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Nothing Substandard about this Subshrub

by Bruce Crawford, Manager of Horticulture, Morris County Park Commission

When the topic of groundcovers is breeched, there are certain plants that inexorably come to mind. These are the plants that were used time and time again until they rose – or should I say sank – to the point of monotony, resulting in a search by gardeners for more interesting options. To my dismay, one of the options seems to have become shredded wood mulch, ultimately not a very sustainable option. Nature prefers to have plants covering the ground, which is why in due time 'weeds' start to grow through the mulch. Ground covering plants often benefit taller shrubs and trees, as they aid in preserving soil moisture and enhance the subsurface mycorrhizal web. Historically, one of the most overly abused groundcovers was Pachysandra, yet within this genus there is an option many gardeners have overlooked!

Zoom in Pachysandra procumbens.
Picture 1: A mature mass of Pachysandra procumbens in August.

Pachysandra is a member of the Buxaceae or Boxwood Family, with 4 species native to Eastern Asia and one species found in Southeastern North America. Surprisingly, it is actually a subshrub or a dwarf woody shrub and not an herbaceous plant, as it is characterized by persistent ground-hugging rhizomes and short upright woody stems. The name was first coined by the French botanist and explorer, André Michaux (1746–1802), who traveled throughout Eastern North America from 1785–1796 in search of plants for King Louis XVI. Pachysandra was published posthumously in 1803 in his manuscript Flora Boreali-Americana (Flora of North America). The name is derived from the Greek Pakys for thick and Anēr or Andrus for man, referring to the prominent and very thick stamens displayed by the flowers. A stamen is the 'stem' that supports a pollen releasing anther.

The plant Michaux discovered in the early 1790's and subsequently named and described is Pachysandra procumbens or Alleghany Pachysandra (Picture 1). The species epithet is from the Latin Procumbo, meaning to lie down, a reference to either the response of the stems to the cold of winter or the woody rhizome (a horizontal stem) that creeps along the ground. As the common name suggests, the plant is primarily found in the Alleghany Mountains, with its native footprint spreading from North Carolina south to Florida and west to Louisiana and Kentucky. It is interesting how its Japanese counterpart, Pachysandra terminalis became far more popular, even though it was not named until 42 years later in 1845. In fact, short of botanic gardens and rare plant nurseries, Alleghany Pachysandra remains scarce in commerce. Undoubtedly, its slow ability to spread is partially to blame, since calling Pachysandra procumbens a restrained groundcover is certainly an understatement. After 36 years of undisturbed growth at the Arnold Arboretum, two plants have slowly expanded with one reaching 3'x7' and the other a mere 3'x4.5'.

Zoom in Pachysandra in August.
Picture 2: Central silver leaf veins in August.
Zoom in Pachysandra in November.
Picture 3: The silver mottling of the foliage come fall.

The slow growth rate of the rhizome or horizontal stem explains its leisurely ability to spread. Growing near the soil surface, the rhizome essentially serves as the trunk, producing the individual unbranched woody stems that grow to 6–8" (10") tall. The foliage emerges from the upper half of the stem, with each stem only persisting for 1½ to possibly 2 years. The stems and accompanying foliage typically collapse to the ground during their first winter and proceed to gradually die back during their second summer once the new stems and foliage have become established. However, they can be killed back to the rhizome if the temperatures dip well below 0 degrees Fahrenheit. The foliage varies from somewhat oval to rounded, with the outer or apical regions strongly toothed, appearing much like the hair of Calvin in the comic strip 'Calvin and Hobbs'! Each leaf is 2–3½" long and nearly as wide. The leaves are arranged alternately along the stems, although they often appear to be whorled as the internodes are tightly compressed. As the foliage unfurls in May (in Picture 7 at the article's end), it is an attractive bright green, transitioning to a deep green or gray green come summer. The deep green of the foliage nicely highlights the light green of the venation throughout the summer months (Picture 2). As autumn approaches, one might say the leaves develop 'age spots', as they become decorated with dusky white spots, upwards of ¼" in diameter that appear around the veins (Picture 3). In some plants, the cooler weather also yields an attractive reddish glow around the margins. Hardy in zones 5–9, the foliage remains evergreen in zones 6–9 and becomes increasing tattered with colder temperatures if snow is not present. Although many authorities find the foliage less attractive than its cousin, I find the seasonal changes both attractive and entertaining! Come winter, the stems arch over and become procumbent, often persuaded by a coating of snow while the foliage assumes tints of deep purple.

Zoom in Pachysandra in March.
Picture 4: Flower buds as they appear in early March.

