How to grow Bearded Iris

Iris mixing with tulips and forget-me-nots in my garden last May.

Bearded iris is a classic cottage garden flower, but I think they are so traditional they often get overshadowed by other perennials. With this post, I hope to remind you just how beautiful they are. 

The earliest bearded iris begins to bloom at the perfect time. Late tulips are flowering and alliums are putting on a show, but many perennials are still in that early stage of green growth. In my garden, I find that bearded iris, with their ruffled blooms, is a welcome addition in May.

A Tall Bearded Iris at the Royal Botanical Gardens in Hamilton, ON.

With satin petals that shimmer in the sunlight, the flower of a Tall Bearded Iris is an exquisite thing to behold. They are perhaps the most iconic iris, but they aren't your only option.

Here's a look at the range of bearded iris available:

MDB: Miniature Dwarf Bearded Irises are the smallest and earliest of the bearded irises to bloom. They grow up to 8 inches tall and have flowers that are three inches or smaller.

SDB: Standard Dwarf Bearded Irises bloom after Miniature Dwarf Iris and usually finish flowering just as the Intermediate Bearded Iris are reaching their peak. SDW reach a height of 8 to 15 inches tall and has blooms that are 2-4 inches in size.

IB: Intermediate Bearded Irises are 16 to 28 inches in height. The flowers are 3.5-5 inches in size and extend up above the foliage for a nice display.

MTB: Miniature Tall Bearded Irises are 16 to 18 inches tall and have flowers that are approximately 6 inches. The flowers are fragrant and are often used as cut flowers.

BB: Border Bearded Irises are 16-27 inches in height and are more resistant to wind damage than Tall Bearded Iris. At 5 inches the flower size is a little smaller than TB.

TB: Tall Bearded Irises is the last of the bearded iris to bloom. They are 27 inches or more in height.

A few other considerations:

Bearded irises flower for 3 or 4 weeks. You can extend the flowering season, however, by selecting early and late flowering varieties.

Some bearded irises have a light fragrance. If you are looking to add fragrance to your plantings, you can shop for scent in a range of iris sizes.

Even if your garden is small, there is still a bearded iris for you. The Miniature Dwarf Bearded Iris (see below) in my mother's old garden kickstarted my passion for this tiny iris.

Border Bearded Iris 'Batik' in my garden.

How to Choose: Bigger is Better...right?

Not necessarily. Selecting a bearded iris is a mix of personal preference and what works best in your garden. Statuesque Tall Bearded Iris has the largest blooms, but they are 27 or more inches in height. The big flowers make them top-heavy. Wind and rain can send their flowers right to the ground. In an open area, Tall Bearded Iris generally requires some form of staking.

The blooms of the shorter types of bearded iris are significantly smaller, but they are less top-heavy and yet still manage to be quite showy. Over the years, I have come to prefer Intermediate Bearded Iris, partly because I am lazy, and don't want to have to stake flowers, and also because I find the metal stakes detract from the flowers. 

You, on the other hand, might prefer the larger flowers and think stakes are a small price to pay for the amazing blooms.

By way of example, here's a comparison of two peachy-pink iris:

Tall Bearded Iris 'Beverly Sills' is one of the most popular peachy-pink tall bearded irises. It has a reputation for being a vigorous grower that blooms heavily mid-season. Full sun. Height: 60-90 cm (24-36 inches).USDA zones: 3-9.

Intermediate Bearded Iris 'Pink Kitten' has much smaller pale pink flowers with tangerine beards. Fragrant. Full sun. Height: 45-60 cm (18-24 inches). USDA zones: 3-9.

A purple Intermediate Bearded Iris in a master gardener's backyard in Mississauga, ON. A step-back view of the same iris can be seen below.

A yellow Intermediate Bearded Iris in the same master gardener's backyard in Mississauga, ON.

The yellow iris in close-up.

A few other considerations:

Bearded irises flower for 3 or 4 weeks. You can extend the flowering season, however, by selecting early and late flowering varieties.

Some bearded irises have a light fragrance. If you are looking to add fragrance to your plantings, you can shop for scent in a range of iris sizes.

Even if your garden is small, there is still a bearded iris for you. The Miniature Dwarf Bearded Iris (see below) in my mother's old garden kickstarted my passion for this tiny iris.

Planting a Bearded Iris:

The best time to plant bearded iris is in July, August and September. To ensure your iris will make it through winter, be sure to plant it at least 4 weeks before the first hard frost. Typically a bearded iris will bloom a year after it is planted.

Iris like full sun (6-8 hours of sunlight). The exception might be a hot climate where iris might benefit from light shade in the afternoon.  The only other requirement is good drainage. If your soil is poorly drained, add organic matter to improve drainage.

