Growing Irises : Planting & Caring for Iris Flowers

Despite the tall, beautiful iris’ divine origins, it is rugged, reliable, and easy to grow. Learn all about planting, growing, and caring for this flower.

About Irises

There are more than 250 species of plants in the Iris genus. The most well-known irises are the tall bearded irises, which can grow to be 2 to 3 feet tall. Their flowers have six petals, three of which hang down (known as “falls”), and three of which stand upright (known as “standards”).

There are two types of irises – bearded and crested (or “beardless”). Bearded irises have soft hairs along the center of their falls, while crested irises have a comb or ridge of hairs.

Irises typically bloom from late spring to early summer, with some varieties (primarily bearded hybrids) flowering a second time later in the summer. Siberian irises usually bloom around the same time as the bearded types.

Irises are a beautiful addition to any garden and attract many different types of animals such as butterflies and hummingbirds. They also make lovely cut flowers. For companion plants in the garden, look to roses, peonies, and lilies.

Propagation

If you want flowers next year, the best way to propagate iris is by rhizome division.

Although it is possible to propagate species plants from seed, the germination rate can be as low as 50 percent. Additionally, it can take two to three years for flowers to appear. Seeds from hybrid varieties will not grow true to the parent plant.

From Seed

Seeds can be sown outdoors in autumn at a depth of half an inch. They will need to experience cold temperatures, between 34 and 41°F, for at least 60 days in order to germinate.

Add three or four seeds to small, four-inch pots, or space seeds an inch apart in a tray of moist peat moss, perlite, or vermiculite. Cover with a quarter to half an inch of medium and place inside a lightly sealed plastic bag.

Vernalization is the process of a plant exposure to cold temperatures in order to stimulate flowering. This process is important for the plant to be able to flower. Seeds need to be exposed to cold temperatures for at least 60 days in order for them to germinate. The ideal temperature for this process is between 34 and 41°F.

After the cold period finishes, put the pots or trays in a place that’s warm and has bright, indirect light. The best temperature for germination is 70 to 75°F.

Once the seeds have sprouted, remove the bag from the tray or pots. Water the seeds lightly when the top of the soil dries out.

When plants have three sets of true leaves, move them to a lightly shady spot for two or three days, then morning sun with afternoon shade for two or three days, then full sun after that.

When it’s warm enough that there’s no more danger of frost, plant your crops outside following the instructions below.

Root Division

When rhizomes get too close together, they may not flower as well. Divide them every 3 to 5 years to give them room to grow.

After the flowers have bloomed and for six to eight weeks after, the roots can be divided to create new plants or to rejuvenate existing clumps.

Insert the fork perpendicular to the ground. Use a garden fork to loosen the soil around the root clumps, six to eight inches deep. Insert the fork perpendicular to the ground.

You will need to carefully pry the roots from the soil, making sure that you don’t damage the rhizomes in the process.

Select healthy rhizomes by hand or cut them with a clean sharp knife. Discard any rhizomes that are soft, rotting, or have evidence of root borers.

Cover the cut pieces with a powdered fungicide such as garden sulfur.

Pruning your leaves will help keep your plants from being uprooted by strong winds.

PLANTING

Irises need full sun to bloom their best, being in direct sunlight for around 6 to 8 hours a day. They canstill bloom with being in direct sunlight for half the day, however it’s not the best conditions for them. Without enough light, they will not bloom as well. Bearded irises specifically cannot be shaded out by other plants and often do the best when they are the only plant in the bed.

Irises prefer to have their feet in wet soil, but their knees need to be in dry soil. To provide this for them, loosen the soil to a depth of 12 to 15 inches and mix in compost or aged manure. It is also important to make sure that the soil has good drainage. Irises will not tolerate wet soil in winter.

When to Plant Irises

Plant most irises in late summer to early fall, when nighttime temperatures remain between 40° and 50°F (4° and 10°C) or above. This gives them plenty of time to get established before the coming winter.
Tall bearded iris varieties are best planted closer to fall because they tend to go dormant in early to mid-summer.
If you receive bare rhizomes or irises in a container at some point earlier in the year, go ahead and plant them as soon as convenient. It’s better to get them in the ground rather than wait until the “ideal” time.

