I have been fascinated with irises for the water garden ever since I was a young boy exploring the Mississippi River in northern Minnesota. The first water-inhabiting iris I ever found was a muddy blue color, a wild-growing cultivar of Iris versicolor whose beauty and appeal only I seemed to appreciate. Over the years I have found more irises that tolerate or require either wet or water-saturated soil. Growers and hybridizers are developing many new, exciting cultivars with larger flowers, greater color, and enhanced hardiness. There is no need nowadays to look along the shores of a local pond, stream, or river to find excellent, sturdy, free-flowering irises for the water garden. Resilient and remarkably pest-free, water-side irises reward the gardener with years of early-summer flowers.
The true water irises, which grow best with water over their crown throughout the year, even in fall and winter, include Iris fulva (red flag or copper iris), I. laevigata (rabbit-ear iris), I. prismatica (cube-seed iris), I. pseudacorus (yellow flag), I. versicolor (blue flag; pictured), I. virginica (southern blue flag), and the Louisiana irises. (Another group, comprising Iris ensata, the Siberian irises, I. missouriensis, and I. setosa, grows best with wet soils for some of the growing season, but in most climates prefers drier conditions for the remainder of the year; they will not be discussed here.)
All the irises discussed here are central landscape plants for the water garden. They bloom in the spring or early summer, depending upon cultivar and climate. Colors range from the whitest whites to the deepest blues, with purple and lavender hues, and reds, yellows, browns, and greens—essentially every color of the rainbow. Certain cultivars even rebloom in the fall, if they are grown in the climates that they prefer.
The substantial root systems of many water garden Iris species often make them excellent for holding back soil erosion along the banks of natural ponds and streams. Some species tolerate seasonal drought as well as seasonal flooding, thus earning them a place in detention or retention areas in the larger, corporate landscape. Even a ditch or a wet spot in the homeowner's backyard is perfectly suited to many moisture-loving Iris species.
Water garden irises are amiable sorts that blend well with many other pondside plants, especially those that grow closer to the water surface and have delicate foliage. Parrot feather (Myriophyllum aquaticum) is an excellent accent plant when grown at the base of a water iris. Its finely cut, emerald-green, fluffy foliage softens the water line and contrasts well with the iris's larger, sword-shaped, architectural foliage. Iris leaves also serve to shade smaller inhabitants, like marsh marigold (Caltha palustris) or candelabra primula (P. japonica), which shun the heat of the summer sun.
SUNLIGHT, WIND, AND WATER DEPTH
Water garden irises perform well if they receive a full day of sunlight. They also grow well and bloom reliably if given at least six hours of sun. Cultivars whose flowers are white or light-colored often prefer some shade to protect their blossoms from drying out too quickly. Generally, water garden irises will not flower if sunlight drops to an average of less than four hours per day. If an iris plant is to be placed in a partially shaded spot, morning sun is usually preferable to afternoon sun.
Gentle breezes can be an aesthetic benefit to a water garden with irises, sending ripples across the water and gently nudging the iris blossoms. Too much wind, however, can cause substantial damage to iris flowers and can even reduce the height of the foliage. Grasses planted around the pond, outside the liner, make a great windscreen.
Most water irises will tolerate a range of water depths, say from moist soil to water a few inches over the crown. Submersion within the range given will provide them with proper growing conditions.
SEASONAL CARE AND MAINTENANCE
Water irises are dependent upon seasonal changes in day length and water or soil temperature to induce them to go dormant in the winter and to prompt them to start growing again in the spring. This being so, it is critical that potted water irises be brought up near the surface of the pond in the early spring where the water is warmer. This awakens the irises from their winter slumber and causes them to begin bud formation. Irises that are planted in soil near the edge of the pond should be cleaned of mulch and debris in early spring to allow the sun's rays to warm the plant and its soil environment.