The flower spikes begin to appear at the base of the arching stem in late March into April. The flower buds are actually formed the previous fall and remain naked or unprotected by bud scales throughout the winter (Picture 4). If the stem dies back due to an unusually cold winter, the flower buds will be lost as well. Fortunately, that rarely happens in NJ and with the previous year's stems and foliage now procumbent, the unobstructed inflorescences appear prominently in the center of the plant (Picture 5). The flower spikes, technically called a raceme, grow upwards of 4" tall and, to my eye, are very attractive. The flowers are monecious, whereby the staminate (male) and pistillate (female) flowers appear separately. The inflorescence consists of a light pink central stem, with 20–30 showy staminate flowers arranged radially around the majority of the upper portion of the floral spike. The 3–5 pistillate flowers are a more demure light red in color and are located near the base. In Picture 5, the arrow points to a pistillate flower. Each staminate flower has 4–7 conspicuous and thickened (Pakys) white filaments, nearly ½" long, with a small brown anther at the tip. In some cases, the filaments have a pink tinge and the central stem is a very attractive red. They are also nicely fragrant! The pistillate flowers are roughly ¼" in diameter and feature 3–4 recurved red bracts. Interestingly, both types of flowers appear without petals (termed apetalous), relying on the sweet fragrance and the showy stamens to attract the various native bees patrolling the woodland floor in the early spring. The plants are self-incompatible, preventing self-pollination and it is rare for the plants to produce seed.

Zoom in Pachysandra.
Picture 5: The flowers in April.

In the wild, plants typically grow on rocky hillsides in forested areas or on banks adjacent to streams. Plants prefer limestone soils with a pH near 7, although they grow perfectly well in more acidic soils. Plants require shade, especially from afternoon sun and will perform well in deep shade. Plants do not require any trimming or cutting back, as the previous year's foliage is quickly hidden by the expanding foliage of the current season. I have noticed that some people insist on 'tidying up' the plant by removing the older foliage (as seen in Picture 5). An unnecessary chore, since the older foliage will serve as a mulch, helping to reduce potential weed competition, retain moisture and protect the rhizome!

Of course, the species familiar to most gardeners is the Japanese Pachysandra, or Pachysandra terminalis. The plant was collected by the German botanist, physician and explorer Philipp Franz Balthasar von Siebold (1796–1866). From 1823 through 1829 Siebold lived and explored parts of Japan, sharing both the insights of western medicine and collecting several thousand specimens of plants and animals. Once settled back in the Netherlands, he worked in collaboration with the German botanist, Joseph Gerhard Zuccarini (1797–1848) on the writing of Flora Japonica, in which they named and described the various plant specimens in his collection. The first edition was published in 1835, but it was not completed until 1870 by the Dutch Botanist Friedrich Anton Wilhelm Miquel (1811–1871). In 1845 Siebold and Zuccarini described Pachysandra terminalis. Unlike its North American cousin, this species features flowers that appear at the terminus (Picture 6) of the shoots rather than the base. Most likely this was the inspiration for the species epithet, although it could also be the rosette of foliage at the tip of the stems. Native to Japan, parts of Korea and East Central China, the foliage is very glossy and alternately arranged on the stem, although it once again appears whirled due to the congested arrangement at the tip of the stem. The leaves are oval or obovate in shape, 1–3" long by ½–1" wide with the apical edges of the leaves once again toothed or dentate.

Zoom in Pachysandra.
Picture 6: The terminal flowers of Pachysandra terminalis.

Japanese Pachysandra spreads by underground stolons or stems, which are similar to rhizomes but located close to or at the soil surface. The individual upright stems grow either from these stolons or sprout from existing stems that have been flattened under snow load or other physical pressures. The plants grow to 8–12" tall and although the new stems only grow 2–4" each year, the stolons will quickly spread to cover vast areas. The flowers are 1–2" long, with 15–25 staminate flowers towards the tip of the inflorescence and 1–2 pistillate flowers near the base (Picture 6). Once again, the staminate flowers have four very pronounced, thick white filaments and a brown anther at the tip. At the base of the filaments are 4 light green tepals. Although they are not as showy as their North American counterpart, they are still of interest and bear a sweet fragrance. The flowers are self-infertile and it is rare for pollination to occur since most of the plants offered at nurseries are undoubtedly propagated from the same genetic strain. If pollination should occur, a small white fruit will result.

Japanese Pachysandra is a good, evergreen groundcover for shade and grows best in zones 4–7. In full sun locations, the foliage will often become yellow due to the solar stress and if combined with droughty soils, may lead to plant loss. Unfortunately, it has been overused to the point of maniacal boredom. It is also susceptible to the fungus Volutella pachysandrae, which creates a canker leading to stem dieback and ultimately an untimely death. Interestingly as well as fortuitously, the fungus does not seem to impact Alleghany Pachysandra, which aside from possible slug issues, appears to have few problems.

Durable and long-lasting ground covers will become an increasingly important tool for homeowners in abating climate change. Their impact may appear minor, but collectively they are incredibly beneficial. They help reduce the use of mulches, soil erosion, soil compaction and actually aid in carbon sequestration when used and grown properly. Additionally, some will also support our native woodland pollinators, whose life depends on gathering nectar and pollen from March through June! Although Pachysandra terminalis is starting to appear in natural areas and on invasive plant lists, Pachysandra procumbens by contrast remains painfully underused. Although a subshrub, there is nothing inferior or substandard about this plant that contributes so much for the garden and potentially for the environment.

Picture 7: Unfurling foliage in May.