Tuesday, August 11, 2015

Passiflora in Kentucky

The genus Passiflora L. contains roughly 500 species, the majority of which are vines of pantropical distribution.  Collectively, they are the passionflowers, and their blossoms are some of the most unique and bizarrely structured in the plant kingdom.  In temperate North America where there are only ~9 spp., the flowers seem especially alien.  The name Passiflora derives from the Latin passio (passion) and flos (flower).  It was a name given by the early missionaries to South America who believed that the parts of the complex flower were different symbols of Christ’s crucifixion: five anthers represented five wounds, three styles the three nails, and the corona (the numerous fringed filaments) the crown of thorns.  

The flora of Kentucky contains just two species of passionflower, the sole representatives of the Passifloraceae family in the state.  Both are perennial vines that climb by means of tendrils, using other vegetation as a means of support to access sunlight.  These sister species occupy discrete ecological niches, and it is interesting to consider those niches as they relate to speciation.  It is also a helpful exercise with cultivation in mind, the ‘right plant, right place’ mantra – as both species can be spectacular garden plants when well-placed.

Passiflora incarnata (2014-120) in the West Knobs region
Passiflora incarnata L. directly translates to ‘flesh-colored passionflower’, though in the literature it is always referred to as ‘purple passionflower/passionvine’ or ‘maypop’.   It is widespread in Kentucky, occurring in each physiographic region but relatively sparse in the Appalachian Plateaus and Cumberland Mountains.  Purple passionflower is close to the northern extent of its range in Kentucky, being far more widespread in the southeastern United States.  At present, it is a species typically seen along rights of way and field edges; pre-settlement habitats remain enigmatic. 
The Walk Across Kentucky staff collecting in the Pennyrile region fall 2014.  The disturbed field and wooded edges here supported populations of purple passionflower.

It seems safe to say that P. incarnata prefers full sun, but will tolerate moderate shade.  Like so many species of vines and lianas, it grows masterfully in disturbed areas – and that is not necessarily a complement to the plant.  Climbers invest far less resources in their stems than other plants, as a result of their nature as structural parasites.  This allows them to grow quickly, overtopping both themselves and other species while forming a thick mass of tangled leaves and stems (think Kudzu). 

Overlapping stems of purple passionflower.  Note the two glands where the leaf meets the petiole - distinctive for this species.
Purple passionflower is especially prolific because of its propensity to sucker adventitiously from roots.  These growth aspects are worthy of note should you consider to cultivate this plant, remember that purple passionflower doesn’t ‘play well’ with other species, and the root suckers may pop up in garden beds and lawn areas nearby.  Getting rid of this plant will be no easy task.  Our plants grow up an old willow stump surrounded by turf where the mowers keep their spread at bay.

Passiflora lutea var. glabriflora (2004-003) in the Shawnee Hills region
Passiflora lutea L. is the yellow passionflower.  In Kentucky, our plants should all be the smooth form (P. lutea var. glabriflora Fern.).  While also distributed across the state, it is most prevalent along the Ohio River Valley and central Bluegrass.  It is the northernmost (and hardiest) of all Passiflora species.  Though tolerant of sun and shade, the yellow passionflower can handle the shadier end of the spectrum as compared to purple passionflower.  Our plants on the Walk Across Kentucky grow in quite dense shade, and still perform admirably.  This species is accustomed to soils more rich and moist than purple passionflower.
Yellow (left) and purple passionflower (right).  Yellow passionflower has smaller features, an entire leaf margin (vs. serrate), and rounded lobes (vs. pointed).
Where purple passionflower is big, flashy, and wild; yellow passionflower is the delicate, modest, and well-behaved of the two sisters.  Flowers of the former can be larger by an order of magnitude, and their bright-purple color is a beacon compared with the unassuming pale yellow of P. lutea.  Still, the temptation to cultivate P. incarnata may be something of a fool’s errand to be regretted later when it overruns a garden.  Yellow passionflower, if not available commercially, should be.  It would be well-placed in a shade garden, as a fine-textured and sparse groundcover, or against a tree/trellis where it may climb up to 20-30’ each season.  Along home walkways and bannisters would be an especially nice location to appreciate the subtle blossoms.
P. lutea ascending the lower limbs of an oak.  Tendrils grasp both the oak twigs and other vine stems for support.
P. lutea young leaf and flower buds

Both of our native passionflowers are valuable to wildlife, particularly the various butterflies that use them for a larval food source.  Bees are the main pollinators, and of special note is the Passionflower Bee (Anthemurgus passiflorae), a rare ground-nesting species that contributes little to pollination, but is fully obligate to the yellow passionflower – this plant is the only known species able to support the insect. 

