Variations on a theme

Features - Plants

Natives are put to the test next to landscape standbys

July 9, 2013
Jessica Lubell

The golf ball-sized flowers of buttonbush often age to reddish-pink fruits.

It is often said, “Use natives, because they are the best adapted plants for their region.” But, is this really a true statement? Different native species are found in different specific environments. Some native species hail from the margins of swamps in full shade and others from dry rock outcroppings in full sun.

Information about the natural habitat in which a particular native is found may be used as a guide to determine what native plants can be used in particular landscape situations. But sometimes plants surprise us and can also grow in environments that are different from where they are typically found. For example, some natives found in wet areas only, can also perform well in dry conditions under cultivation. In the wild, they do not compete well with other species in dry areas, and this is why they come to occupy wet areas where they are more competitive.

Native adaptability
The adaptability of native shrubs has received only limited research attention and we don’t really know how suitable they might be for challenging landscape sites, which present drought, full sun, infertility and pedestrian pressure. To test the adaptability of native shrubs, I planted six species in the ultimate challenging landscape site — a commuter parking lot on the University of Connecticut campus in Storrs, Ct. The six native species were American filbert (Corylus americana), buttonbush (Cephalanthus occidentalis), northern bush honeysuckle (Diervilla lonicera), steeplebush (Spiraea tomentosa), sweet fern (Comptonia peregrina) and sweet gale (Myrica gale). Each species provides ornamental interest, but has not been used extensively for landscaping because their landscape adaptability was unknown. Japanese barberry (Berberis thunbergii) and winged euonymus (Euonymous alatus) were also planted as controls to understand how the natives perform compared to these tough, old landscaping standbys.

To generate sound data, significant replication was built into this native shrub evaluation to be sure that what I observed was not anomalous. At the UConn parking lot site, 30 plants of each species were installed in June 2010 in a replicated experimental layout. Plants were mulched with four inches of softwood bark and hand weeded as necessary. Plants were irrigated three times weekly for the first four weeks after planting, and twice weekly for the next six weeks. In spring 2011, plants were fertilized with a balanced granular synthetic fertilizer at a rate of 2 pounds Nitrogen per 1,000 square feet. Plants were evaluated for aesthetic quality and growth rate.

Five of the six native species had aesthetic quality indices, which were comparable to the control barberry and euonymus (Table 1). Steeplebush was the only native species that did not perform as well as the controls. These pleasantly surprising findings indicate how much potential there is for expanded use of native shrubs by the nursery industry. Let’s take a look at what each of the six native shrubs has to offer our landscapes.

American filbert (Corylus americana)

These plants have a pleasant form, interesting textured leaves, edible fruits with unique involucres and respectable fall color. In the full sun parking lot site, plants produced a dense, uniform rounded habit, which is contrary to the sparse open plants seen in the wild where plants are growing in moderate to dense shade. After three growing seasons in the parking lot plants were 36 inches tall and 38 inches wide. Use plants in groups to create a naturalistic landscape or along the edge of woods to integrate the cultivated yard with the natural surroundings. American filbert can also function well in foundation plantings.

Buttonbush (Cephalanthus occidentalis)
Buttonbush is an upright, spreading, medium to large flowering shrub. Better specimens sport glossy dark green foliage and numerous golf ball size flowers that often age to reddish pink fruits. It was surprising how well buttonbush adapted to the dry, well-drained parking lot soil conditions, considering this plant occupies wet habitats in the wild such as swamps and bogs and along the edges of lakes and rivers. In three years plants reached 64 inches tall and 62 inches wide. Use buttonbush for naturalizing, in the shrub border or for screening. Just be sure when using buttonbush to provide adequate space so that plants can develop to their maximum potential.

Northern bush honeysuckle (Diervilla lonicera)
Plants in the parking lot produced dense mounds, 48 inches tall and 59 inches wide, with coppery red new foliage color, which slowly faded to green. Small tubular yellow flowers in bunches at the tips of shoots appear in June. At the start of the flowering period, plants produced a heavy bloom and flowered more or less continuously throughout the season into late September. Use northern bush honeysuckle in mass plantings or in repetition in the landscape. It can also work well as a facer plant for other more pointedly showy plants. Northern bush honeysuckle spreads slowly by rhizomes and should be used in sites where plants can be allowed to spread and cascade, like on a gravelly bank or above a rock wall.

Steeplebush (Spiraea tomentosa)
Steeplebush didn’t exhibit the same ability to perform under the most challenging landscape conditions but should perform well in less difficult residential landscape sites. The most striking features of steeplebush are its tomentose leaves and large pink flower spires in early July. Use it in mass plantings, mixed in a border with herbaceous perennials. Annual pruning may improve plant form and performance.

Sweet fern (Comptonia peregrina)
Sweet fern is desirable for its dark green fern-like foliage and the sweet fragrance it produces, most noticeably on warm sunny days. Plants in the parking lot performed exceedingly well, producing dense, uniform mounds up to 42 inches tall and 60 inches wide. Sweet fern should be massed or repeated in the landscape. It can be used in foundation plantings or as low hedge along a walkway in full sun to light shade. Plants can also be used to stabilize a gravelly bank by a driveway or road due to its rhizomatous nature, ability to fix nitrogen and salt tolerance.

Sweet gale (Myrica gale)
This shrub has an interesting candelabra-like, multi-stemmed suckering habit that uniformly fills an area. Plants in the parking lot reached 36 inches tall and 55 inches wide. The frosty lime green foliage is scented, especially when bruised, and the aroma was noticeable in the parking lot on warm days. Sweet gale will tolerate light shade and infertile soils, the latter due to its nitrogen fixing ability. Use it in groups, mass plantings or as a low informal hedge. Sweet gale is a good foundation plant due to its compact size and ability to tolerate reflected light. Site plants near a window to enjoy the aromatic foliage indoors.

Steeplebush (Spiraea tomentosa) shows large pink flower spires in early July.

More native species
Due to the overwhelming success of this study, I have selected eight other relatively unknown native shrubs to evaluate for landscape adaptability under similar conditions. These native species are creeping sand cherry (Prunus pumila var. depressa), elderberry (Sambucus canadensis), grey dogwood (Cornus racemosa), high bush blueberry (Vaccinium corymbosum), round leaf dogwood (Cornus rugosa), spicebush (Lindera benzoin), sweetbells (Eubotrys racemosa) and Virginia rose (Rosa virginiana). Based on very early observations during the first growing season (2012), it appears that many of these natives will also prove to be worthy alternatives for use in challenging landscape conditions. Stay tuned for future results. NM

Jessica Lubell is assistant professor, Ornamental Horticulture, Department of Plant Science and Landscape Architecture at the University of Connecticut; If you missed her first story on natives in the April issue, find it here: Follow Lubell on Facebook at Native Plant Gal for help with native shrub propagation and production, as well as landscape use.