Jackie Herald checks out a new way to specify the right trees for projects
When selecting plants, most of us base our decisions on nursery recommendations and catalogues. Yet, according to the Trees and Design Action Group (TDAG) survey of professional specifiers, many catalogues offer conflicting advice, and tend to prioritise aesthetics over hard technical information.
There is, however, help at hand with TDAG’s new digital document Tree Species Selection for Green Infrastructure: A Guide for Specifiers. The guide is co-authored by Dr Andrew Hirons of University Centre Myerscough and Dr Henrik Sjöman, Scientific Curator at Gothenburg Botanical Garden, Sweden, who launched the guide with presentations containing real gems of information for designers.
For example, they say that good street trees are a long-term investment; they usually require three to five years of watering and establishing, compared with two years in a park or garden environment.
When planting by paths and driveways, they suggest it’s essential to ensure there is structural soil under the hardscape, and not just in the tree pit or verge.
And if you like lime trees but want to avoid that sticky black deposit on cars and pavements, instead of the common lime, Tilia cordata, select Tilia tomentosa, whose leaves do not attract aphids and, therefore, emit honey dew.
The guide features 287 trees, most of which are widely available from UK growers, and I tried it out to see which will suit the typical small garden scenarios for which we garden designers regularly specify.
Falling into several scenarios – small, park, coast, transport, paved – are the Callery pear and the Swedish whitebeam.
The versatile Pyrus calleryana is possibly overused, especially as a street tree. It is tolerant of pollution and salt, and its clusters of white flowers in late spring are attractive, but unpleasantly smelly. Most cultivars are thornless. ‘Autumn Blaze’ and ‘Redspire’ display excellent autumn colour; ‘Chanticleer’ has a narrow conical form; while ‘Capital’ is strongly upright. Avoid the widely available ‘Bradford’, because it has weak wood and is prone to biomechanical failure.
Also tolerant to salt and air pollution, Sorbus intermedia ticks the bees, birds and beneficialto- insects boxes, and offers plenty of seasonal interest: white spring blossom, attractive white hairs on the underside of summer leaves, and, in autumn, clusters of red berries (pomes) and yellow-tinted foliage.
Compact trees for small gardens
For a paved courtyard in full sun, Clerodendrum trichotomum, the harlequin glorybower, offers large clusters of white, fragrant flowers wrapped in a purple casing in late summer, followed in late autumn by deep-blue berries with a crimson calyx. The leaves smell of peanut butter when rubbed. Be aware, however, that it forms root suckers.
Another choice is Elaeagnus angustifolia, which has good tolerance to air pollution, wind and salt, and its nitrogen-fixing ability helps it perform well on low-nutrient sites. Excellent for nectargathering insects, its attractive, fragrant, small yellow flowers emerge in early summer in the leaf axils of young shoots, with silvery scales on the undersides of leaves. Buy a large tree, as oleasters can be rather unruly when young.
A third option is Malus baccata (Siberian crab apple) and its cultivars, with highly ornamental blossom and fruits that are good for bees and other pollinating insects – for a narrow crown, choose ‘Street Parade’.
There is a dearth of trees deemed suitable for SuDS in small gardens, so if you are scratching your head for cultivars that will work in front gardens adjacent to driveways, consider Catalpa speciosa (Northern catalpa). This is classified as a SuDS-appropriate large tree, but if you coppice it, this might be worth trying in a small garden. It means you will get extra large leaves but won’t get the benefits of flowers and pods for pollinating insects and late autumn fodder for birds.
Clerodendrum trichotomum buds. Photo: Andrew Hirons
On a windswept coastal site, Cercis siliquastrum, the Judas tree, is one of my favourites. Spectacular, highly ornamental pink flowers appear on old wood in spring before leaves emerge. ‘Bodnant’ has larger flowers, and ‘Alba’ white. Although it’s moderately tolerant to shade, it will perform better in partial shade or full sun.
Crataegus laevigata is also worth considering – its spring flowers and red pomes in autumn attract a range of wildlife – however this species is vulnerable to fireblight (Erwinia amylovora). For double flowers and no fruit choose ‘Plena’.
Ilex aquifolium is a good holly for coastal gardens. Most of the widely available cultivars are female, so bear the characteristic drupes (berry-like fruit) – usually red, but in some cultivars, yellow or orange. Be aware that holly is a high emitter of biogenic volatile organic compounds (BVOCs), so avoid planting alongside busy roads as this can lead to a reduction in air quality.
For colour and interest
Speaking of which, for a garden by a busy highway, road or large paved areas, you can’t beat Cornus mas, with bright yellow flowers in late winter, before the leaves and red ovoid drupe fruit in early autumn. One caveat is that shallow rooting may cause problems with hard surfaces, particularly when combined with small or poorly aerated soil volumes.
Another option is Rhus typhina (stag’s horn sumac). It is dioecious, so select female plants for the highly attractive crimson fruit clusters that mature in late summer and persist well into winter. The species produces root suckers so it should be planted where it is able to form thickets. Individual trees are quite short-lived (less than 50 years) but clonal colonies can live much longer. For cut leaves, choose ‘Dissecta’ and ‘Laciniata’.
Finally, Sorbus commixta offers multi-seasonal interest, and it’s excellent for bees and other pollinating insects. The fruits are attractive to birds, and it is easy to establish and fast growing as a young tree. It is, however, extremely sensitive to poor soil aeration, so should not be planted in heavy, frequently waterlogged or compacted soil.
For naturalistic schemes
For naturalistic planting within a lawn, meadow or woodland, Abies koreana (Korean fir) is a late successional species that is tolerant to shade and shallow soils, but slow growing, even when well established. Young trees are sensitive to weed competition (including turf) and require humidity and ample soil moisture.
Cornus florida (flowering dogwood), though noted to be tolerant to shade, will perform better in partial shade or full sun. It is vulnerable to mildew, anthracnose and sunscald, but is coveted for its highly ornamental white bracts; for red bracts, choose the ‘Cherokee Chief’.
Heptacodium miconioides is a rare, but useful, small tree because of its late flowering and fruiting, benefitting pollinators. Its fragrant, creamy white flowers are followed by clusters of highly attractive fruits that turn from green to purple, then tan during early winter.
Magnolia ‘Yellow Bird’ tends to bloom later than ‘Elizabeth’, which helps to avoid later frost damage to the lemon-yellow flowers. Its young leaves have a coppery colour, and the bark is light grey and relatively smooth. Like all magnolias, it is known to be a high emitter of Biogenic Volatile Organic Compounds (BVOCs).
Another choice for naturalistic settings is medlar, Mespilus germanica, with attractive russet-brown fruits that are edible when over-ripe and have partially decayed – so better to plant in a grassy orchard or edge of woodland, as the fruit would become an issue on paved or gravel surfaces.
Lastly, Styrax japonicus, the Japanese snowbell tree, has pendulous fragrant white flowers in early summer, followed by dry ovoid drupes that mature by early autumn, which are covered with star-shaped hairs and not edible. This tree is sensitive to salt and air pollution, but partially tolerant to shade, and moderately sensitive to drought and waterlogging.
These are just a few common examples, but whatever your situation, the guide, Tree Species Selection for Green Infrastructure: A Guide for Specifiers, has a search tool, so you can enter your particular requirements and receive recommendations of trees, presented as highly detailed, informative and practical profile pages. It uses hyperlinks to be as dynamic and flexible as possible and enable users to flick back and forth easily. As it is a digital guide, the latest research findings, new cultivars and other data can be incorporated in response to emerging needs.
Styrax japonicus. Photo: Duncan Slater