Know Your Natives – Water-willow

Water-willow (Justicia americana) of the Acanthus (Acanthaceae) family is an herbaceous aquatic perennial with willow-like leaves. The genus name recognizes James Justice, an 18th-century Scottish author of horticultural books. The specific epithet denotes the plant’s area of primary occurrence. In the U.S., water-willow occurs primarily from eastern Oklahoma and Kansas in a broad sweep that extends to the Atlantic Coast, as well as extensive populations in Alabama and central Texas. In Arkansas, it occurs throughout a large portion of the state except for Crowley’s Ridge and portions of the Mississippi Alluvial Plain and the West Gulf Coastal Plain. It occurs throughout the Interior Highlands portion of the state. Habitats are sunny areas in wetlands, especially those with sandy to gravelly soils in the shallow portions of perennial streams and the muddy margins of streams, ponds and lakes.

Photo 1: Shallow areas of this rocky perennial stream, with flow from left to right, provide a favorable habitat for water-willow. Stems of colonies, all of equal height, become densely packed. Photo – August 3.

Plants have long ropy roots, short fibrous roots, and slender, occasionally branched runners (stolons). The white descending ropy roots provide nutrients and stability to the plant in its aquatic habitat. The oxygen-absorbing fibrous roots are in dense tangles at the stem’s base, below and above the soil surface, as well as at submerged leaf nodes. The creeping runners, at and just below the soil surface, produce new stems at their tips. Runners have short, linear-lanceolate, alternate bracts which may subtend a secondary runner. In a favorable site, large dense colonies develop with all stems tending to be of equal height. Plants are adapted to varying water levels up to 3 feet deep, with varying water flow rates.

Photo 2: This stem, which grew from the tip of the older brown runner at right, has long ropy roots and short fibrous roots. A new runner, growing at left, would have produced several new stems. Stem shown in this photo is from a site similar to that in Photo 1.

The light green stout stems, to 3 feet tall with a base to ⅜ inch wide, have a gradual taper to the apex, which terminates with a pair of small leaves. Stems are hexagonal with rounded corners in the lower portion, the corners becoming ribs as the stem narrows above. The erect stems are typically not branched, although separate stems may grow from a stem’s base and a few short leafy axillary branches may be present. The linear to lanceolate, glabrous leaves are medium-green above and yellowish medium-green below, from lighter green swollen nodes. They are produced in opposite pairs arranged cyclically about the stem, the largest occurring about mid-stem where they may be up to 4¾ inches long and ½ inch wide. Blade margins are slightly wavy and entire (uncut); blade surfaces may be undulating, especially along the margins. Leaves, with their gradually tapering bases, are sessile to short-petiolate (subsessile) and are up-folded along the midrib. Stems, with a white corky interior and a small hollow center, are buoyant.

Photo 3: Plants are adapted to perennial streams where flow may be nil to deep and swift. These plants are growing in a shallow-water portion of a perennial stream as compared to those in deeper water as shown in Photo 1.

The rather thick leaves have pinnate venation, with the primary (midrib) and secondary veins being expressed above and below. Venation of younger leaves is a lighter green than the blade, while veins and blades of older leaves are the same color. Secondary veins arc from the straight midrib to where they closely parallel the margin. Tertiary veins are obscure.

Photo 4: The sessile to sub-sessile willow-like leaves have upper and lower veins which are both expressed. Separated leaves show the upper surface (left) and lower surface (right).

The inflorescence, present primarily in June to July, consists of a stubby (to ¾ inch long) tight cluster of six to eight flowers at the tip of slender, erect, light green peduncles. The glabrous, ribbed peduncles, to 3+ inches long, are axillary from several uppermost leaf pairs. Elongate light green flower buds have several tiny triangular bracts appressed at their base. Lower flowers of a cluster bloom first with several closely spaced flowers often in bloom at the same time, so that multiple flowers may even appear to be a single flower. Flower buds are ascending and flowers in bloom face the sky. The loose sepals surrounding the buds are pushed open at anthesis, but return to an upright position after the flower has bloomed.

Photo 5: Long axillary peduncles bear a tight cluster of flowers. As shown by the cluster at left, lower flowers bloom first. Color contrasts of leaves, stems and nodes diminishes later in the growing season. Photo – June 5.

Flowers, although relatively small, are rather showy on close examination. The calyx is 5-lobed. The corolla is bilabiate (2-lipped) with a broad centrally-notched upper lobe and a lower lip of 3 lobes, 2 broadly elongate out-flared laterals and a distally broadening lower lobe. Corollas are about ¾ inch wide and ½ inch long. Mostly white, they have light lavender shading across the upper lobe and small to large splotches of dark lavender on both upper and lower lobes, with the splotches increasing into the throat (bee guides).

Flowers have two stamens and a single pistil (ovary, style and stigma). The white stamens, well-spaced to left and right of the style, become flattened distally, with noticeably asymmetrical anther sacs, one just below the other. Anthers open to expose dark purple oblong disks that release white pollen. The slender white style is reflexed tightly against the upper lobe and extends above the lobe to a minutely 2-lobed stigma.

Photo 6: These four flowers radiated from their rachis at the top of the peduncle so that their lavender upper lobes are positioned together. Several styles (with pointed stigmas) and stamens (with asymmetrically paired anthers) can be seen. Photo – June 5.

Ovaries of fertilized flowers develop into 2-valved, club-shaped capsules with acutely pointed apexes. The hardening capsules are rounded in cross-section with two flattened sides and two “lumps” (representing growing ovules) extending around the capsules. The flattened sides have a longitudinal seam where dry capsules split. The glabrous capsules, with persistent sepals, are initially a light green, but become a greenish brown with maturity and finally tan. The dry ½-inch long and ¼-inch wide capsules dehisce with explosive force so that the one to four round, flat seeds are forcibly ejected. Seeds, to ⅛ inch diameter, have notched bases and papillose surfaces. After seed dispersal, the dried peduncles and now-spiky cluster of empty capsules quickly drop off.

Photo 7: The hardened fruit is a pointed club-shaped capsule with green leaf-like sepals at the base. The lumps along the capsule mark two sets of developing seeds. (The two peduncles shown here are not attached to each other.) Photo – July 24.
Photo 8: The fruit cluster, peduncles and sepals dry simultaneously. When the hardened capsules dry, the valves spring back and up to four wafer-like seeds are forcibly ejected. Photo – August 4.

In its natural aquatic environment, water-willow is an important species, not just for insects, also providing shelter to snakes, amphibians and spawning fish. Water-willow may also provide some erosion control and sediment filtering. With the plant’s tendency to colonize, most home gardens would probably not have an appropriate site for water-willow, other than a container set in a sunny site, which might attract mosquitoes. Although the bloom period is fairly short, flowers are interesting and orchid-like.

In addition to water-willow, lance-leaf water-willow (J. ovata var. lanceolata) also occurs in Arkansas (southwestern two-thirds of the state). Lance-leaf water-willow, with a somewhat similar growth habit to water-willow (although often a bit shorter in stature), can be distinguished by its habitat (swamps and marshes, often shaded), inflorescence structure (loosely flowered spikes), and shorter but broader leaves. The corollas, although also having two lips, are smaller, with the lower lip divided into 3 equal-sized, down-trending and non-flared lobes. The color pattern of lance-leaf water-willow corollas is usually simpler, more uniformly white to pale lavender with a few darker lavender splotches in the throat of the lower lip.

Article and photographs by ANPS member Sid Vogelpohl

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