How to Plant & Grow Water Iris (Iris laevigata)


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Iris variegata in bloom
One of the most popular water iris varieties is the ‘Variegata’ (pictured). Stefan.lefnaer, CC BY-SA 3.0 AT, via Wikimedia Commons

Iris laevigata is known as a true water iris due to its tolerance for a semi-aquatic habit. It is also referred to as rabbit-ear iris and kakitsubata (Japanese). This species is often confused with other Japanese irises, but is ecologically set apart by its preference for having its crown submerged in water all year round.

Water iris belongs to the Iridaceae family, which includes more than 2000 species of colorful plants found worldwide. The Iris genus contains some of the most notable species under this family due to their sizes and eye-catching features.

Native to Japan, I. laevigata has gained international popularity as its varieties have been cultivated for hundreds of years. One of its most popular varieties is the ‘Variegata’ or variegated water iris, which has striped green and white leaves. The colors and features of this plant’s stunning flowers depend on the cultivar. While those of ‘Variegata’ are pale and occur singly, those of ‘Alba’, ‘Mottled Beauty’, and ‘Elegante’ are mostly white and may have mauve or blue-colored streaks. The parent I. laevigata plant produces blue-violet petals that gracefully contrast its lengthy green leaves every summer.

Facts, Benefits & Uses of Water Iris

The water iris is the perfect marginal plant for medium to large-sized ornamental or wildlife ponds. Its foliage can pleasantly blur the edges of your pond and aid in hiding pond liner or artificial pond features. Juvenile fish, tadpoles, and insect larvae may benefit from the protection that the submerged structures are likely to provide. Moreover, the stunning iris flowers attract a host of pollinators. Even hummingbirds are hard-pressed to ignore the blooms of this plant.

If your garden is frequently visited by deer, you’ll be happy to find that they generally avoid water irises. For natural water features, a few patches of water iris along the shoreline can even help prevent erosion and serve as a buffer against waves. Not yet convinced? Apart from its suitability as a backdrop for ponds, water irises can even help keep your water clean, provide shelter to visiting waterfowl, and are generally avoided by hungry fish.

Perhaps, one of the only downsides of this plant is its tendency to spread quickly. When conditions are optimal, mass plantings of this herbaceous perennial can expand their ground cover area via rhizomatous growth. However, I. laevigata seems to be more manageable compared to other invasive water irises, such as I. pseudacorus (yellow flag).

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Water Iris Fact Sheet:
PLANT TYPE
Herbaceous perennial
HARDINESS ZONES
USDA 5 – 9
LIGHT REQUIREMENTS
Full sun to partial sun
BLOOM COLOR
Blue, white
BLOOM PERIOD
Late spring to early summer
MAXIMUM GROWTH
3 feet (90 cm)
PLANTING DEPTH
2 – 8 inches (5 – 20 cm) in water
WATER QUALITY
pH 5.5 – 6.5

 


Water Iris Growth, Hardiness & Climate

Iris laevigata flower above the water
If conditions are perfect, water iris can bloom twice a year! Leonora (Ellie) Enking / CC BY-SA 2.0

Water iris grows best in organic, slightly acidic soil, but it can withstand slightly alkaline conditions as well. It prefers clay or loamy soil types as its root systems are averse to quick drainage. For this reason, this species would prefer a pond marginal rather than an edge placement. Ideally, a few inches of water should cover the base of the plant throughout the year. However, complete submergence will, unfortunately, reduce the chances of survival.

Hardy to USDA zones 5 – 9, water iris can tolerate seasonal temperature fluctuations but would thrive best in 12 – 25˚C (53 – 77˚F). If temperatures fall outside of this range during summer, the plant may fail to have a high flowering rate. In contrast, if conditions are perfect, your water iris may surprise you with a second round of blooms in late summer or early fall. Erect clumps of this species will typically spread to a width of 0.5 meters (20 inches) after just 2 – 3 years of proper care.


How to Plant Water Iris

Water iris in a pot with its roots submerged in water
The root systems of Iris laevigata should be submerged under a maximum of 4 inches (10 cm) of water. サフィル, CC BY-SA 4.0, via Wikimedia Commons

Water iris is usually planted using its bulbs and propagated further via division. It can also be grown from seed, but may be more challenging to germinate and will require a few transplantation phases prior to being outplanted. Seeds should be sown on a setup with consistently moist soil. Plastic containers or small pots (with bottom holes) placed on a tray with few inches of water should work. If the seeds are viable, they should sprout within 3 – 4 weeks.

