Growing Irises

How to Plant, Grow, and Care for Irises

Iris Flowers
Pixabay

The tall, beautiful iris, named after the Greek goddess who rode rainbows, comes in many magical colors. Here’s how to grow irises in your garden!

Every gardener wants this pretty perennial. Despite its divine origins, it is hardy, reliable, and easy to grow. Irises also attract butterflies and hummingbirds, and make lovely cut flowers.

There are some 300 species in the genus Iris. The most familiar irises are the tall (at least 28 inches) bearded irises (Iris germanica).

The distinctive flowers have three large outer petals called “falls” and three inner upright petals called “standards.” The falls may have beards or crests. Bearded iris are so-called because they have soft hairs along the center of the falls. In crested iris, the hairs form a comb or ridge.

Most irises flower in early summer. Some—mostly bearded hybrids—are remontant, flowering again later in the summer.

Planting

When to Plant Irises

  • Plant irises in mid- to late summer. This gives them plenty of time to get established before the coming winter.
  • 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.

Choosing and Preparing a Planting Site

  • Irises will bloom best in full sun, but can tolerate as little as half a day of sun. Without enough light, they won’t bloom.
  • Bearded irises must not be shaded out by other plants; many do best in a special bed on their own.
  • They prefer fertile, neutral to slightly acidic soil. Learn more about preparing soil for planting and organic soil amendments.
  • Soil drainage is very important; irises prefer “wet feet, but dry knees.” Loosen the soil with a tiller or garden fork to a depth of 12 to 15 inches, then mix in a 2- to 4-inch layer of compost.

How to Plant Irises

  • Plant rhizomes singly or in groups of three with the fans (leaves) facing out, 1 to 2 feet apart, depending on the size.
  • Dig a shallow 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 roots down both sides. Fill the hole with soil and firm it gently, leaving part of the rhizome and the foliage uncovered.
    • Tip: It’s easy to make the mistake of planting irises too deeply. The rhizomes of these plants should be partially exposed to the elements, or thinly covered with soil in hot climates. If they’re buried too deeply, they won’t do well.
  • Water thoroughly.
  • At time of planting, top-dress with a low-nitrogen fertilizer, and again in early spring, 6 to 8 weeks before blooms are expected to appear.

Striped iris flower

Care

How to Grow Irises

  • Avoid applying high-nitrogen fertilizers to the surface or carelessly mulching with organic matter, which may encourage rhizome rot.
  • Keep rhizomes exposed. Unlike bulbs, which thrive deep underground, iris rhizomes need a bit of sun and air to dry them out. If they’re covered with soil or crowded by other plants, they’ll rot. Irises may benefit from shallow mulching in the spring.
  • Don’t trim iris leaves after they have finished blooming. Leaves carry on photosynthesis for next year’s growth. Cut off brown tips—and cut the flowering stalk down to the rhizome to discourage rot.
  • If iris foliage is hit with heavy frost, remove and destroy it to eliminate borer eggs. See your local frost dates.

Dividing Irises

Over time, it’s not unusual for plantings of iris to become overcrowded, which causes the rhizomes to lose vitality and stop blooming. When this happens (usually every 2 to 5 years), it’s time to divide and replant healthy rhizomes in fresh soil.

  1. Shortly after blooming (mid-summer), carefully dig up the clump of irises. You’ll find that the original rhizome that you planted (the “mother”) has produced several offshoot rhizomes.
  2. Separate these rhizomes from the mother with a sharp knife and discard of the mother, as it will no longer produce blooms. 
  3. Inspect the rhizomes for any rotting tissue or other signs of disease, removing and discarding infected parts or entire unhealthy rhizomes.
  4. Trim the iris foliage down to 3 to 5 inches in height so that the plants can focus on establishing new roots.
  5. Plant these fresh rhizomes in a new bed, replant them where they were before (after adding new soil), or share them with friends and spread the joy of irises!

Pests/Diseases

  • Irises are deer-resistant and drought-tolerant. However, they are susceptible to borers, so check the rhizomes (fleshy roots) yearly for holes, discarding any infested ones.
  • Verbena bud moth, whiteflies, iris weevil, thrips, slugs and snails, aphids, and nematodes may also be troublesome.

Recommended Varieties

Wit & Wisdom

  • The iris is depicted in the French royal standard fleur-de-lis and is also the symbol of Florence, Italy.
  • Oral root, taken from the dried roots of Iris ‘Florentia’, was considered a cure for blood and lung diseases, and teething babies were encouraged to gnaw on a “finger” of dried root for its natural fluoride.

Growing Irises

Botanical Name Iris germanica
Plant Type Flower
Sun Exposure Full Sun, Part Sun
Soil Type Any, Loamy, Sandy
Soil pH Slightly Acidic to Neutral
Bloom Time Summer
Flower Color Blue, Multicolor, Orange, Pink, Purple, White, Yellow
Hardiness Zones 4, 5, 6, 7, 8, 9
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