Plant your rhizomes at least 12 inches apart. Crowding them can create an impressive display, but you'll have to dig your iris up and divide them after just a couple of years. Spacing irises properly also encourages good air circulation and helps prevent disease.

If your iris is in a nursery container, remove it from the pot without disturbing the soil. Plant it at the same level or even slightly higher in the ground. Be careful not to cover the rhizome with soil. Water well. Continue to water every few days for about a week. Then water weekly until the iris has rooted.

Planting a bare-root bearded iris is a little more tricky. If the roots are looking a little wrinkled you can rejuvenate them by soaking the rhizome in a shallow pan of water for a couple of hours just before you do your planting.
Irises like to have the top of their rhizomes visible to the sun. Dig a planting hole and fashion a hill of soil in the middle. The mound of soil should come up to ground level. Centre the rhizome on top of the mound and spread out the roots down the sides of the hill. Bury the roots taking care not to cover the rhizome. Water well. Continue to water every few days for about a week. Then water weekly until the iris has rooted.

A Tall Bearded Iris with Columbine just in behind it. My garden in May.

Ongoing Care

Water
Established clumps of bearded iris do not need supplemental water. They should be fine with natural rainfall unless there is an extended period of drought.

Fertilizing 
Generally speaking, Bearded iris does well in average garden soil and do not need regular fertilizer. If your soil is really poor, a light application of fertilizer can be added in early spring and again a month or so after bloom. Superphosphate or a well-balanced fertilizer (with an NKO ratio of 10-10-10 or 5-10-10) are two good options.
There are a couple of fertilizers that should be avoided. A fertilizer that is high in nitrogen can encourage lush growth that is susceptible to bacterial soft rot. A weed and feed fertilizer should also be avoided.

Do not mulch!
Mulch locks in moisture and can cause the soft rot of your rhizomes.

Avoiding Potential Problems
To avoid problems with disease and pests keep your iris clear of garden debris. Remove any dead foliage and after your iris finishes flowering, snap or cut the flower stalk off at the base. In the late summer/fall prune back the foliage to discourage over-wintering pests.

Bearded iris mixing with Peonies in a private garden in Caledon, ON.

Dividing

A decline in the number of flowers is a sign that your iris needs to be divided. As a rule, you should divide your irises every 3-4 years in late July or early August.
To divide your irises, dig up the entire clump with a garden fork. Gently pull apart the tangle of rhizomes with your hands. Cut off the larger, healthy-looking young rhizomes with a sharp knife. Throw away the old core "mother" rhizome. It's past it's prime and won't bloom again. At the same time, you should also discard any sections of rhizome that have been adversely damaged by pests. Cut back the leaf fans of your divisions to six or eight inches. This will ease the stress of replanting by allowing the plant to focus on growing new roots. Allow the rhizomes to dry overnight to seal up any cuts before replanting them.

Pests and Problems

Iris Borers
Adult borers are nocturnal moths that lay their eggs on garden debris in last summer or fall. They hatch into one-inch sized larvae that chew into the leaves and then eat their way down to the rhizomes. Borer damage is often seen as notched wounds or slimy, wet-looking areas on the leaves. Once they eat their way down to the base of the plant, they begin to hollow their way through the rhizome. In August they pupate in the soil and hatch into more adult moths.
To deal with this pest, I have learned to keep the rhizomes clear of any debris throughout the growing season. I also try to catch the larvae in the spear-shaped foliage by removing any slimy leaves.

Bacterial Soft Rot
It is hard to imagine anything more putrid-smelling than mushy rhizomes infected with this fungal disease. Too much nitrogen in the soil, garden debris around the base of the plants and too much water are all possible causes of this problem.
Dig up the infected plant/s and cut away the rotten parts of the rhizome (throw the infected sections in the garbage–do not compost them). Allow the cut areas to sit in the open air for a day or two. You can also disinfect the wounds with a solution of 1 part bleach to 9 parts water.

Bacterial Leaf Spot
Small pale spots on the foliage are a sign of Bacterial Leaf Spot. Sadly there is no cure. Remove any infected plants and wash your tools with a 10% bleach and water solution.

Planting Ideas

I always like to end on a positive note. Pests and disease can be an issue, but these are wonderful perennials.

Here are a few ideas on using iris in the garden and how to combine them with other plants.

 Royal Botanical Gardens in Hamilton.

Try planting three or five complementary colors together.

 Royal Botanical Gardens in Hamilton.

Mix bearded iris with Peonies.

 Royal Botanical Gardens in Hamilton.

Mix bearded iris with other types of iris (above and below).

Royal Botanical Gardens in Hamilton.

Private garden in Mississauga, ON.

Let a shrub or tree create the perfect backdrop.

Private garden in Mississauga, ON.

A bearded iris in my garden with Sweet Rocket in the background.

No matter what type of bearded iris you choose, it is bound to be a great addition to your spring garden.

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