Living Wall Plant Selection Guide

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1 While vertical gardens provide a great opportunity for visual creativity, with a diverse range of plant flower and leaf colours and textures available, the vegetation must also thrive in the space chosen for them. In other words if you put the wrong plant in the wrong location it will not grow regardless of how much care it is given. Depending on the scale of the wall, plantings can range from ground covers to larger herbaceous species. This plant selection guide has been designed for use with the Royal Horticultural Society Find a Plant web page at: which is specifically written around the UK climate. You can search by plant name, attribute or both to find detailed plant information. Below are the key attributes to consider for living wall plant selection: Plant Resilience Hardiness: H1a (>15ºC) to H7 (<20ºC) The selection of species will firstly depend on the local climatic conditions. It is important to realise that plant growth form, sun and shade exposure as well as wind exposure, is notably different on vertical surfaces compared to growing conventionally at ground level. In urban environments temperatures tend to increase with elevation, due to the increased thermal mass of built structures and the commensurate heat gain, while the potentially stronger winds at higher elevation can influence temperature as well. Enclosed spaces such as urban canyons can create their own microclimate where wind turbulence, pooling of pollution, humidity and temperature can be intensified. The localised climate of these areas will change the growing conditions for plants and needs to be considered when planning and designing green walls. First establish the expected temperature range the plants will be exposed to, and then select plants with suitable hardiness levels to cope with this based on the RHS hardiness scale (see table 1).

2 Table 1: Guide to RHS temperature and hardiness ratings Ratin g Temperature ranges ºC (ºF) Category Definition H1a warmer than 15 (> 59) glasshouse - tropical Needs to be grown under glass or as a house plant all year. H1b 10 to 15 (50 59) 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 all year. H1c 5 to 10 (41 50) glasshouse warm temperate Can be grown outdoors in summer throughout most of the UK while daytime temperatures are high enough to promote growth. H2 1 to 5 (34 41) 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, but can be grown outdoors once risk of frost is over. H3-5 to 1 (23 34) Half-hardy unheated glasshouse/mild Hardy in coastal/mild areas, except in hard s 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 s, particularly with no snow cover or if potted. Can survive with artificial protection. H4-10 to -5 (14 23) Hardy average Hardy through most of the UK apart from inland valleys, at altitude and central/northerly locations. May suffer foliage damage and stem dieback in harsh s in cold gardens. Some normally hardy plants may die in long, wet s in heavy or poorly drained soil. Plants in pots are more vulnerable. H5-15 to -10 (5 14) Hardy cold Hardy in most places throughout the UK even in severe s. May not withstand open/exposed sites or central/northern locations. Many evergreens suffer foliage damage, and plants in pots will be at increased risk. H6-20 to -15 (-4 5) Hardy very cold Hardy across the UK and northern Europe. Many plants grown in containers will be damaged unless given protection. H7 colder than -20 (< -4) Very hardy Hardy in the severest European continental climates including exposed upland locations in the UK.

3 Sunlight Light Levels: Full Sun, Partial, Full Shade Aspect: North facing, South facing, East facing, West facing Wall aspect influences light intensity. South facing walls receive most sunlight so suit plants that prefer full sun and warm temperatures. North facing walls receive the least sunlight so are best for plants that thrive in shade and cooler conditions. East and West facing wall have the benefit of getting a mixture of light and shade so are less influential in plant choice. Light intensity tends to be greater at height than at ground level. At height there are fewer structures, no vegetation to absorb solar radiation and increased reflection from adjoining building and surfaces (such as glass and light-coloured walls). Conversely, there are some roofs and walls that may receive significantly less solar radiation, due to intense shading by nearby buildings. Shadowing and shading analysis can be used to assess areas of light and shade on a site and possible changes over the year (for example, at the equinox) and over time (for example, adjoining new building development). Size Height: 0.10 to 0.50m recommended Spread: Range: 0.10 to 0.50m recommended As living wall plant density is much higher (typically 30 plants/m²) than conventional flowerbeds, greater care must be taken in considering how the various plants will grow and interact with their neighbours. Look for low growing varieties, ideally 50cm tall at maturity. A neat tapestry of plants could quickly deteriorate if, for example, larger, or more vigorous, plants shade or smothered surrounding slower growing companions. Vigorous growth increases maintenance requirements and slow growing plants are often preferred. However, vigorous species can be used to create protected niches for the inclusion of sensitive species in high exposure areas. This can help provide light or shade for other species, wind protection or humidity conservation. Consideration of where each species will be placed in relation to others (the mixture of plants), and their growth habits, helps to develop a working artificial ecology on the wall. Understanding how the ecology will change as the plants mature is important, as niches will evolve beneath, next to and above certain species. Plant Characteristics Foliage: Evergreen, Semi-Evergreen, Deciduous For vertical gardens to look good all year round, with minimal maintenance, it is best to choose about 95% evergreen plants with plenty of different textures, then, for seasonal added colour, disperse semi-

