Friday, December 17, 2010

Under the Norway Maple

One of the most common trees in the residential setting is the Norway Maple, Acer platanoides. It was first introduced into the United State in 1756 by John Bartram. It is well adapted to the climate in the Northeast and grows well under a wide range of growing conditions, making it a preferred tree in urban and suburban landscapes. It’s tolerance of for almost any environmental condition, ability to grow in the shade, prolific seed production and overuse in the landscape has led to this becoming a dangerously invasive species in the Northeastern US.

Working in residential landscapes I often encounter the difficulty of working around this species. Its dense shade and greedy roots that suck moisture out of soil make it difficult to underplant. While many consider the Norway Maple as allelopathic (producing compounds that retard the growth of other plants), there are studies that indicate that this is not the case. (One study found no experimentally measurable allelopathic effects from Norway Maple.)  I think the problems encountered under a Norway maple are mainly due to the shade it casts and the dense network of roots that scavenge moisture and nutrients from the soil.

Norway Maple stump 6 years after a native species
restoration at Mount Auburn Cemetery

This photo shows the best method for dealing with a Norway Maple. Despite my disdain for this tree, I have not taken that step on my own property, yet. Since there is not another tree in the area to provide shade around the house for the hot afternoon sun I am hesitant to open up the canopy that much. Instead I have been exploring which native plants will grow under its canopy.

To get some ideas, I tried to learn about what grows the natural forest community along with Norway Maple in its native range (Europe and Asia). I thought I could find native equivalents to those European species. This tree is naturally found in mesic deciduous forests and mature riparian communities. While I did not find a definitive description of the other plants in this community (I’m sure that information is out there) I did get some clues from a site on the Plant Formations in the Central European BioProvince.  One shocking conclusion jumped out.

Many of the perennials and shrubs that grow in forests along with the Norway Maple are also invasive, or have tendencies toward invasiveness, in North American forests. These include goutweed (Aegopodium podagraria), fig buttercup (Ficaria verna), Tartan honeysuckle (Lonicera tartarica also L. xylosteum), yellow archangel (Lamium galeobdolon), and lily-of the valley (Convallaria majalis).  So I would be hesitant to recommend any new non-native species with the possibility of making a bad situation worse. However there were some others that are better behaved such as sweet woodruff (Galium odoratum), European ginger (Asarum europaeum), Siberian squill (Scilla sibrica), Tartan dogwood (Cornus alba) and fumewort (Corydalis solida) and some really neat plants like common hazel, Corylus avellana (from which comes ‘Harry Lauder’s Walking Stick’ ). 

In my own testing, I have been selecting native plant species that are tolerant of dry shade. So far I have found a number of natives that survive but do not flourish. It seems to be a general trend that all plants grow smaller and slower in that environment. However, there are a few that are more than holding there own. My plant list is as follows:

Smooth Aster and Showy Goldenrod,
both about 1/2 size of those
in other parts of the garden
American Bellflower (Campanulastrum americanum), American Pennyroyal (Hedeoma pulegioides), Smooth Aster (Symphyotrichum laeve), Heartleaf Aster (Symphyotrichum cordifolium), Virginia Creeper (Parthenocissus quinquefolia), Wafer Ash (Ptelea trifoliata) , Wild Bleeding Heart (Dicentra eximia), Witchhazel (Hamamelis virginiana ).

Holding their own:
Male Fern, still green after a couple of frosts.

Male Fern (Dryopteris filix-mas), Christmas Fern (Polystichum acrostichoides), Rosey sedge (Carex rosea), Alumroot (Heuchera villosa), Barren Strawberry (Waldsteinia fragariodes), Hairy solomon’s seal (Polygonatum pubescens), Largeflower bellwort (Uvularia grandiflora), Rosinweed (Silphium integrifolium), Showy Goldenrod (Solidago speciosa), Twinleaf (Jeffersonia diphylla), and Possumhaw Viburnum (Viburnum nudum).  This particular Viburnum was selected to improve pollination of the 'Winterthur' Viburnums nearby.

Promising, but still early:
Allegheny vine (Adlumia fungosa), Black Huckleberry (Gaylussacia baccata), Black-eyed Susan (Rudbeckia hirta), Inland Sea Oats (Chasmanthium latifolium), Lowbush Blueberry (Vaccinium angustifolia), and Pennsylvania sedge (Carex pensylvanica).

Failures, faded away or died outright:
Canada Mayflower (Maianthemum canadense), Fetterbush (Leucothoe fontanesiana), Foamflower (Tiarella cordifolia), Green and Gold (Chrysogonum virginianum), and Squirrel Corn (Dicentra canadensis). (When I moved the Leucothoe to a different shady location, without the root competition, it perked up after a couple of weeks.)

Plants that should work, planned for next season:
Labrador violet (Viola labradorica) and Maple-leaf viburnum (Viburnum acerifolium),

In addition to using plants that are strong competitors that can get their share of moisture there are some maintenance practices that will help the understory plants. Limbing up and thinning the canopy to let in more sunlight has helped a lot. Also, new plants should be irrigated deeply to get them established as well as under droughty conditions (mid-summer). Returning leaf mulch to the understory area helps to build the soil.

I’d like to hear what other natives have worked for you under Norway Maples, or ones that have failed desperately.

Thursday, December 2, 2010

Mistaken Identity

Oops, both are actually Meadowsweet

On looking back on an older post from May 2010, I realized that I had misidentified a native Spiraea.  While I had initially planted both Meadowsweet and Steeplebush in this spot, only the Meadowsweet had survived.  This became evident while visiting my sister last week.  I had given her a couple of pots of Steeplebush that I had grown from seed last year because she has a moister site, preferred by that particular species.

In this photo of Steeplebush, Spiraea tomentosa, gone to seed, you can see the the tight, upright flower plumes (typically pink to purple) and the deeply veined leaves, indicative to that species.  While I think it would be happier growing out in a moister area of the yard (growing to 4' tall), it appears to have a very nice form growing in a pot.  Maybe this is another 'Natives in Pots' candidate.  I'll have to see if I still have some leftover seed.

Meadowsweet, Spiraea alba, on the other hand has looser flower panicles (white to pale pink) and the leaves are not as deeply veined.  The specific variety I have, Var. latifolia, is native to drier upland sites, which explains how well adapted it is to growing around my house.  Mine bloomed continuously from early June until October and was a favorite of the bees.

It just goes to show, no matter how badly you want to have a plant, sometimes it just isn't there.