Iris flowers have around 300 varieties in the Iris genus. These famous flowers are available in two main forms: those that grow from rhizomes and those that grow from bulbs. They feature unique blooms that are made up of two different kinds of petals, falls and standards. The falls form the lower petals, which droop downward or fall. The standards are the upper three petals of the flower.
The characteristics of an iris petal further segment the plant into three types: bearded, crested, and beardless. Bearded iris plants have soft hairs along their falls, resembling a beard. The flowers of a crested iris have a ridge-like crest on their falls. Beardless have neither hairs nor crests.
Irises that grow from bulbs include Iris reticulata, Spanish (Iris xiphium) and Dutch varieties (Iris x hollandica) and bloom earlier than the rhizomatous irises. However, most irises show their famous flowers in the early summer, while some also bloom for a second time in the late summer. They are known for attracting butterflies and hummingbirds and make perfect cutting flowers. The iris varies greatly in size, from the smallest dwarf variety, which only grows to 6 inches tall, to the tallest variety which is up to 4 feet tall.
|Mature Size||6 in. to 4 ft. tall|
|Sun Exposure||Full sun|
|Soil Type||Rich, moist, well-draining|
|Soil pH||Neutral to slightly acidic|
|Bloom Time||Late spring, early summer, and late summer|
|Flower Color||Varying shades of purple, blue, white, yellow|
|Hardiness Zones||3 to 9, USA (depending on variety)|
|Native Area||Europe, Asia, North America|
|Toxicity||Toxic to humans, toxic to pets|
Different species of irises require slightly different methods of and timing for planting. Bulbous irises, which includes Dutch, Spanish, and reticulata irises, are planted in the fall in full sun in well-draining soil.
To plant your bulbs, loosen the soil, then mix in compost and 1/4 cup of all-purpose granular fertilizer according to the directions on the bag. Situate the bulbs 4 to 5 in. deep, depending on the type of bulb. For bearded irises, position rhizomes horizontally in the soil, leaving the top of the rhizome partially exposed. For other varieties, position the crown of the plant 1/2 to 1 in. below the the soil line.
Once flowers are spent, deadhead the blooms. When the whole stalk of flowers is spent, cut the stalk to the ground to direct energy to the roots, instead of forming seed heads. Once the first heavy frost comes or your leaves yellow for the season, you can cut your iris foliage to the ground to prevent iris bores from overwintering in the leaves. However, do not be tempted to remove the foliage before then, as the greenery is still performing photosynthesis, providing the plant with energy necessary for next year’s blooms.
Once the foliage is trimmed back for the winter, be sure to cover the rhizomes with something to help protect them, such as sand or mulch. Remove this in the spring. Irises are a great choice for areas with wildlife as they are deer resistant. Common pests include iris borers.
Most iris varieties do best in full sun. Some varieties can tolerate partial shade, but too much shade will prevent them from flowering.
Rich, well-draining soil is best for iris plants. Though they like moist soil, too much water can be damaging. If you are worried about too much standing water, try planting your irises in raised beds, as this will allow for optimal water drainage. Japanese and Louisiana irises can tolerate moist soil and are excellent for areas near ponds. Siberian irises prefer acidic, moist soil.
Because the iris likes both moisture and well-draining soil, watering consistently and deeply is very important. Just be sure not to overwater, as too much water in the soil can cause problems such as root rot. Though they appreciate consistent water, most iris varieties are drought-resistant and won’t die quickly if they are deprived of water for a short time.
Temperature and Humidity
With its wide range of varieties and growing zones, the iris is a hardy plant that can tolerate fluctuations in temperature and humidity. As long as the soil is well-draining and they get plenty of water and sunshine, these flowers can thrive in a large variety of gardens. Siberian, Bearded, and Japanese irises typically are hardy in USDA zones 3-9; Iris reticulata and Dutch iris are hardy in zones 5-9; and Louisiana iris prefers zones 6-9.
Because irises prefer rich soil, compost makes a perfect amendment. Loosening the soil in the spring and adding a healthy layer of compost will help give your irises the nutrients they need to grow healthy and lush.
If you do not have compost, a well-balanced fertilizer for flowers works well. Just beware of too much nitrogen, which can lead to rot. Because some varieties bloom twice, once early in the season and once later in the season, these varieties will appreciate another dose of fertilizer before their second bloom.
- Yellow Iris: Also sometimes known as "flag," this variety of beardless iris is highly tolerant of moisture. It is an abundant spreader, which sometimes deters gardeners from planting it. However, it makes a great container flower variety and produces beautiful blooms and bright green foliage.
- Louisiana Iris: This iris is native to the U.S. and hardy to zone 6-9. They boast a wide variety of colors and petals that are reminiscent of lilies.
- Japanese Iris: The Japanese iris boasts large, broad petals, and a stunning array of colors. This variety also does well with increased soil moisture, making it the perfect choice in areas with high water tables or a greater chance of standing water.
- Siberian Iris: The Siberian iris provides smaller, more delicate-looking blooms than many other iris species and adds a beautiful pop of color in the late spring.
Irises spread underground through rhizomes or bulbs and will need to be divided every 3 to 5 years, creating the perfect opportunity to spread your irises to new landscaping areas. You will know when it is time to divide when you have fewer blooms or there are rhizomes popping out of the ground. Follow these basic steps to propagate iris plants:
- Wait for 6 to 8 weeks after your irises have finished blooming. Then, with a garden fork or shovel, slowly work around each plant to loosen the rhizomes or bulbs.
- Gently lift the iris out of the ground and shake out the dirt.
- Once the dirt is removed you will be able to see the rhizomes or bulbs. You will find smaller rhizomes spreading from the larger mother rhizome. Some may come away naturally while others will need to be cut. Either way, divide these smaller rhizomes and toss any old, shriveled rhizomes.
- Once you divide the rhizomes and remove any that are spent or diseased, simply plant the divided iris plants in a new location.