Water garden irises are heavy feeders and benefit from early-spring fertilizer to supplement the soil's nutrients. The more fertilizer they receive, the more they will grow and flower. As a general rule, begin fertilizing irises when the pond temperature reaches 65°F and continue to fertilize at one-month intervals through the season, well after they have started to sprout. Stop fertilizing about a month before the last frost-free date in the area, to allow the irises to harden off for the winter. In climates without frost, withhold fertilizer once the plants show signs of going dormant.
Irises are carefree plants and need minimal attention through the summer. Deadheading spent flowers will curtail the impulse of those especially energetic strains to seed themselves throughout the pond. Other more restrained varieties benefit from deadheading simply for the sake of appearance. For the same reason, trim back older leaves as they develop a tan or brown color. Trimming off old or unsightly foliage reduces the threat of pests and diseases, another excellent reason to keep the plants well tended. Fall cleaning of irises also prevents diseases or insects from overwintering on the plant.
Generally, irises that like to grow in water year-round are easily prepared for winter's cold winds. As the foliage dies down in the fall, trim the leaves back to just an inch or so above the crown of the plant. Leave the plants in the pond, making sure that they will stay wet throughout the winter.
Irises grow best in pots that are wide and shallow. The rhizomes grow close to the surface of the soil, much like German bearded irises, in more or less a straight line. Plant the rhizome toward the edge of the pot with the growing tip pointing toward the center, to maximize the pot's space. Spread the roots out on a mound of soil, so that they are not all tangled together underneath the rhizome. Then add soil over the rhizome and growing tip. The rhizome should be underneath the soil, with the growing tip just barely below the soil surface.
When water-tolerant irises outgrow their living quarters, they have a tendency to push against the edge of the pot and then “jump” out of the pot entirely. Finding a pot that is at least a foot in diameter will help keep this from happening too frequently. Louisiana irises are especially prone to running and easily jump from their pots in a single season. Make sure to use a pot that is wide enough to accommodate at least a year's growth. Better yet, grow them in a large, shallow tray to avoid the necessity of replanting them in midseason.
Water garden irises prefer a soil that is high in clay, to provide structural support and to allow the plants' roots to move freely for adequate strength and nutrition. Although organic matter, such as decomposed cow manure, may be added to the soil, be very careful. In a water garden, organic matter breaks down quickly once the water temperature warms, creating salts that can burn the roots of water plants. Too much organic matter can also cause an algae bloom in the pond. I recommend keeping the soil somewhat “lean” in organic matter and supplementing it with regular feedings of fertilizer.
I recommend planting and transplanting irises immediately after they have finished flowering, when they are actively growing new roots. Although other sources recommend planting irises in late summer or early fall, we in our midwestern (USDA Zone 5) climate have suffered severe losses through the winter with irises planted out in August or September. In such colder climates, if transplanting must wait until later in the summer, it is imperative that the plants be mulched heavily to withstand the winter cold.
Iris fulva (copper iris, red flag) This species is native to the central southern United States east of the Mississippi River, ranging from Illinois and Missouri southward to Georgia and Louisiana. It is a smaller iris in both plant and flower, growing to 12 inches or so in height. Its slender leaves droop slightly at the tips, giving the plant a graceful air. The falls and standards of the three-inch flower also hang downward, causing the blooms to resemble miniature, copper-colored I. laevigata or I. ensata.
Iris fulva will grow in full sun or part shade. It usually flowers in mid-June, about the same time as other Louisiana irises, of which it is one of the parents. Preferred water depth is generally from evenly moist soil to water about three inches over the crown of the plant. It is the most water-tolerant of the irises for the water garden. It performs well in sites that dry out during the summer, and also tolerates seasonal flooding.
Copper irises are perfectly hardy in colder climates, Zones 5 to 11, and easily survive a winter freeze, provided they remain in the pond water. In very warm climates, the plant should be permitted to spend some time in dormancy—cease fertilizing and allow it to dry out to just damp.
