Plant of the Month


Plant of the Month Archive

Our Wondrous Native Irises

To the garden enthusiast and even the novice, spring often conjures up thoughts of Iris blooms gently swaying in the warm breezes of May.  Many of the paintings by Monet, van Gogh and others repeatedly feature Iris as the symbol of spring.   As iconic as the flower is for the spring season, there is much to understand about the complexity of the flower, as well as the diversity of North American species that are readily available for your garden.  

Not surprisingly, Iris is found within the family Iridaceae and contains upwards of 300 species found around the world.  The name was first coined by the Swedish botanist Carl Linnaeus (1707-1778) in 1753.  The name comes from the Greek Ris, meaning rainbow and describes the glorious rainbow of floral colors showcased within this group of plants.  The ‘Type Species’, or species to which all the other species are compared is the Bearded Iris, botanically known as Iris x germanica.  Again it was named by Linnaeus and as the species epithet infers, it is found throughout central Europe and the Mediterranean region.  It is thought to be a naturally occurring cross between Iris pallida and Iris variegata, whose native stands overlap around present day Croatia.  The Iris genera has since been subdivided into 6 subgenera, based upon presence or absence of a ‘beard’ on the falls or the physical appearance of the bulb for those plants that do not produce rhizomes. 

The fascination that repeatedly lures gardeners and painters alike to an Iris lies in the complexity and beauty of the flower.  The flowers consist of 3 horizontal or slightly drooping ‘petals’, which are actually not petals but POMmodified leaves called sepals and 3 upwardly oriented true petals.  The sepals are called the falls, while the petals are the standards.  The sepals typically have a brightly colored central blotch that is called a signal with lines of varying colors running parallel with the length of the sepal.  Varying in size from species to species is an attractive, petal-like style arm that partially projects out over the sepals (see image at left).  The style is the female portion of a flower that connects the pollen receptive tip or stigma to the ovary.  In most flowering plants the style has no floral interest, but Iris is an exception!  The outer tip of the style arm is split into 2 upwardly arching lips, which contain the pollen receiving stigmas.  The stigmas are protected or covered by a stigmatic flap that is hinged at its lower or basal side.  Hidden beneath the style arm and closer to the center of the flower is the pollen releasing anther and at the base of the stem or filament that supports the anther are the nectar glands.  The entire flower design is structured to be inviting to pollinators, while preventing self-pollination and ensuring pollination from another flower.  The horizontally oriented sepals acting as a pollinator landing pad and the signal along with the colorful lines help to direct and move the pollinator under the style arm towards the nectary glands.  The nectar glands continue to lure the insect further in and under the anther, allowing pollen to be deposited on the back of the insect.  Upon backing out from under the style arm, the stigmatic flap is pushed shut, preventing the flower from self-pollination.  Upon landing on a new flower, the forward motion of the insect will push open the stigmatic flap as it moves under the style arm, transferring pollen from its back to the sticky surface of the stigma and pollination is complete.  An engineering marvel!

Most Iris need to bask in a full day of sunshine to flourish, but the Crested Iris, Iris cristata prefers shaded woodland conditions.  It was first discovered in Western Pennsylvania and initially named Pittsburgh Iris by the renowned botanist John Bartram (1699-1777), who sent samples to England around 1756.  However, it was not officially named and described until 1789 by the Scottish botanist and Director of Kew Gardens, William Aiton (1731-1793).  Crested Iris lie in the subgenus ‘Limniris’, which are characterized by an absence of hairs on the falls.  The species epithet refers to the 3 yellow or sometimes orange crests or ridges that appear in the central white signal.  The plants are relatively short, with the flowers ranging from 4-6” tall and 2-3” in diameter.  After flowering the foliage continues to grow to 6-12” long and effectively hides the seeds from predation.  The flowers appear in early May and range in color from light to dark blue, purple, white and veryPOM rarely pink.  One of my personal favorites is ‘Powder Blue Giant’ (image at right) whose large, yet soft blue flowers look great in the shade garden!  On average, foliage is a rich green throughout the summer, turning yellow come autumn before vanishing for the winter.  The plants create dense rhizomatous mats, but can spread rapidly by producing long stolons that terminate with the production of a new plant.  As a result, Crested Iris can rapidly spread into a broad groundcover.  Native to the central portion of Eastern United States, the plants are typically found growing in mountainous regions in rich woodland soil, in ravines or on bluffs at higher altitudes where it is cooler.  They are tolerant of calcareous or acidic soils and are hardy from zones 3-8. 

For sunnier locations, consider either Iris versicolor, Blue Flag Iris or Iris virginica, the Southern Blue Flag Iris.  Once again, both are found in the subgenus ‘Limniris’. They prefer water retentive soils and are even tolerant of standing water for periods of time.  The primary difference, as the common names infer is their native regions.  Iris versicolor is located in Eastern North America north of Virginia, while Iris virginica is Native throughout Eastern North America, stretching from Florida to Quebec!  Both plants were named and described by Linnaeus in 1753.  The species epithet of versicolor is from the Latin Versi meaning various and color for color, describing how the flowers range from various shades of blue to violet, rose and white.  A blue form is pictured below.  The epithet virginica refers not to the current state of Virginia, but to the Virginia Territory, POMwhich includes the current states of Virginia, West Virginia, Kentucky, Indiana and Illinois, along with portions of Ohio and Western Pennsylvania.  The flowers of both species are 3-4” in diameter and once again, the flowers are varying shades of blue, but violet and pink forms do appear.  Below right is a light blue form.  In both images, notice the two ‘lips’ on the style arm, the bright yellow signal and the dark purple lines directing the pollinator to the nectar glands.  From my observations in Central NJ, Iris versicolor is first to bloom, appearing in mid-May while its cousin starts to bloom a week or so later.  The common name of Flag originates from the Middle English word Flagge, meaning Rush or Reed and refers to the narrow green, rush-like leaves.  The foliage of Iris versicolor is slightly shorter, growing to 2 ½’ tall compared to 3’ of Iris virginica.  The biggest difference between the two actually lies in how the seeds are arranged in the seed pods.  Iris versicolor has a double row of seed inside each chamber orPOM carpel, while its cousin has but one.  Both increase gradually in size by rhizomes, although those of Iris versicolor are poisonous and gloves should be worn as a precautionary measure should one wish to divide the rhizomes.  Despite the potentially negative health risks, the rhizome was used as a laxative and for relieving fluid retention and bloating. Arguably, these three species may not be the plant that most people envision as they conjure up images of Iris blossoms in May.  However, they are fantastic garden plants and all have shown respectable resistance to deer browse.  The Blue Flag Irises, in combination with many of our native sedges have great potential for use in rain gardens and all three species certainly support and feed our native pollinator populations!  For dry shade or those seasonally wet location, consider enhancing your Garden with one or all of our North American natives.  Requiring minimal attention besides cutting back the foliage, the beauty and complexity of their flowers make them ‘Wondrous’ additions for your Garden!
Bruce Crawford
Director, Rutgers Gardens