Woody > Ilex > Ilex crenata > Ilex crenata 'Sky Pencil'
Ilex crenata
'Sky Pencil'
Sky Pencil Holly
Origin: In 1898, the Japanese Box-leaved Holly (Ilex crenata) was introduced to North America by the Arnold Arboretum. Beloved for its lustrous foliage and glossy black drupes, the holly has become a staple in North American landscapes. Almost 100 years after I. crenata’s original introduction, the holly would again capture American interest as a new form came to market. In the wilds of Mount Daisen on Honshu Island, Japan, a man named Norihiro Shibamichi found a female Box-leaved Holly with a uniquely upright and stiff branching habit. Shibamichi took cuttings of the wild holly which would find their way into the private collection of notable botanist Dr. Masato Yokoi of Kawaguchi City, Japan. The columnar holly would quietly remain in Dr. Yokoi’s garden until the arrival of a plant collection team from the United States National Arboretum (USNA) in 1985. During this trip, Dr. Yokoi generously provided tissues from his stock to two members of the collection team, Sylvester March of the USNA and Frederick Darke of Longwood Gardens. March and Darke brought the cuttings back to the United States, allowing the USNA to propagate more hollies through their Elite Plant Program. In 1992, the USNA brought the holly to market under the name Ilex crenata 'Sky Pencil'. It proved to be an incredibly popular introduction.
Mike's
Opinion
"
Described by the USNA as “...a living exclamation point in the landscape!”, Ilex crenata 'Sky Pencil' is an agreeable and elegant plant named for its fastigiate form. Gardeners with any sized landscape can enjoy this beautiful holly. Where the straight species would dominate a small landscape with its mounded, bushy habit, ‘Sky Pencil’ exists comfortably in the narrowest of growing spaces. The holly is excellent for making small spaces feel larger because its height and vertical branching habit draws the eye skyward. It is also excellent for framing doorways and pathways as specimen plantings or as hedges. The Sky Pencil Holly is perfect for beginner or busy gardeners because it tolerates varying climatic conditions, is rarely attacked by pests or disease agents, and it requires little pruning. With minimal education on mulching and winter protection, a gardener can enjoy their slow growing, delicate shrub for many years. Additionally, gardeners looking for alternatives to boxwood hedging should consider planting Sky Pencil Holly. The holly has dense, deep green foliage and a stiff branching pattern that provides a similar effect to boxwood. It is easier to maintain than boxwoods given its minimal pruning requirements and pest-resistance.
Michael Pascoe, NDP., ODH., CLT., MSc. (Plant Conservation)
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| Family |
| Aquifoliaceae |
| Genus |
| Ilex |
| Species |
| crenata |
| Cultivar |
| 'Sky Pencil' |
| Category |
| Woody |
| Type |
| Shrub (evergreen) |
| USDA Hardiness Zone |
| 5a - 7b |
| Canadian Hardiness Zone |
| 6a |
| RHS Hardiness Zone |
| H6 |
| Temperature (°C) |
| (-26) - (-23) |
| Temperature (°F) |
| (-20) - (-10) |
| Height |
| 2.5 - 4 m |
| Spread |
| 0.5 - 1.0 m |
Photographs
Description and Growing Information
Flowering Period
| General Description |
| Aptly named for its slender, skyward-facing stems and finely-toothed leaf margins, Ilex crenata ‘Sky Pencil’ is a valuable ornamental shrub in the Aquifoliaceae family. With glossy evergreen foliage and a stiff, vertical branching pattern, it is commonly used as hedging, screening, and as a vertical accent or for foundational planting. As one of the most compact and fastigiate evergreens available, it is suitable for lining narrow walkways, small gardens and patios. The holly’s excellent tolerance of varying soil textures, light exposure, and pollution make this a popular urban shrub. It requires minimal pruning and care to maintain its columnar and tight form. |
| Landscape |
| As a beloved cultivar of I. crenata, the Sky Pencil Holly is valued for the texture and structure its form, foliage, and fruit bring to the landscape. Combined with dense foliage, its pencil-thin stems and vertical branching pattern draw the eye skyward, creating a pleasant vertical accent. Its columnar and compact form makes it effective as a specimen, foundational planting, hedging, or a border planting for walkways. Furthermore, its glossy evergreen leaves provide year-round greenery and consistency to the landscape. The holly is well-suited for structured and formal garden styles, such as those seen in English, Japanese, and Contemporary gardens. |
| Cultivation |
