How to Plant, Grow, and Care for Irises
Irises will bloom best in full sun, meaning at least 6 to 8 hours of sunlight per day. They can tolerate as little as half a day of sun, but it’s not ideal. Without enough light, they won’t bloom well. Bearded irises must not be shaded out by other plants; many do best in a special bed on their own.
Provide well-draining, fertile, neutral to slightly acidic soil. Loosen soil to a depth of 12 to 15 inches, then mix in compost or aged manure. Good drainage is critical: Irises prefer âwet feet, but dry knees.â They will not tolerate wet soil in winter. Learn more about preparing soil for planting and organic soil amendments.
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 âidealâ 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 plantâs 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.
How to Grow Irises
- In the early spring, remove winter mulch and any old foliage to allow for fresh, new growth.
- Fertilize in early spring, scratching an all-purpose fertilizer around the plants. Avoid high-nitrogen fertilizers. Reblooming irises perform best if fertilized again after the first wave of flowering is finished.
- Do not overwater irises; too much moisture in the soil can cause the rhizomes to rot. Water consistently and deeply, especially during summer drought.
- Keep rhizomes exposed. Unlike bulbs, which thrive deep underground, bearded 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.
- Taller irises may need staking or they will fall over.
- Deadhead (remove spent blooms) consistently. Bearded Irises flower sequentially on buds spaced along the stems.
- After blooming is finished, cut flower stems down to their base to discourage rhizome rot, but do NOT trim the irisâ leaves. The plant’s foliage carries on with photosynthesis and generates energy for next year’s growth and flowers. Only prune off brown leaf tips, if desired.
- After a hard frost in the fall, cut foliage back hard, remove any foliage that appears spotted or yellowed, and dispose of all debris in the trash.
- If iris foliage is hit with heavy frost, remove and destroy it to eliminate borer eggs. See your local frost dates.
- For winter protection, cover the rhizomes with an inch or two of sand topped with a light layer of evergreen boughs, applied after the ground freezes and removed when the forsythias bloom in the following spring.
- Iris borer, a common iris pest, overwinters as eggs in spent leaves. Signs include vertical streaks in the leaves. If apparent, look for the pests and squash them! If you see rot in the rhizome, dig it up and remove the affected parts. See pest tips below.
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.
When to divide? Do this task after flowering finishes and then trim the foliage back to six inches.
- Shortly after blooming (usually around mid-summer), carefully dig up the clump of irises. You’ll find that the original rhizome that you planted (the âmotherâ) has produced several offshoot rhizomes.
- Separate these rhizomes from the mother with a sharp knife and discard the mother, as it will no longer produce blooms.
- Inspect the rhizomes for any rotting tissue, soft spots, or other signs of disease, removing and discarding infected parts or entire unhealthy rhizomes.
- 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!
The tall bearded irises, Iris germanica, come in many flamboyant colors. They are generally planted in the fall. Rebloomers (remontant) varieties include:
- âImmortalityâ: fragrant white flowers that appear in early summer and again in late summer; hardy to Zone 4
- âFeed Backâ: fragrant dark purple flowers; Zones 4 to 9
- âEarl of Essexâ: white flowers, with purple edging; Zones 3 to 10
- âJennifer Rebeccaâ: mauve pink flowers; Zones 4 to 9
Photo: Bearded Irises. Credit: Pixabay.
Siberian irises, I. sibirica, also comes in a range of colors. They have a more delicate beauty than the stately bearded irises, but are equally as rugged. They also tend to be more pest and disease resistant.
- âBlueberry Fairâ: ruffled blue flowers; Zones 3 to 8
- âFond Kissâ: white flowers with pink flush; Zones 3 to 8
Photo: Siberian irises. Credit: Pixabay.
Japanese irises, I. ensata, bear huge, flat blooms. These heavy feeders thrive on moisture during the growing season and do well around ponds; move to drier ground for fall and winter.
- âCohoâ: pink flowers with golden flush; Zones 4 to 9
- âVariegataâ: dark purple-reddish flowers; Zones 4 to 9
Photo: Japanese iris. Credit: Pixabay.
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I was given some rhizomes 2 years ago, & planted them around my river birch tree. Theeaves have come up these last 2 seasons, but still no blooms. Any ideas why?
Hi, Mary, You say you planted the rhizomes âaroundâ a tree; they may be hindered by the tree and its roots. Iris will not bloom if there is too much shade or too little water, space, or nutrition. It’s also possible that the rhizomes are planted too deep; too deep and they will not bloom. We hope this helps!
I just love Irises! They remind me of my grandma who took special care of hers. She had a variety that grew 4-5 feet tall. She dug them up every fall and planted them again in the early spring (I'm not sure why she did this, we didn't live in a harsh climate). My first landlord cultivated Irises for competition and created several new colors. These are the best smelling flowers; not too perfumey or over powering, just a nice clean fresh scent.
We live in Tucson. AZ. I was given a 6"-8" pot with Iris. Today 4/5/2022 hi temp will be about 90 degrees only to climb in the weeks to come. (105 maybe). What do I do to save these wonderful plants.
Water deep once a month. I too love in Tucson mine are under tree . I didn't plant them there was there when we bought house. I fertilize in spring after bloom with low nitrogen fertilizer like 6-8-8. Than again after Labor Day. We have been here 25 years so I just think them out for first time. They weren't flowing well . Hope to have more blooms in March .Mine do well even in cold weather without protection but you can cut out anything that does get damage..They seem very hardy ,no diseases or pest.Good lucky with yours.
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