Ecosystem Gardening and Living: Part I, The Piedmont Mafic Barren

Looking to Local Habitats: models for creating vibrant self-sustaining urban ecosystems.

You know that spot in your garden on the south side of your house or office building where nothing will grow without significant watering? Maybe it’s against the house with full sun exposure and maybe some hot and hard surface is nearby. Maybe it’s one of the thousands of cracked concrete and asphalt surfaces scattered about town. There are countless little cracks and divots where sidewalks meet buildings, where parking lots meet sidewalks, where little islands of “weeds” and very well-drained sandy soils stand in the hot sun, or in other areas where the hard surface has begun to weather a bit. Maybe it’s the water-shedding heat-generating roof on your house, with absolutely nothing growing on it. Well, all of these sunny, hot and dry spots present wonderful opportunities for low maintenance and interesting native plant gardening. These locations are perfect places to plant species that thrive on and around local rock outcrops.

Feature: The Albemarle County “Piedmont MAfic Barren”

Outcrop Conditions: Outcrops with large surface area are typically fully exposed to the elements because they don’t have enough soil to support forest canopy and understory growth. Therefore, temperatures tend to be extreme in summer and winter. Because of the impermeable surface of the rock outcrop there is very little opportunity for soil deposition. Rainwater and snow melt water run off readily. In areas of small depressions and cracks, or in areas where boulders or smaller rocks rest upon the outcrop surface, soil may accumulate. Soil tends to be very well drained sandy organic loam with angular sand, pebbles, and cobbles. Because of the lack of moisture retention the site remains hot and dry, or cold and dry, for extended periods of time during summer and winter. When temperatures are more moderate in spring and autumn, the site retains the quality of being very well drained and dry.

What parts of urban human habitats resemble the Piedmont Mafic Barren habitat? Granite outcrops on south facing slopes very near Charlottesville provide examples for a multitude of sustainable solutions for what flora to plant on south facing roofs, barren areas in your yard that receive lots of sun and little water, concrete and asphalt surfaces, and other exposed, hard surfaces in urban areas. There are challenges presented when dealing with those spots that reach summertime surface temperatures exceeding 120 degrees! Local rock outcrop plants are up to the challenge!

Why use local habitats as models for creating artificial but self-sustaining urban ecosystems? Local habitats contain plants that have had millions of years to adjust to local conditions. These conditions are frequently quite habitat-specific. These local conditions include very important determining factors like latitude, longitude, aspect, slope, elevation, soil drainage, geologic substrate, soil chemistry, and local rainfall patterns. This means that they don’t need much of your help to survive and thrive! Yes, no watering! Furthermore, we are in a world where we must return to the ways of understanding local systems and local species. Our well-being is tied directly to the extent to which we understand these things and the connections we have to them in our day to day lives. When we explore local habitats and consider using them as models for ecosystem gardening, we become engaged with our immediate local and regional woods, fields, and streams. The benefit of this approach is that it encourages we humans to pay attention to our surroundings, look closer, understand more, and to nurture the health of human ecosystems for the sake of our health, our survival and the well-being of future generations. So, give it a try! It’s educational, fun, and healthy. And of course, include your friends and family.

An interesting result: you could end up creating habitats for rare species! Habitat is the most limiting factor for many rare and endangered species. While protecting natural habitat is vital, a serious risk is that we can end up with wild islands of biodiversity that are too vulnerable and decline over time. The good news is that sometimes just the creation of habitat is enough to attract rare native plants and animals. In Switzerland a green roof on a water filtration plant was even spontaneously colonized by thousands of a rare native orchid (Orchis morio). One other study in Switzerland discovered that green roofs are also a haven for rare and endangered spiders and beetles. (http://urbanhabitats.org/v04n01/introduction.html)

Features like “green roofs”, and living walls are still relatively new to the United States, so while use of native plants that are considered local is standard in many parts of the world we are still catching up to identify and use more of the species that actually thrive in our region. In Europe and other countries, the use of native plants on roofs goes back to ancient times. Sod roofs were common in Scandinavia, and in Japan, Japanese roof iris even got its name from the ancient practice of growing it on roofs.

Application: The obvious challenge is to find practical solutions for building (or renovating) spaces that can accommodate plants in a way that accomplishes the same thing as the local habitat example. One must be able to find parallels between the conditions of their urban space and those of specific local wild habitats. In other words, which habitat model and set of solutions is right for your urban space…your roof, yard, or park? The answer resides in a combination of matching existing or planned urban site conditions with those of local habitats while considering the special use requirements of the human habitat being created or renovated.