Iris laevigata (rabbit-ear iris) The common name of Iris laevigata is quite appropriate, given its rounded, short, upright petals. Originating in eastern Asia, I. laevigata is especially grown, cultivated, and appreciated in Japan.
Iris laevigata requires moist soils throughout the entire year; it does not tolerate seasonal drought. Hardy in Zones 4 to 9, it grows to about 36 inches in height and has a spread of approximately 12 inches. Because it prefers cooler summers, it usually reaches only 12 to 18 inches in our midwestern gardens. This preference for milder climates has made I. laevigata a much-favored plant in English water gardens and in the Pacific Northwest. It can be more difficult to establish in the South and Midwest. Preferred water depth is from moist soil to six inches of water over the crown.
The blossoms of I. laevigata, usually three to four inches wide, are stunning additions to the pond garden. They appear in midsummer, sparkling white, elegant blue, or royal reddish purple blooms that dance over arching green foliage that sways in the breeze.
Iris prismatica (cube-seed iris) Native to the eastern seaboard of North America, Iris prismatica is a sod-forming plant adorned with early-summer flowers of white, light blue, dark blue, purple, or even pink. Unlike other irises, it has rhizomes that are small and more rounded. It grows by sending out thin extension rhizomes that sprout and form new clumps of fans. Because of this growth habit, I. prismatica competes well with grasses and forbs in the wet meadows, damp fields, and open marshes that it prefers.
Cube-seed iris grows to about 8 to 14 inches or so in height, and forms a tight clump about 12 inches wide. Its blooms are diminutive, only one to two inches in diameter. It is hardy in Zones 2 to 9 and will tolerate the salty conditions of the seaside. It prefers soil that is moist or only slightly submerged. It can withstand some summer drought and occasional seasonal flooding. The plant should not be submerged to the bottom of the pond for overwintering, but will last until spring quite well if simply left in place at the margin of the water garden.
Iris pseudacorus (yellow flag) This species is commonly known as yellow flag owing to its bright yellow flowers that appear in early spring. It originates in Europe, where it was often used as a dye plant and a medicinal herb. Yellow flag has now naturalized over most of North America, growing in full sun or part shade, in shallow or deep waters. It is an exuberant seeder and should never be planted in a natural lake or on the edge of a natural stream.
Generally hardy in Zones 4 to 9, I. pseudacorus usually grows to about three to four feet tall and has a spread of 24 to 30 inches. Blooms are usually three inches wide. It tolerates water over its crown, up to six inches, and withstands seasonal flooding. It also survives periods of drought during the summer.
Iris versicolor (blue flag) and I. virginica (southern blue flag) These two species are best known as blue flags, because of their light blue flowers that bloom in mid-spring. Native to North America, both blue flags are naturalized throughout the eastern United States. Iris versicolor is hardy in Zones 3 to 9, but I. virginica requires a slightly warmer climate, being hardy in Zones 4 to 9.
Iris versicolor grows from 24 to 30 inches tall, and I. virginica reaches up to 36 inches or more. Both have a spread of approximately 12 inches. Iris versicolor prefers water that is up to three inches or so over its crown. Iris virginica enjoys deeper water, up to six inches over its crown. On the other hand, I. virginica tolerates drought better than I. versicolor, making it a perfect plant for retention areas or wet spots in the lawn. The flowers of I. virginica are usually two to four inches wide, slightly larger than the two-inch blooms of I. versicolor, but I. virginica has only about half as many as I. versicolor.
Louisiana irises These plants are hybrids of the five to eight Iris species in the series Hexagonae. Louisianas will grow from drought conditions to water as deep as four to six inches, and they tolerate seasonal flooding. They do well in the garden with supplemental watering. Most are not hardy north of Zone 5 and many only to Zone 6, but they need no special care to overwinter where hardy. Plant height varies from 18 to 36 inches. For more information, see The Louisiana Iris: The Taming of a Native American Wild-flower, second edition, by the Society for Louisiana Irises (Timber Press, 2000).