Once the seedlings are large enough to handle, you may transplant them into individual pots. If you intend to plant them in shared water baskets, make sure that they are placed at least a few inches apart. If propagating water iris divisions, place them 18 – 24 inches (46 – 61 cm) apart from one another or in their own pond baskets. Aquatic soil can be used, but this species will benefit from additional organic matter. The root systems of plants should be submerged under a maximum of 4 inches (10 cm) of water. They can gradually be moved further into the pond as they mature.


How to Care for Water Iris

Rice root aphids on a leaf
Make sure to check your water iris plant regularly for glasshouse pests such as rice root aphids (pictured). Whitney Cranshaw, Colorado State University, Bugwood.org, CC BY 3.0, via Wikimedia Commons

Once your water iris is established, you’ll find that it is relatively easy to care for. It should be placed in an area that receives around 6 hours of full sunlight each day, preferably during the morning hours. Intense afternoon light can cause leaf scorching, so a partially shaded area would be best if you are located in a hardiness zone with particularly hot summers.

This species may benefit from a broad-spectrum aquatic fertilizer during its growth periods, especially if it is cultivated outside of a pond setting. If cultivated in the margins of a fish pond, it will not require additional fertilizer due to the nutrient composition of pond water. Regularly inspect the leaves of the plant and remove or treat any parts that may have been infected by pests or disease. Spent blooms can be pruned to maintain the appearance of your iris clusters and to encourage a second flowering period.

Be on the lookout for the following glasshouse pests that typically occur on the organs of water iris: rice root aphid (Rhopalosiphum rufiabdominalis) and common blossom thrips (Frankliniella schultzei). Once spotted, the spread of these insects should be manually or chemically controlled as they may quickly infest entire water iris colonies and nearby plants.


How to Winter Water Iris

Iris laevigata is adapted to winter temperatures that dip to below 0˚C (32˚F). Colonies that have become naturalized in the Lake Baikal region of Siberia may even tolerate temperatures nearing -35˚C (-31˚F). As this plant is cold hardy, it need not be brought indoors for winter. Simply cut back the foliage to a length that allows them to protrude to just above the waterline. This should be done in autumn or before temperatures drop to below zero. Remove any decaying leaves to prevent decomposition in the pond water.

Once temperatures begin to rise once more, the established root systems should produce new foliage. If you are located in an area with fairly moderate to warm winters, the foliage need not be cut down as it will remain green through winter.


Is Water Iris Invasive or Toxic?

Iris laevigata has the potential to be an aggressive spreader when reared in optimal conditions and rich soils. It is listed under the Global Register of Introduced and Invasive Species of the US, Great Britain, and Australia. Fortunately, its potential to outcompete native species is not as severe as that of other Iris species. To manage your water iris colonies, grow them out of aquatic baskets or containers.

Like all other Iris species, this plant’s organs contain toxins that have mild effects on humans but pronounced symptoms in livestock and pets. Wild animals generally stay away from this plant as it contains pentacyclic terpenoids and resinoids. These compounds are sometimes called irisin or iridin, and the highest concentrations are found in the rhizomes and bulbs of the plant. If large amounts are ingested, symptoms include nausea, vomiting, and diarrhea. Keep curious cats and dogs away from this plant to prevent poisoning.

Note that this species does not cause contact dermatitis, so it is safe to touch and handle the plant with bare hands. If sap-oozing tissues are exposed, consider using gardening gloves as a precaution.


Is Water Iris Edible? Will Fish Eat it?

Due to its toxic composition, I. laevigata is not an edible plant, even when cooked or dried. Though dilutions of Iris plants may traditionally be used to treat ailments, this plant should by no means be harvested for consumption or use in infusions.

Fish don’t typically consume any parts of this plant. It should be safe to place along the margins of a koi pond, as koi are unlikely to graze its organs.


Where to Buy Water Iris & Seeds? (UK & US)

Iris laevigata and its cultivars can be purchased from ornamental plant stores and nurseries. If located in the US or UK, you may have more luck acquiring this species through online plant portals. Keep in mind that this species is synonymous with I. albopurpurea and I. phragmitetorum.

If this species is unavailable and you’d like to acquire a mature water iris locally, consider purchasing the following alternatives: I. versicolor, I. fulva, I. ensata, and I. virginica. These alternate species can likewise be cultivated in a few inches of water. Do make sure to check that none of these species are prohibited in your area before making a purchase.

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