| Needing little maintenance and space, with tolerance of many growing conditions, the Sky Pencil Holly is an agreeable plant for an urban environment. A versatile shrub, the holly can be planted in open ground or in a container. Easily cultured and transplanted, the holly is happiest when given partial shade to full sun, and is rooted in light, well-drained, moist, and slightly acidic soils. The holly is tolerant of a degree of moisture levels and thrives with wet feet. Its moisture tolerance makes it suitable for clay, loamy, or sandy soil textures. However, it will experience chlorosis if the soil pH is too alkaline. While the holly rarely splays apart, some light springtime trimming may be necessary to remove dead and diseased branches. Gardeners may also remove some new shoots that diverge from the shrub’s form, but regular pruning is not advised. Since the shrub is slow growing, it does not possess the vigour necessary for constant pruning. In fact, pruning should be limited to a single bout of light clipping in springtime to prevent stress and disease. Additionally, gardeners should take care to protect their Sky Pencil Holly from drought conditions, especially in cold temperatures and high winds. Despite its mountainous origins, both observations of Fanshawe College’s specimen and research suggest this cultivar has some hardiness issues when left exposed to winter winds. To prevent winterburn and dieback, ‘Sky Pencil’ requires wind protection and sufficient mulching to ensure adequate moisture levels. |
| Shape |
| Narrowly columnar and compact, its stiffly erect branches are abundantly laden with evergreen foliage. Its density and upright habit give the holly a formal appearance. While little pruning is required for the plant to maintain its shape, some patience is required in order to experience the shrub’s full effect. This holly is slow growing, and according to the Royal Horticultural Society, the holly reaches maturity in 10 to 20 years. |
| Growth |
| Slow |
| ID Characteristic |
| Colloquially known as the inu tsuge or false boxwood in Japan, many gardeners confuse Ilex crenata with plants in the Buxus genus. Boxwoods and Japanese Hollies share a similar horticultural niche due to their dense shape and ovate, emerald-coloured evergreen foliage. However, close examination of leaf arrangement and marginal pattern differentiate the plants. As opposed to the oppositely arranged, smooth margined boxwood leaves, Japanese Hollies have alternately arranged leaves with crenate margins. Additionally, Japanese Hollies produce inconspicuous springtime white blooms and autumnal black drupes, unlike the fruit and flowers of boxwoods. The Ilex crenata ‘Sky Pencil’ cultivar shares all the characteristics of its parent species, differing only in its narrowly upright habit. |
| Pests |
| Besides preventing foliar chlorosis in alkaline soils or winterburn, gardeners should occasionally inspect their hollies for insect damage. True to the straight species, the Sky Pencil Holly is susceptible to holly leaf miner, scale, spider mites, and nematodes. Young stems and leaves may experience aphid damage. These pests generally cause cosmetic, non-lifethreatening foliar issues in stressed plants. The holly may also suffer from diseases such as holly leaf blight, Phytophthora root rot, and honey fungus. These diseases are more serious than insect infestations; diseases can cause defoliation, twig dieback, and death. However, disease incidence is rare and is directly related to poor conditions and improper care. When gardeners follow proper pruning practices, provide cold and wind protection, and ensure soil is adequately acidic and moist, the Sky Pencil Holly is relatively pest and disease free. |
| Habitat |
| Although it is a cultivar, Ilex crenata ‘Sky Pencil’ was not developed in the cultured landscape. Recall that tissue samples were wild collected on Mount Daisen in Japan. Like the straight species, this cultivar calls temperate mountainous regions home, and is adaptable to inconsistent moisture, sun exposure, and soil textures. |
| Bark/Stem Description |
| Although many species in the Ilex genus have spiked stems, Japanese Hollies and the ‘Sky Pencil’ cultivar lack prickly armature. Instead, the delicate pencil-thin stems and branches are somewhat pubescent in youth and become smooth throughout the plant’s lifetime. Young stems are a pale green and become grey-brown after two to three years. Bark near the plant’s base is dark grey and smooth. By the author’s observations, stems are under 4 cm in diameter. |
| Flower/Leaf Bud Description |
| The holly’s tiny buds possess three or more overlapping scales. They are alternately arranged along the full length of the holly’s stems. |
| Leaf Description |
