Blooming in Early July: Great Rhododendron

With the gasoline and gunpowder gang’s biggest holiday of the year now upon us, wouldn’t it be nice to get away from the noise and the enduring adolescence for just a little while to see something spectacular that isn’t exploding or on fire?  Well, here’s a suggestion: head for the hills to check out the flowers of our native rhododendron, the Great Rhododendron (Rhododendron maximum), also known as Rosebay.

Great Rhododendron
The Great Rhododendron is an evergreen shrub found growing in the forest understory on slopes with consistently moist (mesic) soils.  The large, thick leaves make it easy to identify.  During really cold weather, they may droop and curl, but they still remain green and attached to the plant.

Thickets composed of our native heathers/heaths (Ericaceae) including Great Rhododendron, Mountain Laurel, and Pinxter Flower (Rhododendron periclymenoides), particularly when growing in association with Eastern Hemlock and/or Eastern White Pine, provide critical winter shelter for forest wildlife.  The flowers of native heathers/heaths attract bees and other pollinating insects and those of the deciduous Pinxter Flower, which blooms in May, are a favorite of butterflies and Ruby-throated Hummingbirds.

Pinxter Flower in bloom
A close relative of the Great Rhododendron is the Pinxter Flower, also known as the Pink Azalea.

Forests with understories that include Great Rhododendrons do not respond well to logging.  Although many Great Rhododendrons regenerate after cutting, the loss of consistent moisture levels in the soil due to the absence of a forest canopy during the sunny summertime can, over time, decimate an entire population of plants.  In addition, few rhododendrons are produced by seed, even under optimal conditions.  Great Rhododendron seeds and seedlings are very sensitive to the physical composition of forest substrate and its moisture content during both germination and growth.  A lack of humus, the damp organic matter in soil, nullifies the chances of successful recolonization of a rhododendron understory by seed.  In locations where moisture levels are adequate for their survival and regeneration after logging, impenetrable Great Rhododendron thickets will sometimes come to dominate a site.  These monocultures can, at least in the short term, cause problems for foresters by interrupting the cycle of succession and excluding the reestablishment of native trees.  In the case of forests harboring stands of Great Rhododendron, it can take a long time for a balanced ecological state to return following a disturbance as significant as logging.

Birds of Conewago Falls in the Lower Susquehanna River Watershed: Ruffed Grouse
Ruffed Grouse (Bonasa umbellus) may be particularly sensitive to the loss of winter shelter and travel lanes provided by thickets of Great Rhododendron and other members of the heather/heath family.  (Vintage 35 mm image)

In the lower Susquehanna region, the Great Rhododendron blooms from late June through the middle of July, much later than the ornamental rhododendrons and azaleas found in our gardens.   Set against a backdrop of deep green foliage, the enormous clusters of white flowers are hard to miss.

Great Rhododendron Flower Cluster
Great Rhododendrons sport an attractive blossom cluster.  The colors of the flower, especially the markings found only on the uppermost petal, guide pollinators to the stamens (male organs) and pistil (female organ).
Bumble Bee Pollinating a Great Rhododendron Flower
To this Bumble Bee (Bombus species), the yellowish spots on the uppermost petal of the Great Rhododendron may appear to be clumps of pollen and are thus an irresistible lure.  

In the Lower Susquehanna River Watershed, there are but a few remaining stands of Great Rhododendron.  One of the most extensive populations is in the Ridge and Valley Province on the north side of Second Mountain along Swatara Creek near Ravine (just off Interstate 81) in Schuylkill County, Pennsylvania.  Smaller groves are found in the Piedmont Province in the resort town of Mount Gretna in Lebanon County and in stream ravines along the lower river gorge at the Lancaster Conservancy’s Ferncliff and Wissler’s Run Preserves.  Go have a look.  You’ll be glad you did.

Great Rhododendron along Route 125 near Ravine
Great Rhododendron along Route 125 along the base of the north slope of Second Mountain north of Ravine, Schuylkill County, Pennsylvania.
Great Rhododendron along Swatara Creek
Great Rhododendrons beginning to bloom during the second week of July along Swatara Creek north of Ravine, Schuylkill County, Pennsylvania.  Note how acid mine drainage continues to stain the rocks (and pollute the water) in the upper reaches of this tributary of the lower Susquehanna.

Blooming Now in the Lower Susquehanna Region: Mountain Laurel

Mountain Laurel (Kalmia latifolia), designated as Pennsylvania’s state flower, is a native evergreen shrub of forests situated on dry rocky slopes with acidic soils.  As the common name implies, we think of it mostly as a plant of the mountainous regions—those areas of the Susquehanna watershed north of Harrisburg.  It is indeed symbolic of Appalachian forests.  But Mountain Laurel can also be found to the south of the capital city in forested highlands of the Piedmont.  There, currently, it happens to be in full bloom.  Let’s put on a pair of sturdy shoes and take a walk in the Hellam Hills of eastern York County at Rocky Ridge County Park to have a look.

