How to care for irises

What are the needs of irises?

Irises are hardy perennial plants, but to ensure they produce vibrant blooms year after year, it’s essential to understand their needs. There are many varieties of iris, each with its own preferences in terms of growing conditions. The most well-known and widely cultivated is the bearded iris (Iris germanica), prized for its large, colourful flowers and sword-shaped foliage.

Bearded iris requires full sun exposure (at least six hours of direct sunlight per day) to flower well. A lack of light can result in fewer blooms and weaker stems. It also thrives in well-drained soil, as stagnant moisture is its enemy—it can cause rhizome rot. Slightly alkaline and moderately fertile soil is ideal. Unlike other flowering plants, bearded iris does not appreciate overly rich soil, which promotes foliage growth at the expense of flowers.

If you grow other iris varieties, such as the Siberian iris or the yellow flag iris, be aware that their needs differ. The Siberian iris (Iris sibirica), more delicate and refined, produces elegant flowers, often in shades of blue or violet. It tolerates slightly moist soil better and adapts to less sunny conditions. As for the yellow flag iris (Iris pseudacorus), it is ideal for damp areas such as pond edges or bog gardens, with its striking yellow flowers that catch the eye. There are also dwarf irises (Iris pumila), perfect for rockeries or borders, which bloom early in spring and are highly drought-resistant.

How to care for irises? (Note: In British English, the plural of "iris" can be either "irises" or "iris" – both are acceptable. I've used "irises" here as it's more commonly used in gardening contexts.)

Watering

Bearded Irises are drought-resistant and particularly sensitive to excess moisture. It is therefore important not to overwater them, especially if planted in poorly draining soil. On average, water them only once every three weeks during summer. When watering, avoid wetting the foliage to prevent the development of diseases.

Fertilisation

Irises are not demanding plants, but applying fertiliser will enhance their flowering abundance.

We recommend using well-balanced, mature compost or a commercial complete fertiliser low in nitrogen (high-nitrogen fertilisers would promote foliage at the expense of flowers). Fertilisation can be done twice a year: before flowering (in March) and after flowering.

Weeding and Mulching

Weed carefully and regularly around your irises. Allowing grass to grow at the base of your plants would create constant moisture, which is harmful. We recommend weeding by hand rather than using sharp tools that could damage the rhizomes.

To prevent weeds from regrowing, you can use mulch, but it must always be suitable: organic mulch made of dead leaves or dry grass is acceptable in hot, dry regions but should be avoided in more humid areas as its decomposition could cause your irises to rot. In such cases, opt for a mineral mulch like pumice or gravel.

Pruning

Many gardeners ask us whether irises need pruning. Here’s our advice:

• During flowering, regularly remove spent flowers before they produce seeds, which could unnecessarily exhaust the plant.
• When an entire flower stalk has faded, you can cut it back with secateurs, trimming it at the base of the clump.
• After flowering, leave the leaves in place, as this is when the plant replenishes its reserves in the bulb or rhizomes.
• Only when the leaves have completely dried out can you remove them, gradually.

When and how to divide irises?

Dividing irises is an essential step to maintain vigorous plants and abundant flowering. Over time, the rhizomes accumulate, creating dense clumps that limit flower production. Dividing irises helps rejuvenate them, improve their health, and encourage spectacular new blooms.

When to divide irises?

The ideal time to divide irises is between July and September, after flowering. At this point, the plant has replenished its reserves, and the new divisions will have time to root before winter. You can divide your irises roughly every four years.

How to divide irises?

Here are the steps for successful division:

• Dig up the iris clump using a garden fork, gently lifting the root ball to avoid damaging the rhizomes.
• Clean the rhizomes by removing the soil around the roots to assess their condition.
• Separate the rhizomes by breaking or cutting sections by hand or with a clean, sharp knife. Keep young, firm rhizomes with healthy roots and a few leaves.
• Remove damaged or dried parts, as well as older rhizomes that have become less productive.
• Trim the foliage to about 15 cm in height to reduce evaporation and help the plants recover.
• Replant the rhizomes, placing them half-buried with the upper part slightly exposed to light. Space them 30 cm apart to give them room to grow.
• Water lightly after planting to encourage rooting, while avoiding excess moisture that could cause rot.

