The pollution soakers: Colorado natives to test

Cotoneaster franchetii  plant beat all others in a Royal Horticultural Society study to determine how much car pollution a hedge formed plant removed from the air and captured within leaf materials.  Leaf samples were measured for heavy metals: Zn, Cu, Pb, as well as Na.  before and after being exposed to roadside traffic for 9 days without rain. The study found that those with dense canopies and rough and hairy leaves, such as cotoneaster captured the most Pb, none other heavy metals were present enough in quantity.
From the Blanusa, Qadir et al study:
 ”  The traits for leaf-level particulate capture and retention include
presence of leaf hairs (trichomes)
scales, ridges and generally rough epidermal surfaces
along with canopy size (i.e., larger leaf area index and canopy density)
More detailed information is beginning to emerge on the importance of the length and density of hairs, their location on the leaf (ab-vs. adaxial), as well as leaf thickness (i.e., specific leaf area) on leaves’ capacity to capture particles [32]. Greater capacity of leaf surfaces to trap and retain particles is seen as a positive trait, but the need for the canopy to avoid the ‘saturation’ of surfaces and ‘regenerate’ this capacity by effective flushing from rain, is also acknowledged [13]. “
 Cotoneaster is not a candidate for the northern and central front-range.  Its classified in USDA zones 6-11 and native to China.  Its deeply veined, elliptic to oval leaves (to 1 1/2″ long) are glossy gray-green above with a white felty pubescence beneath. In zone 6, the shrub becomes deciduous.
So what would be a more cold hardy hedge with similar characteristics? They would need to be evergreen as well as well as have hairy leaves.
In Colorado winter we can have extended periods of dry, so the study period could be much longer than 9 days as they had in UK.
Top Candidates to study based on leaf morphology.
Taller Hedge
  1. Deciduous – Ribes cereum – wax currant sticky-hairy shrub under 6′ tall, leaves lightly fragrant. Small fan shaped, waxy leaves.  Leaves have numerous, short, stiff hairs with visible resin glands, particularly around the edges. Pink or white flowers form in dense clusters.  (resin may not release pollutants).  Note ribes aureum or odoratum have smooth leaves not conducive to capturing particulates.
  2. Evergreen – Ceratoides lanata winterfat-
  3. Evergreen –   Atriplex canascens – Four-wing saltbush up to 8′ tall, Its leaves are simple, alternate, entire, linear-spatulate to narrowly oblong, canescent (covered with fine whitish hairs) and ½ to 2 inches long. Its root system is branched and commonly very deep (to 20 feet) when soil depth allows.  These may not form a nice thick hedge. But could be spaced closely to allow overlap.
  4. Deciduous –  Shepherdia argentea – Silver buffaloberry  a tall thicket forming shrub up to 15′ with silver, willow-like  leaves and thorn tipped twigs.  Common along streams.   The willow like leaves have fine hairs (densley-pubescent) and silver-scale.
Low Hedge
  1. Symphoricarpos oreophilus – Mountain snowberry.  Forms dense thickets 1-3′ tall.Mountain snowberry is a native, deciduous, montane shrub. It is low growing, erect and sometimes trailing, with spreading to arching branches [11,25]. Although averaging 2 to 4 feet (0.6-1.2 m) in height, plants on good sites can grow up to 5 feet (1.5 m), while those on poor sites are barely a foot (0.3 m) tall.
  2. Jamesia americana – waxflower. Stiffly branched shrub with oppposite twigs and branches. up to 4′.  Opposite Leaves deeply veined, oval with sharp teeth.  Dark green. Seed heads persist through winter.
Perennial swaths
For winter gathering of particles that are then removed at spring for new growth.
  1.  Rabbitbrush and Stiff Goldenrod  is often seen in winter full of the dank of winter.
The original pollution soaker.
Common Name: cotoneaster 
Type: Deciduous shrub
Family: Rosaceae
Native Range: Southwestern China, Tibet, and Burma
Zone: 6 to 9
Height: 6.00 to 10.00 feet
Spread: 4.00 to 8.00 feet
Bloom Time: July
Bloom Description: Pink and white
Sun: Full sun to part shade
Water: Medium
Maintenance: Low
Suggested Use: Hedge, Naturalize
Flower: Showy
Fruit: Showy
Tolerate: Drought, Clay Soil

More about the RHS study     Open AccessArticle

Evaluating the Effectiveness of Urban Hedges as Air Pollution Barriers: Importance of Sampling Method, Species Characteristics and Site Location

1 Science and Collections Division, Royal Horticultural Society, Wisley, Woking GU23 6QB, UK
2 School of Agriculture, Policy and Development, University of Reading, Reading RG6 6AS, UK
3 Chemical Analysis Facility, School of Chemistry, Food and Pharmacy, University of Reading, Reading RG6 6AP, UK
* Author to whom correspondence should be addressed.
Current address: Natural England, North Gate House, 21–23 Valpy Street, Reading RG1 1AF, UK.
Environments 2020, 7(10), 81; https://doi.org/10.3390/environments7100081
Received: 7 July 2020 / Revised: 18 September 2020 / Accepted: 25 September 2020 / Published: 1 October 2020

An amateur gardener who loves to watch the garden grow.

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