The showy flower clusters of blooming Mountain Laurel are conspicuous throughout Rocky Ridge County Park right now.
Mountain Laurel flowering in the utility right-of-way south of the main parking area at Rocky Ridge…
…and to the north of the parking area at the hawk-watch platform.
Strolling a forest trail, particularly west of the utility right-of-way, can take you on a path through a thicket of flowering Mountain Laurel.
The majority of Mountain Laurels one might encounter will sport white flowers.
Others vary, exhibiting shades of spectacular pink.
Look closely and you’ll see flowers with curled filaments on some of the stamens.  When a bee or other insect makes contact, they spring into an extended position to assure pollen transfer to the visiting pollinator.
Absolutely spectacular.

Rain or shine, do get out and have a look at the blooming Mountain Laurel.

Living in the Shadows

They get a touch of it here, and a sparkle or two there.  Maybe, for a couple of hours each day, the glorious life-giving glow of the sun finds an opening in the canopy to warm and nourish their leaves, then the rays of light creep away across the forest floor, and it’s shade for the remainder of the day.

The flowering plants which thrive in the understory of the Riparian Woodlands often escape much notice.  They gather only a fraction of the daylight collected by species growing in full exposure to the sun.  Yet, by season’s end, many produce showy flowers or nourishing fruits of great import to wildlife.  While light may be sparingly rationed through the leaves of the tall trees overhead, moisture is nearly always assured in the damp soils of the riverside forest.  For these plants, growth is slow, but continuous.  And now, it’s show time.

So let’s take a late-summer stroll through the Riparian Woodlands of Conewago Falls, minus the face full of cobwebs, and have a look at some of the strikingly beautiful plants found living in the shadows.

Oxeye (Heliopsis helianthoides) is common on the interior and along the edges of Riparian Woodland.  Specimens in deep shade flower less profusely and average less than half the height of the five feet tall inhabitants of edge environs.
Pale Touch-Me-Not (Impatiens pallida) is one of two species of native Impatiens found in the river floodplain.  Both are known as Jewelweed.  The stems and leaves of the indigenous Impatiens retain a great quantity of water, so life in filtered sunlight is essential to prevent desiccation.  Contrary to popular folklore, extracts of Jewelweed plants are not effective treatments of Poison Ivy (Toxicodendron radicans) contact dermatitis.
Spotted Touch-Me-Not (Impatiens capensis) is typically found in wetter soil than I. pallida.  Both Jewelweeds develop popping capsules which help to distribute the seeds of these annual wildflowers.  “Touch Me Not”, or you’ll be wearing tiny seeds.
Green-headed Coneflower (Rudbeckia laciniata) grows to heights of eight feet in full sun, hence its alternate common name, Tall Coneflower.  In deep shade, it may not exceed two feet in height.  Floodplains are the prime domain of this perennial.
Wingstem (Verbesina alternifolia) normally flowers no earlier than late August.  The bases of the leaves are continued onto the stem of the plant to form wings which extend downward along its length.  This wildflower tolerates shade, but flowers more profusely along the woodland edge.
Great Lobelia (Lobelia siphilitica), or Great Blue Lobelia, is a magnificent wetland and moist woodland wildflower, usually attaining three feet in height and adorned with a plant-topping spike of blossoms.  Invasive Japanese Stiltgrass (Microstegium vimineum) can be seen here competing with this plant, resulting in a shorter, less productive Lobelia.  Stiltgrass was not found in the Susquehanna River floodplain at Conewago Falls until sometime after 1997.  It has spread to all areas of woodland shade, its tiny seeds being blown and translocated along roads, mowed lots, trails, and streams to quickly colonize and overtake new ground.
American Bladdernut (Staphylea trifolia), a shrub of shaded woods, develops inflated capsules which easily float away during high water to distribute the seeds contained inside.
Buttonbush (Cephalanthus occidentalis) is a shrub of wet soils which produces a strange spherical flower, followed by this globular seed cluster.
Common Pawpaw (Asimina triloba) is a colony-forming small tree which produces a fleshy fruit.  It is the host plant for the caterpillars of the Zebra Swallowtail.  The plant and the butterfly approach the northern limit of their geographic range at Conewago Falls.
Common Spicebush (Lindera benzoin) is a widespread understory shrub in wet floodplain soils.  It is the host plant for the caterpillars of the Spicebush Swallowtail (Papilio troilus).
Sweet Autumn Virgin’s Bower (Clematis terniflora) is an escape from cultivation which has recently naturalized in the edge areas of the Conewago Falls Riparian Woodlands.  This vine is very showy when flowering and producing seed, but can be detrimental to some of the understory shrubs upon which it tends to climb.

SOURCES

Long, David; Ballentine, Noel H.; and Marks, James G., Jr.  1997.  Treatment of Poison Ivy/Oak Allergic Contact Dermatitis With an Extract of Jewelweed.  American Journal of Contact Dermatitis.  8(3): pp. 150-153.

Newcomb, Lawrence.  1977.  Newcomb’s Wildflower Guide.  Little, Brown and Company.  Boston, Massachusetts.