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Plant Stewardship Index
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Plant Stewardship Index
How close to nature is your landscape?

Free online tool can help you tell how natural is your landscape

By George Weigel, Pennsylvania Certified Horticulturist (PCH)

Bowman's Hill Wildflower Preserve, a 134-acre public garden in Bucks County, has a free, new online tool that will measure just that by assessing the kinds of plants growing on a particular tract.

The Preserve's "Plant Stewardship Index" rates 3,400 plants based on their value in native plant communities. Gardeners and landowners check off which plants they've got growing on their land, and the index computes a score that tells them how much — or how little — the site has been impacted by human activity.

By reassessing scores from year to year, landowners are able to determine if their plant conservation efforts are working.

"The index is free, and anybody can use it," says Jeannine Vannais, the PSI coordinator for Bowman's Hill. "We're already seeing landowners make use of it for things like assessing the health of their meadows."

To set up an account to use the index and its plant lists at no charge, visit

"It's designed to assess the naturalness of an area," says Miles Arnott, executive director of Bowman's Hill, which is Pennsylvania's official wildflower preserve located 2½ miles south of New Hope along the Delaware River. "With the PSI, you can get a good idea of the quality of an ecosystem ... The presence or absence of a particular plant tells us a lot about the nature of a habitat."

That's because some plants, such as white trout lilies and dwarf crested iris, will grow only in very particular and undisturbed areas. In other words, they're picky.

Then there are plants such as pigweed, bindweed and dandelions that will grow just about anywhere, including roadside ditches, cracks in curbs and those heavily graded clay fields masquerading as residential yards.

Rating plants
On the Plant Stewardship Index scale, plants are rated from 0 to 10. Plant thugs and other undesirables rate a 0. "Generalists" that aren't exactly unwelcome but that tend to show up even in highly disturbed sites rate somewhere between 1 and 3.

Scores rise along with each plant's demand for increasingly pristine conditions, up to a 9 or 10 for plants you're only likely to find in sites unblemished by human activity.

"Just because a plant gets a 9 or 10 doesn't mean it's endangered or rare," says Arnott. "But it does mean it's indicative of a good quality ecosystem."

A team of botanists from Pennsylvania and New Jersey rated the 3,400 plants, so this index is local to the Piedmont region. Vannais says plans are to convene a second team of botanists that will rate plants for the rest of Pennsylvania.

The online tool calculates two different scores. One — a "coefficient of conservatism" — is a measure of how close the land is to what would have existed before roads, buildings and other human impact occurred. The second — a "Floristic Quality Assessment Index" — is a measure of how far nonnatives have elbowed their way into the native plant community.

"This gives you a numerical sense of the impact of nonnative plants," says Vannais.

The online plant list itself will help gardeners and landowners determine exactly what's a native plant and what's not. Natives show up in green type online while nonnatives are in black.

"That information can be used to help people identify appropriate choices for a restoration," Vannais says.

One thing the index doesn't do is assess how worthy a plant is for a home garden.

Putting it in perspective
Just because a plant has a high PSI rating doesn't mean it's a "good" one that everyone should plant, and just because a plant scores a 0 or 1 doesn't mean it's a "bad" one to be shunned.

For instance, all plants that are not native to the Piedmont region are automatically assigned a 0.

It doesn't matter whether the nonnative is a horrible imported weed like the mile-a-minute weed or a garden center favorite like a lilac, Japanese spirea or purple coneflower. If it was introduced here by man, it's not something you should find in a natural setting and therefore is of no value in your site's naturalness rating.

That will throw you a few curveballs if you're looking at this from a pure gardening standpoint. Even some nasty plants that most of us would call weeds outscore plants we buy for their ornamental value.

For example, giant ragweed (Ambrosia trifida) might be a sneeze hazard, but it's a native that manages to rate a 1 on the PSI scale. By comparison, the nonnative butterfly bush rates a 0.

Wild strawberries (Fragaria virginiana) might plague our lawns, but they're natives that score a 2 on the PSI scale. By comparison, the white-blooming groundcover sweet woodruff (Galium odoratum) — a nonnative here — rates a 0.

Even the dreaded but native poison ivy (Toxicodendron radicans) comes away with a 1 while Japanese maples, Japanese hollies and bearded irises — all nonnatives — score zeroes.

Remember, the point is to assess the naturalness of a site, not the garden-worthiness of plants.

Arnott says it's important to get a handle on what's out there and how to preserve it because so many of our native habitats are being lost to development.

Also a big problem is how nonnative invasive plants are increasingly displacing native species — an immigration problem of a botanical sort.

The usefulness of the index will depend on a gardener's ability to correctly identify the plants on their property. That's especially critical because even two closely related species may have markedly different scores, such as Hibiscus moscheutos (the native rose mallow) that rates a 7 vs. Hibiscus syriacus (the nonnative rose-of-sharon) that rates a 0.

To score a site, the owner jots down the names of as many species as he or she can identify. (See below for sources of help identifying plants.)

"You don't have to count everything," Arnott says. "Just write down what you have. You don't need to do an inventory. Even if you count 30 to 40 percent of the species you have, it'll give you a good reading."

"This lets you see what you have and where the trouble spots might be," says Vannais. "And it'll show you over the years if what you're doing is working."

Once your plants are logged in on the site, you can call up that list later, make changes and quickly get a new reading instead of having to re-enter everything from scratch.

"It's not a magic bullet or an end-all," says Arnott, "but it's a very important piece that we haven't had before."

Sources to help identify the plants on your property:
* County Extension offices, local Master Gardener clinics and some garden centers have staff that can identify plant clippings.

