USDA system

The following links from ‘PlantMaps’ provide interactive plant hardiness zone maps for Europe based on the USDA system. Also provided are maps indicating records of high/low temperatures and dates of first and last frosts.

Plant hardiness zone comparison chart

There is no direct conversion between the systems but the chart below provides a rough comparison.

Table 4: Plant hardiness zone comparison

Temperature °C	USDA Zone	RHS Zone
-20 and below	6	H7	Very hardy in the severest European continental climates including exposed upland locations in the UK.
-20 to -15	7	H6	Hardy - very cold winter across the UK and northern Europe. Many plants grown in containers will be damaged unless given protection.
-15 to -10	7b	H5	Hardy - cold winter in most places throughout the UK even in severe winters. May not withstand open/exposed sites or central/northern locations. Many evergreens suffer foliage damage, and plants in pots will be at increased risk.
-10 to -5	8	H4	Hardy - average winter through most of the UK apart from inland valleys, at altitude and central/northerly locations. May suffer foliage damage and stem dieback in harsh winters in cold gardens. Some normally hardy plants may die in long, wet winters in heavy or poorly drained soil. Plants in pots are more vulnerable.
-5 to 1	9	H3	Half-hardy in unheated glasshouse / mild winters in coastal/mild areas, except in hard winters and at risk from sudden (early) frosts. May be hardy elsewhere with wall shelter or good microclimate. Likely to be damaged or killed in cold winters, particularly with no snow cover or if potted.
1 to 5	10	H2	Tender - cool or frost free glasshouse. Tolerant of low temperatures, but will not survive being frozen. Except in frost-free inner-city areas or coastal extremities, requires glasshouse conditions in winter, but can be grown outdoors once risk of frost is over.
5 to 10	11	H1C	Heated glasshouse - warm temperate. Can be grown outdoors in summer throughout most of the UK while daytime temperatures are high enough to promote growth.
10 to 15	12	H1B	Heated glasshouse - subtropical. Can be grown outdoors in summer in warm, sunny and sheltered locations (such as city-centre areas), but will generally perform better under glass or as a house plant.
15 and over	12b	H1A	Heated glasshouse - tropical. Needs to be grown under glass or as a house plant all year.

Climate classifications of Europe

In Europe there are five types of climate classification (Koppen classification system) [6, 7]:

Cfa (includes Slovenia) Humid subtropical climate;
Cfb (includes UK and NL) Oceanic climate;
Csa (includes Rome, Madrid, Istanbul) Hot-summer Mediterranean climate;
Csb (includes most of Iberia except Madrid) Warm-summer Mediterranean climate; and

Dfb (Oslo, Berlin) Warm humid continental climate.