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Arum italicum subspecies italicum: One Hot Flower!

by Bruce Crawford, Program Leader in Home and Public Horticulture

With the chilly temperatures of winter well entrenched in our Gardens, evergreen foliage clearly plays an important role in providing interest throughout this season. Conifers serve an important role in providing the bones for the garden, but there is still a need for detail plantings as we stroll our walkways and paths. One plant I have greatly enjoyed over the past 30 years for winter foliage and form, as well as for its 'hot' flowers come spring, is the Italian Arum, Arum italicum subspecies italicum.

Italian Arum is a member of its own family, the Araceae, and is native to Southern Europe, Northern Africa and the Middle East. The genus name was penned in 1753 by the Swedish Botanist Carl Linnaeus (1707–1778) from the Greek Aron, the Greek term for this family of poisonous plants. Plants within this genus contain calcium oxalates, which when ingested result in the swelling of the throat and tongue, eventually causing difficulty in breathing and even death for children. The species epithet was crafted by the English botanist Philip Miller (1691–1771), meaning 'of Italy,' giving homage to one of its native regions. Arum italicum subspecies italicum is the most commonly cultivated form as the foliage has very distinctive and attractive white venation, as pictured at right in early December.

Typical to the genus, Italian Arum grows from a corm, with the sagittate or arrow-shaped foliage beginning to emerge in late September or early October and typically remains attractive until the following June. The photo to the left was taken on February 10th. During severe winters, the foliage can scorch if located in an exposed location, but fret not, as it will be refreshed with new foliage in the spring. The straight species features glossy deep green foliage, or foliage with white splotches, while the subspecies italicum is laced with the more attractive white venation. The leaves usually grow to 12" tall by 6-8" wide, but can grow larger if conditions are ideal. The foliage certainly provides great winter interest, but when spring arrives, it is the flower that makes the plant all the more fascinating!

The flowers appear in May. They consist of a central club-like spadix, appearing in front of, and slightly surrounded by, a creamy white bract or modified leaf called a spathe. The spathe grows 8–10" tall while the spadix reaches a slightly shorter 4–5" height. The urn-shaped base of the spathe wraps around the lower portion of the spadix and is called the spathe tube. This spathe tube encloses the fertile flowers which appear in whirls, encircling the base of the spadix. The fertile pistillate, or female flowers, are located at the very base of the spadix, with the pollen releasing staminate flowers perched above. They are separated by a ring of filamentous sterile female flowers called pistiloide florets, as seen in the image below. Atop the staminate flowers, and located near the narrow neck of the urn-like spathe tube, are the infertile staminode flowers that consist of slightly downward-oriented filaments that stretch outwards and nearly touch the neck of the spathe tube. Arum flowers are termed protogynous, whereby the female flowers mature first and are no longer receptive to pollen when the staminate flowers mature and begin to release pollen.

This staged maturation process prevents the flowers from self-pollinating, resulting in inbreeding depression. To further aid in pollination, the male flowers are thermogenic, whereby they go through periods of releasing heat and can become upwards of 50 degrees warmer than the ambient air. The flowers evolved to do this because they are pollinated by flies and the heating of the flowers releases volatile chemicals that smell like rotting meat or even stale urine! Not to worry though, the odors are too faint to be noticed by the passing gardener. Attracted by the odor, small flies are able to push past the filaments of the staminode flowers in search of the source of the odor but, due to the slightly downward orientation of the filaments, the flies are not able to escape. The filaments remain resilient against any attempts of escape for around 24 hours, allowing the incarcerated flies to hopefully deposit pollen from a previously visited flower onto the stigmas of the female flowers. After one day passes, the stigmas become unreceptive to pollen while the staminate flowers begin to shed pollen. At this stage, the staminode filaments become limp, allowing the pollinators to move upward and out of the spathe tube to the next flower. As the flies move upward, they collect new pollen and the process is repeated.

Shortly after the flowers fade, the foliage withers, leaving the fruit to develop. The peduncle or stem of the flower proceeds to elongate to 6–12" tall and is topped by an oblong cluster of developing fruit that is covered by a papery tunic. As the tunic dries, it splits open revealing the attractive cluster of red fruits that remain ornamental well into August (pictured right at the Cornell Botanic Gardens). In the southeastern and northwestern parts of North America, along with Australia, New Zealand and the UK, the dispersal of seeds by birds and even ants have caused this plant to be listed as an invasive plant. Invasive plants outcompete native plants, disrupting the natural insect-plant community. I have always kept a watchful eye on this plant in garden settings and although I have seen seedlings appear around the mother plant, it has not proven to be a worrisome self-seeder in New Jersey. Of course, planting one plant or removing the fruit before it matures will negate any worries about invasiveness and allow the gardener to simply enjoy its winter and early spring beauty

Hardy from zones 5–8, the plants prefer to be located in a humus-rich, shaded location where the soil does not become excessively dry throughout the growing season. They can even be located in fairly wet soils, as I witnessed at the Cornell Botanic Gardens, where they are planted adjacent to a stream. The corms readily produce offsets during the late fall, allowing an individual plant to gradually increase into a substantial clump with time. It is also interesting to note the corm produces two types of roots: vertically oriented contractile roots during the early fall that help to pull the corms ever deeper into the soil, followed by a mat of horizontal roots whose purpose is to absorb nutrients and water.

Available as a container plant or as corms from some bulb companies, Italian Arum is not an easy plant to find. Most nurseries do not like to stock container plants since there is typically nothing to see after June, which is certainly a detriment to sales. Fortunately, there are a number of specialty nurseries that carry this plant, along with forms showing ever more incredible marbling to the foliage. One combination that I found very attractive was at Chanticleer Gardens where the Arum was combined with the lower growing, yet similarly shaped foliage of Asarum splendens, the Chinese Ginger, as seen below. The fern nicely compliments the composition!

The Italian Arum is an amazing plant for the garden, providing nearly year-round interest with a flower that literally becomes warm to the touch during pollination. Truly one hot plant that needs to grace more mid-winter gardens!