* Numerous books have pictures, illustrations and keys to ID plants. For serious scholars, the best is The Plants of Pennsylvania by Ann Fowler Rhoads and Timothy A. Block (University of Pennsylvania Press, $75, 2000). Good layman books include Armitage's Native Plants for North American Gardens by Allan M. Armitage (Timber Press, $49.95, 2006); Newcomb's Wildflower Guide by Lawrence Newcomb (Little, Brown, $19.95, 1989); the New England Wildflower Society Guide to Growing and Propagating Wildflowers of the United States and Canada by William Cullina (Houghton Mifflin, $40, 2000), and the Peterson Field Guide A Field Guide to Trees and Shrubs by George Petrodes and Roger Tory Peterson (Houghton Mifflin, $19, 1973).

* Numerous Web sites have searchable databases and plant photos. Start with the Morris Arboretum's Pennsylvania Flora Project at Also try the Pennsylvania Native Plant Society at; the U.S. Department of Agriculture's 30,000-image database at, and for identifying invasive plants, the state Department of Conservation and Natural Resources at

* Bowman's Hill Wildflower Preserve offers plant identification classes through its ongoing "Knowing Native Plants" series. Details: or 215-862-2924.

* If you don't have a clue or don't have the time to research plant identity, Bowman's Hill offers a Specimen ID Service in which its staff identifies mailed-in plant specimens for a fee. Or it offers on-site plant identification by a botanist at a daily rate. For more on those services, call 215-862-2924 or email

(insert image here)
Dwarf crested iris (Iris cristata)

How 40 common landscape plants rate on Bowman's Hill Wildflower Preserve's new Plant Stewardship Index.

Plants with higher numbers on a scale of 0 to 10 are indicative of a healthy, native ecosystem. "N" stands for plants that are native to the United States. "I" stands for plants that were introduced here after European settlers arrived.
* American dogwood (Cornus florida) (N): 5
* Arborvitae (Thuja occidentalis) (I): 0
* Arrowwood viburnum (Viburnum dentatum) (N): 3
* Beebalm (Monarda didyma) (N): 4
* Blazing star (Liatris spicata) (N): 8
* Burning bush (Euonymus alata) (I): 0
* Butterfly bush (Buddleia davidii) (I): 0
* Cardinal flower (Lobelia cardinalis) (N): 6
* Eastern red cedar (Juniperus virginiana) (N): 2
* Elderberry (Sambucus canadensis) (N): 3
* English ivy (Hedera helix) (I): 0
* Flowering pear (Pyrus calleryana) (I): 0
* Garden phlox (Phlox paniculata) (I): 0
* Goldenrod (Solidago canadensis) (N): 1
* Hemlock (Tsuga canadensis) (N): 6
* Japanese holly (Ilex crenata) (I): 0
* Japanese maple (Acer palmatum) (I): 0
* Japanese spirea (Spiraea japonica) (I): 0
* Japanese yew (Taxus cuspidata) (I): 0
* Lilac (Syringa vulgaris) (I): 0
* Linden viburnum (Viburnum dilatatum) (I): 0
* Maidenhair grass (Miscanthus sinensis) (I): 0
* Mountain laurel (Kalmia latifolia) (N): 6
* Pachysandra (Pachysandra terminalis) (I): 0
* Pin oak (Quercus palustrus) (N): 3
* Plantain lily (Hosta) (I): 0
* Purple coneflower (Echinacea purpurea) (I): 0
* Redbud (Cercis canadensis) (N): 7
* Red-osier dogwood (Cornus sericea) (N): 3
* River birch (Betula nigra) (N): 7
* Smooth hydrangea (Hydrangea arborescens) (N): 7
* Stonecrop (Sedum) (I): 0
* Summersweet (Clethra alnifolia) (N): 8
* Sweetbay magnolia (Magnolia virginiana) (N): 8
* Switchgrass (Panicum virgatum) (N): 1
* White oak (Quercus alba) (N): 4
* Winterberry holly (Ilex verticillata) (N): 4
* Witch hazel (Hamamelis virginiana) (N): 6
* Woodland sage (Salvia nemerosa) (I): 0
* Yarrow (Achillea millefolium) (I): 0

Ten plants that score a 0 on the Plant Stewardship Index — ones you would NOT want to see in a healthy, native Pennsylvania ecosystem:
1. Tree-of-Heaven (Ailanthus altissima)
2. Japanese barberry (Berberis thunbergii)
3. Crown vetch (Coronaria varia)
4. Burning bush (Euonymus alata)
5. Rose-of-sharon (Hibiscus syriacus)
6. Spearmint (Mentha spicata)
7. Orange daylily (Hemerocallis fulva)
8. Scotch broom (Cytisus scoparius)
9. Butterfly bush (Buddleia davidii)
10. Rugosa rose (Rosa rugosa)

Ten plants that score a 9 or 10 on the Plant Stewardship Index — ones you would WANT to see in a healthy, native eastern-Pennsylvania ecosystem:
1. White trout lily (Erythronium albidum)
2. White lady's slipper (Cypripedium candidum and C. reginae)
3. Harebell (Campanula rotundifolia)
4. Rock harlequin (Corydalis sempervirens)
5. Round-leaved sundew (Drosera rotundifolia)
6. Dwarf huckleberry (Gaylussacia dumosa)
7. Dwarf crested iris (Iris cristata)
8. Possomhaw viburnum (Viburnum nudum)
9. American larch (Larix laricina)
10. Washington hawthorn (Crataegus phaenopyrum)
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