"It has often been vaguely asserted that plants are distinguished from animals by not having the power of movement. It should rather be said that plants acquire and display this power only when it is of some advantage to them; this being of comparatively rare occurrence, as they are affixed to the ground, and food is brought to them by the air and rain. We see how high in the scale of organization a plant may rise, when we look at one of the more perfect tendril-bearers. It first places its tendrils ready for action, as a polypus places its tentacula. If the tendril be displaced, it is acted on by the force of gravity and rights itself. It is acted on by the light, and bends towards or from it, or disregards it, whichever may be most advantageous. During several days the tendrils or internodes, or both, spontaneously revolve with a steady motion. The tendril strikes some object, and quickly curls round and firmly grasps it. In the course of some hours it contracts into a spire, dragging up the stem, and forming an excellent spring. All movements now cease. By growth the tissues soon become wonderfully strong and durable. The tendril has done its work, and has done it in an admirable manner."  

Charles Darwin, 1875.

Monday, July 27, 2015

Notes on Aronia

Aronia x prunifolia (WAKY accession 2012-018)
The Rose Family (Rosaceae) is well known for genera indispensable to both gardeners and wildlife.   The fruits of this group typify their importance ornamentally and nutritionally.  Among just native rosaceous genera, most people are familiar with Amelanchier (serviceberries), Crataegus (hawthorns), Fragaria (strawberries),   Malus (crabapples), Prunus (cherries and plums), and Rubus (blackberries and raspberries).    Perhaps less-familiar is the genus Aronia (chokeberries), not to be confused with chokecherry (Prunus virginiana) – another rare Rosaceous species in Kentucky.

Aronia taxonomy is not for the faint of heart.   The type species [A. arbutifolia (L.) Pers.] was first placed in to Mespilus by Linnaeus.   Various authors have gone on to classify Aronia Medik. into Adenorachis, Crataegus, Halmia, Photinia, Pyrus, and Sorbus.  Some references continue to treat Aronia (and other genera) as Photinia, apparently due to floral and fruit traits deemed indistinguishable.   Nevertheless, recent phylogenetic evidence clearly refutes the sensu lato Photinia hypothesis.  A clear method by which Aronia (s. str.) may be riven from the aforementioned genera is by examining the adaxial midvein - only among Aronia will the presence of small red glands be visible.
note the dark-colored glands on the midrib

In Kentucky, as in the rest of Eastern North America, there are generally three Aronia species recognized:  Aronia arbutifolia (red chokeberry), Aronia melanocarpa (black chokeberry), and Aronia x prunifolia (purple chokeberry).   Red chokeberry is readily distinguished from black chokeberry based on red fruit (vs. black) and strongly pubescent leaves/pedicels/twigs (vs. glabrous).  Purple chokeberry is typically regarded as an interspecific hybrid between red and black chokeberry.  Evidence to support this theory comes from geographic distribution of species, leaf chemistry, cytogenetics, and reproductive biology, but a hybrid origin is also reinforced when morphology is considered.  Purple chokeberry exhibits several characters intermediate between each putative parent, including purplish/black fruits whose size is <black chokeberry and > red chokeberry.  Leaf pubescence of purple chokeberry is likewise intermediary between the strongly pubescent A. arbutifolia and glabrous A. melanocarpa.  
Aronia arbutifolia, red chokeberry

Aronia melanocarpa, black chokeberry

Aronia x prunifolia, purple chokeberry (a purported hybrid)
From a much more practical perspective, all of the aforementioned Aronia species make fantastic ornamental specimens, and it is curious why they are not more widely cultivated.  Chokeberries are all deciduous spring flowering shrubs with small simple leaves, nice fall color, and persistent fruit.  Rhizomatous  suckering is frequent in Aronia, but never in an aggressive manner.  The red chokeberry is common in wetter environments like bogs, seeps, and moist edges.  Black and purple chokeberry may occupy similar environments but can also be found in drier/thinner soils.  Partial shade to full sun is suitable for Aronia, though the latter yields more abundant flowers and fruit, and better fall color.  
Black chokeberry...who needs burning bush?

Aronia fruits are consumed by a number of bird species, and are edible to humans.  Black chokeberry in particular is widely cultivated in Europe and Asia where the fruits are used for jelly, juice, and wine, as well as eaten whole.   Several cultivars have been selected for fruit characteristics, and are available commercially.  It seems typical to observe chokeberries holding fruit for extended periods – to the point where the fruit dries into raisin-like pomes still attached to the plant.  It has been speculated that the astringent taste or low protein content may make the fruit less appealing to birds.  Nevertheless, the fruit is high in vitamins, minerals, and folic acid, which has led to increased human cultivation.  

Red and purple chokeberries can be found in the Appalachian Region on the Walk Across Kentucky.  After some taxonomic verification this month we determined that our black chokeberries had been misidentified – so this species has been put back on the WAKY wishlist.  As of this writing, the purple chokeberry fruit is nearly ripe while the red chokeberries are 1-2 months away.  As always, our specimens can be located by searching the Arboretum Explorer Website.