4 evergreen and deciduous plants among them so that you are not left with large empty patches during die down. Choose plants that are perennial (long-lived) rather than annual (need replacing every year). While regular maintenance is necessary, normally you do not want to replace large numbers of plants every year. Also consider that some plants suffer from die back even as they produce new grow (for example some grasses can look untidy because they become thick with dead blades). Colour Foliage and Flower: Colour Choices Finally turn your attention to creating the artistic tapestry by finding a balance of colour and textural contrast, flowering highlights, seasonal variations, stem colour and fruit. Or by plant theme such as herbs or edibles. Other Considerations Additional Benefits: Certain plants will be better for water purifying, air filtering, producing nectar for pollinators and habitat provision. Exposure: External green walls are often exposed to strong and frequent wind as average wind speeds are greater at height. Recognise that the tops, corners and sides of the wall will have greater wind exposure. In highly exposed locations, select robust species that can tolerate sun and wind. Look for species that have shallow, fibrous root systems to promote strong anchorage in the limited volume of growing medium available. Disease and pest resistance: When plants are crowded diseases and pests can spread rapidly. So be aware that some plants are more susceptible to likes of rust, mould, aphids, slugs and vine weevil so treat these with caution. Maintenance: Some plants species require ongoing aftercare such as dead-heading to prolong flowering or pruning to maintain healthy growth or control spread. Water requirements: These can be minimised by selecting species that are low water users. Recognise that less water may be available at the top of the wall due to dehydration from greater exposure, so species should be selected and positioned on the wall with moisture gradients in mind. If the system recycles water, species selection may have to cater for elevated salt levels and amended ph levels.

5 Examples of Popular Outdoor Species Achillea varieties Heuchera varieties Acorus varieties Heucherella varieties Agapanthus Hosta varieties Ajuga Reptans Houttuynia Cordata Armeria Maritima Hyacinthoides Non-scripta Artemisia Dracunculus Hyssopus Officinalis Asplenium Scolopendrium Iberis Sempervirens Aubrieta Argenteovariegata Iris varieties Azalea Juncus Inflexus Bergenia varieties Lamium Maculatum Calibrachoa (annual) Lavender Campanula Liriope Muscari Carex varieties Lobelia varieties Cerastium Tomentosum Lonicera Pileata Convolvulus Cneorum Luzula Nivea Coreopsis verticillata Luzula Sylvatica Cyrtomium varieties Omphalodes Verna Dianthus Origanum Vulgare Delasperma Pachysandra Terminalis Dryopteris erythrosora prolifica Perovskia Atriplicifolia Epimedium Grandiflorum Petrorhagia Saxifraga Erica Carnea Petunia varieties (annual) Erigeron Glaucus Albus Phlox Subulata Erigeron Karvinskianus Polystichum varieties Escallonia Primula varieties Erysimum varieties Prunella Vulgaris Escallonia Rosmarinus officinalis Euonymus Fortune varieties Rhododendron Euphorbia varieties Salvia Officinalis Fern varieties Sarcococca Festuca glauca Vill. Satureja Montana Fuchsia varieties Saxifraga x Urbium Geranium varieties Scirpus Cernuus Geum Rivale Sedum varieties Haberlea Rhodopensis Seslaria Caerulea Hebe varieties Silene varieties Hedra varieties Soleirolia Soleirolii Helianthemum Tellima Grandiflora Helleborus varieties Thymus Vulgaris Tiarella varieties Uncinia Rubra Vinca Minor Viola (annual) Herbs: Basil Bay Caraway Chamomile Chives Coriander Curry Plant Dill Lavender Lemon Balm Lemongrass Marjoram Mint Oregano Parsley Rosemary Sage Sorrel Tarragon Thyme Edibles: Chard Cherry Tomatoes Dwarf Cabbages English Spinach Lettuces Radishes Rocket Silverbeet Small Chillies Strawberries Watercress