Yellow Iris
Iris pseudacorus
Iris family (Iridaceae)

Description: This perennial wildflower is about 2–3' tall, consisting of large erect basal leaves and a flowering stalk. Usually, the larger basal leaves are a little taller than the flowering stalk. The stiff basal leaves are pale blue-green to green, sword-shaped, glabrous, and glaucous; they have parallel venation and smooth margins. The basal leaves are joined together at the base, from which the flowering stalk emerges. The erect flowering stalk is stout, glabrous, and either unbranched or sparingly branched; it may have a few alternate leaves along its length that are small and sword-shaped.

Each branch of the stalk terminates in a tight cluster of 2-4 flowers and their spathes; the latter resemble small sword-shaped leaves. The showy flowers are 3-4" across. Each flower consists of 3 small erect petals, 3 large petal-like sepals, 3 flattened petal-like styles (directly above the sepals), 3 anthers, and a developing ovary. The petals, sepals, and styles are yellow. The oblanceolate-obovate sepals are widely spreading and recurved; each petaloid sepal has a patch of golden yellow at its base with fine maroon lines. The blooming period occurs from late spring to mid-summer and lasts about a month. Each flower develops an oblongoid-angular seed capsule about 3" long. Each capsule contains several rows of seeds. The root system is fibrous and rhizomatous; vegetative offsets are often formed from the thick rhizomes.

Cultivation: The preference is full or partial sun, wet to moist conditions, and rich soil containing organic matter or muck. This robust iris is easy to grow if it receives enough moisture. It is able to survive occasional dry spells.

Range & Habitat: The introduced Yellow Iris is still uncommon as a naturalized plant in Illinois, but it is probably becoming more common. It is more common in the heavily populated NE section of the state than elsewhere (see Distribution Map). Habitats include low areas along rivers, borders of ponds, marshes, swamps, calcareous seeps, and ditches. Both degraded and higher quality wetlands can be invaded. Yellow Iris was introduced from Europe as an ornamental plant. It is still cultivated in gardens, from where it occasionally escapes.

Faunal Associations: The flowers are cross-pollinated primarily by bumblebees, which seek nectar from the flowers. The caterpillars of Macronoctua onusta (Iris Borer Moth) bores into the rhizomes of Iris spp. Another insect, Iridothrips iridis (Iris Thrips), feeds on young leaves and the inner surface of leaf sheaths. The foliage is toxic and avoided by most mammalian herbivores, although muskrats sometimes eat the rhizomes and lower stems.

Photographic Location: A ditch in Champaign, Illinois.

Comments: Among the introduced Iris spp., Yellow Iris is the only one that can be considered invasive (or at least potentially invasive). Sometimes the commonly cultivated Iris germanica (Bearded Iris) will escape, but it doesn't appear to be nearly as aggressive. Another introduced species, Iris flavescens (Bearded Yellow Iris), also has yellow flowers, but it rarely escapes. However, like the Bearded Iris, the Bearded Yellow Iris has patches of hair on its sepals. In contrast, Yellow Iris never has hair on its sepals. In this regard, it is similar to the native Iris virginica shrevei (Blue Flag Iris), although the latter has blue-violet flowers. Yellow Iris differs from the preceding Iris spp. by the small size of its erect petals – they are even smaller than the petaloid styles and rather inconspicuous.