How to Plant Irises

Plant bare-root rhizomes (the thick stems) horizontally, with the top exposed and only the roots underground. In areas with particularly hot summers, set the rhizome just below the soil surface.
Dig a hole 10 inches in diameter and 4 inches deep. Make a ridge of soil down the middle and place the rhizome on the ridge, spreading the roots down both sides. Fill in the hole and firm it gently, leaving part of the rhizome and the foliage uncovered.
Plant singly or in groups of three, 1 to 2 feet apart, depending on the fully grown plant’s size.
Soak Siberian iris rhizomes in water overnight before planting, then set them 1 inch deep (2 inches, if the soil is sandy), 2 feet apart. Over a period of years, they will form clumps; divide when blooms get smaller and vigor declines.
Do not mulch around the rhizome as this may encourage rot.
Water thoroughly.

Managing Pests and Disease

Although they are deer resistant and drought tolerant, irises are not immune to pests, bacterial infections, and fungal infections.

Pests

Aphids are small insects that can be either green or gray. They suck sap from leaves and spread disease from plant to plant.

You can remove aphids from your plants by spraying them with water from a garden hose.

The caterpillars of the Macronoctua onusta moth bore into the rhizomes of the tall bearded varieties of I. germanica.

You should check rhizomes regularly for small holes or water-soaked leaves. Any plants that are infested should be removed and discarded.

Caterpillars typically cause little damage to foliage, but can be controlled by handpicking them.

Snails and slugs are also known to eat leaves and flowers, creating irregular holes.

Pick the pests by hand and get rid of them. Remove anything nearby that they could hide in, like boards, bricks, and garden debris.

A barrier made of diatomaceous earth can also keep them from getting to plants.

Diatomaceous Earth

Food grade diatomaceous earth is available at Arbico Organics.

If the seed pods of I. versicolor and purple Siberians, I. siberica have multiple small holes, remove them to prevent weevils from breeding.

Disease

The bacteria Erwinia carotovora causes soft rot, which can be a problem for bearded iris, I. germanica. The most likely time for it to occur is during warm, wet springs. Symptoms include soft, smelly rhizomes, decay at the base of leaves, and fan wilting.

If soft rot appears, remove the affected areas with a spoon and let the sun dry the remaining tissue.

After the wound has been sun-dried for several hours, sprinkle it with garden sulfur.

Bonide Sulfur Plant Fungicide is available at Arbico Organics.

Fungal crown rot, caused by Sclerotium rolfsii, is a decay that spread in the form of a soft, gray mat. It often appears in warm, moist conditions and covers rhizomes and the base of leaf fans.

To avoid crown rot, keep your garden tidy, use new and clean tools when handling rhizomes, and get rid of any infected plants immediately.

To prevent crown rot, which can be a regular problem, dip the rhizomes in a solution of bleach and water before planting. The solution should be a 10:90 ratio of bleach to water.

The infection known as leaf spot is caused by the Cladosporium iridis fungus, and is most common on the I. germanica plant. It appears as brown spots on the leaves that were originally water-soaked, and in wet weather, these spots may be covered in a soot-like substance made up of spores.

Any plant parts that are infected with leaf spot should be removed and disposed of.

Rust shows up as yellow spots that turn rusty brown, which then turn black. This causes the leaves to yellow and wither from the tip down.

If there are any leaves with rust on them, remove them and throw them away in the trash. This will stop the rust from spreading.

Best Uses

Irises are a diverse group of flowers that come in many colors, shapes, and sizes. They can be planted in many different areas of the garden.

Make sure to plant some flowers next to pathways and patios, so you can enjoy their fragrance and watch the butterflies and hummingbirds they attract.

Intermediate and taller varieties of plants are great for adding some pizzazz to mixed perennial beds, foundation plantings, cottage gardens, courtyard gardens, and cutting gardens.

Dwarf and intermediate-sized varieties of this plant do well when grown in containers or pots on patios, and all sizes—dwarf, intermediate, and tall—look great as part of a border planting.

Crested varieties of Iris, such as I. crestata, do well in light shade gardens because they can get by with less full sun than most other varieties.