Another important factor in choosing your plants would be soil, or more appropriately, substrate, since many roofs and urban hard surface areas contain no actual soil. For example, semi-intensive “green roofs”, with a slightly deeper soil, can support more diversity than an extensive roof which may only have a soil depth of a few inches. Additionally, the alkalinity of soil (or lack thereof: pH) plays a big role in what species will grow on a rock outcrop or not. In Albemarle County, we tend to see more biodiversity on and around mafic outcrops that contribute calcium and magnesium to the soil. Also, in a granite outcrop, soil depth and type will vary dramatically over a small area. Creating variations in the depth of the media, and including different types of substrate (crushed greenstone, oystershell, crushed shale, sand, permatill, peat, etc.) can help mimic the kinds of natural variation seen over one site.

Benefits: Watering and fertilizing are work intensive and can have adverse impacts on the local environment. With plants that are adapted to local environmental conditions, maintenance is very low! Aside from the obvious value of having low maintenance self-sustaining plant growth, those millions of years of adapting to site-specific conditions have also resulted in a sustainable web of interconnections between a multitude of dynamic systems and organisms. They are all reliant upon one another. Local insects are reliant upon specific local plants, and local native food webs extend outward in countless directions. Native plants on a living roof will attract the life they are co-adapted with. With few exceptions, non-native plants will not support local fauna and will tend to occupy space otherwise suited for native plants. The net result is typically a reduction in biodiversity. These non-native plants are not adapted to local conditions and have not undergone co-adaptation with local species of plants and animals. And, in many cases, they are quite difficult to maintain, since they are hundreds or thousands of miles away from their co-adapted habitats. Low maintenance urban ecosystems with local native species of plants would reduce costs while supporting truly vibrant native plant and animal ecosystems. This type of gardening is exciting for both kids and adults, and can produce a rich learning environment for those that try it.

In addition to the biodiversity and horticultural benefits, there are some financial reasons to use local native rock plants as well. LEED awards points for using native plants, and features like green roofs already get points for reducing heating and cooling costs, meaning effectively that you can maximize your point total towards certification. Also, localities and the EPA are developing tougher regulations for stormwater management. Using drought tolerant native plants can help you meet or exceed the stormwater requirements for your project and reduce water consumption.

Species observed on June 14, 2011 at an Albemarle County granite rock outcrop community:

LOW GROWING PLANTS

annual ragweed, Ambrosia artemisiifolia

American Alumroot, Heuchera americana

common dittany, Cunila oreginoides
whitemouth dayflower, commelina erecta 

dwarf dandeolion, Krigia virginica

eastern prickly pear, Opuntia humifusa
upland boneset, Eupatorium sessilifoium

St. Andrew’s cross, Hypericum hypericoides

venus looking glass, Triodanis perfoliata

fameflower, Phemeranthus teretifolium

woodland sunflower, Helianthus divaricatus

Solomon’s seal, Polygonatum biflorum

smooth forked nailwort, Paronychia canadensis 
blunt-lobed woodsia, Woodsia obtusa

VINES

Virginia creeper, Parthenocissus quinquefolia

poison ivy, Toxicodendron radicans

SHRUBS

deerberry, Vaccinium stamineum

black haw, Viburnum prunifolium

blackberry species, unidentified

raspberry species, unidentified

dwarf hackberry, Celtis tenuifolia

hercules club, devil’s walking stick, Aralia spinosa

lowbush blueberry, Vaccinium pallidum

mountain laurel, Kalmia latifolia

persimmon, Diospyros virginiana

TREES

post oak, Quercus stellata

chestnut oak, Quercus prinus

eastern red cedar, Juniperus virginiana

pignut hickory, Carya glabra

white ash, Fraxinus americanus

ailanthus, Ailanthus altissima (non-native invasive)

Virginia pine, Pinus virginiana

MOSS

grimmia dry rock moss, Grimmia laevigata

FAUNA

eastern fence lizard, Sceloporus undulatus

great crested flycatcher, Myiarchus crinitus

red-bellied woodpecker, Melanerpes carolinus

scarlet tanager, Piranga olivacea

white-breasted nuthatch, Sitta carolinensis

wood thrush, Hylocichla mustelina

summer tanager, Piranga rubra

bluejay, Cyanocitta cristata

eastern wood-pewee, Contopus virens