This simple task not only ensures the health of your irises but also allows you to easily multiply them to beautify other areas of the garden or share them with others.

For more details on dividing your irises, check out the guide: Multiplying Iris germanica!

Diseases and Pests of Irises

Irises are robust and vigorous plants, but they can sometimes be affected by certain diseases, particularly when the planting guidelines are not properly followed.

Among them:

• Heterosporium leaf spot is a fungal disease that manifests as oval, brown spots on the foliage, which eventually dries out completely. This condition is encouraged by a combination of heat and humidity. If you notice these symptoms, cut off and burn the affected leaves to limit the spread of the disease, then treat with a fungicide, such as Bordeaux mixture or sulphur.
• Rhizome rot is also sometimes observed in gardens with damp soil. To prevent it, plant the rhizomes shallowly and on a mound, in well-drained soil, and avoid damaging them during maintenance (weeding, division, etc.). At the first signs of rot, remove the affected parts with a clean cut using a disinfected knife.
• Iris rust can cover the foliage with small orange-red spots. Once again, a fungus is the culprit. To treat it, promptly remove damaged leaves and apply a fungicide to eliminate it. For more details, check out our guide: “Getting Rid of Rust”.

Finally, slugs and snails enjoy nibbling on iris leaves and rhizomes. To protect your plants, you can spread wood ash or sawdust around them as a barrier. However, using an organic slug repellent is often the most effective solution.

Why isn't my iris flowering?

An iris that fails to flower or grows poorly is often suffering from planting or maintenance issues. Fortunately, by identifying the cause, you can correct the situation and enjoy beautiful blooms once again.

Here are the main reasons why an iris may not flower:

Lack of sunlight

Irises need plenty of light to flower abundantly. At least 6 hours of direct sunlight per day is essential. If planted in partial shade or a too-shady spot, their flowering will be compromised. Solution: move your irises to a sunnier location, ideally facing south or southwest.

Unsuitable soil: too heavy, too poor, or too acidic

Irises dislike compacted, waterlogged soils, as well as overly acidic or nutrient-poor soils. Poor drainage can also lead to rhizome rot, preventing the plant from thriving.
Solution: dig up your irises, loosen the soil by adding sand or gravel to improve drainage, and incorporate well-rotted compost to enrich the soil. If the soil is too acidic, gradually add lime, such as dolomite, to rebalance the pH.

Using a fertiliser too rich in nitrogen

If your irises produce lush foliage but no flowers, this may be due to excess nitrogen, often found in overly rich fertilisers. Nitrogen promotes leaf growth at the expense of flowering.
Solution: stop applying nitrogen-rich fertiliser. There’s little else to do but wait—once the plant uses up the excess nitrogen, it will naturally resume its flowering cycle. In the future, opt for a low-nitrogen fertiliser higher in phosphorus and potassium, which encourage blooming.

Infrequent division

Over time, irises form dense clumps where rhizomes compete for space, limiting flower production. This is a common cause of reduced flowering in neglected irises.
Solution: remember to divide your irises every 3 to 5 years, preferably in late summer after flowering. This process revitalises the plant, gives it room to grow, and stimulates new flower production.

Why is my iris turning blue?

It can happen that an iris changes colour over the years, with flowers gradually turning blue or purple when they were originally a different shade. This phenomenon may seem strange, but it is not a sign of the plant degenerating. The most common cause is related to self-seeding.

Indeed, if you don’t remove the faded flowers, they will produce seeds that fall to the ground and germinate, giving rise to new plants. These young irises grown from seed are not clones of the parent plant: they result from genetic recombination, often dominated by genes responsible for blue or purple hues—colours very prevalent in the genetic heritage of irises. This is why the new flowers may differ in colour, reverting to “wild” or dominant shades.

This phenomenon is entirely natural and does not affect the plant’s health. However, if you dislike the new colour, the only solution is to dig up the rhizomes of the affected plants. If you still have irises of the original variety, it’s advisable to sort them during flowering, removing only the plants with undesirable colours. To prevent this issue in the future, remember to deadhead the faded flowers before they go to seed. This will limit the spread of self-seeding and preserve the original colour of your irises.