| Alternately arranged along stems, the simple evergreen leaves emerge singly at leaf nodes. A glossy dark green above, and a lighter green below, the foliage persists through all seasons. The holly is classified as a broadleaf evergreen, which may be surprising given leaf blade size ranges from 15 - 50 mm in length and 5 - 15 mm in width. Leaf blades are elliptic in shape, and are slightly convex at the midrib. Leaf margins are crenate to serrulate, a distinguishing feature from its landscape lookalike, the boxwood. Short-stalked, petioles are under 2 mm in length. |
| Flower Description |
| Appearing in between May and June, the Sky Pencil Holly produces clusters of inflorescences at the nodes of the current season's growth. Japanese Hollies are dioecious, with pistillate cymes and staminate solitary flowers held on female and male plants, respectively. Each inflorescence is no more than 0.5 cm in length and has 4 to 5 petals. Due to their size and dull, greenish-cream colouring, the flowers are considered ornamentally insignificant. However, true to the Ilex genus, this cultivar’s flowers are beloved by bees, particularly the specialized Colletes banksi. Japanese Hollies have a reciprocal relationship with bees as they are not self-fertile and require bee-mediated pollination. |
| Fruit Description |
| As a fully female selection, all Sky Pencil Holly plants have the capacity to produce fruit if pollinated by a nearby male holly of its species. Several small pyrenees are encased in the fleshy, globose, and shiny purplish-black drupes. The berry-like fruits are 0.5 cm in diameter and reach maturity within one season. They emerge in the autumn and often persist through the winter into early spring. While wildlife enjoy the fruit, it is not fit for human or domesticated animal consumption. The drupes are minorly toxic due to their saponin content; consumption can lead to vomiting, diarrhea, and other illnesses. |
| Colour Description |
| An evergreen shrub, the Sky Pencil Holly is a medium to dark green year round. Its stems are a greenish-grey depending on their age. While it possesses spring greenish-cream inflorescences and autumnal purplish-black drupes, both flowers and fruits are small, and therefore do not contribute to the shrub’s overall colour. |
| Texture Description |
| Like the straight species, Ilex crenata ‘Sky Pencil’ is a finely textured shrub through all seasons. It is an excellent detail plant with its small, shiny leaves, pencil-thin stems, and uniformly vertical branches. |
| Notable Specimens |
| Currently, a specimen is accessioned under the number 20290077 in Dwarf Conifer Collection at the Botanic Gardens of Fanshawe College in London, Ontario, Canada. Additionally, many gardens in the United States possess specimens, such as the Chicago Botanic Garden in Chicago, Illinois, the Seattle Japanese Tea Garden in Seattle, Washington, and the U.S. National Arboretum in Washington D.C., Maryland. |
| Propagation |
| The holly is best propagated via semi-hardwood cuttings, starting mid to late summer. Using pruning equipment that has been alcohol-sterilized, new growth is cut at a node. Leaves adjacent to the cut are removed, while those farther away remain intact. The USNA recommends applying “1000-3000 ppm IBA quick dip” rooting hormone to cuts before placing stems cut-side down in a growing media, such as perlite. Over 4 - 6 weeks, cuttings will root if frequently misted. Seed propagation is also possible, but not advised as it is incredibly slow to grow and it is tedious to remove the flesh from the drupe to get at seeds. Research into micropropagation techniques are currently under investigation for their potential to speed up the propagation process. |
| Ethnobotanical Uses (Disclaimer) |
| Ilex crenata and its cultivars are commonly planted as ornamentals in English and Japanese gardens. Traditionally, these gardens use the hollies as hedges, screens, or topiary, depending on their shrubs’ habits. However, Ilex crenata does not seem to possess cultural value nor is it used in cuisine. This is surprising given Japanese Hollies are ubiquitous in wetlands and mountainous regions across Asia. Perhaps their insignificance is due to its inedible foliage and berries or their slow growth. Additionally, there is a stark gap in research on its medicinal properties. For example, a scoping review by Yi et al. (2016) compiled substantial literature supporting the ethnopharmacological benefits of other Ilex species. Reviewer found a single 1987 study which investigated the anti-allergic potential of I. crenata’s ilexoside content. This evidence is weak at best, and confirming the shrub’s medicinal properties requires more attention, given its mild toxicity. |
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