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Garden advice: How to care for bearded iris after they bloom

Carol Savonen
Special to the Statesman Journal
Siberian Iris is a moisture-loving perennial.

Question: I have mature bearded, Siberian and Japanese irises in the garden at the older house we bought this year. These have all finished blooming. How do I best care for them now through the summer?

Answer: Bearded iris are the largest in size. You should be able to see the tops of the rhizomes (underground fleshy stem) all summer, so do not mulch, or these will rot. Cut the spent blossoms off close to the base of the flowering stem after all buds have bloomed. Leave all the healthy leaves on to photosynthesize and feed the rhizome for next year. Remove diseased or insect-infested leaves.

If you want to divide your iris to thin an overcrowded patch or transplant, do so in the late summer. In the fall, trim the leaves to about six inches tall. Be sure to weed and keep grass out of your bearded iris patch. You can mulch lightly in the winter with leaves or bark. 

Beardless irises include the Siberian and Dutch irises, which grow from rhizomes and the Dutch iris, which grows from a bulb. Our native iris here in the Pacific Northwest are beardless as well and can interbreed with the Siberians. 

Siberian iris - Divide in late summer and fall if your patch is several years old and getting sparse in the center. Can get by with deep watering once per week. Little other maintenance needed. Does best in slightly acidic soil that is seasonally wet. A thin cover of mulch is good. Resent transplanting may take a few years to spread and set a lot more stems. Deadheading is not necessary for plant health. 

Japanese iris - These have bigger, showier flowers than the Siberians. Divide in late summer and fall if your patch is several years old and getting sparse in the center. Does best in more acidic and richer soil than bearded or even Siberian irises. Do not fertilize with lime or bone meal. Likes seasonally wet soil and regular moisture all summer. Mulch with 2 to 4 inches of mulch to help maintain dampness. Cut back after frost and transplant about every three years, as the rhizomes tend to heave out of the soil over time.

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Oregon has an invasive species of iris— a gorgeous, large yellow species of iris called "yellow flag" or "yellow water iris" (Iris pseudacorus). Sold in local nurseries and garden stores, these Old World iris have escaped into wetlands, along riverbanks and near ponds all over the state. I’ve seen large patches while paddling the Marys River, which flows through my backyard. 

The big problem with this lovely perennial iris: It is invasive and out-competes native riparian vegetation, including cattails, sedges and rushes, and it degrades native fish habitat, as well as bird nesting and rearing sites. To learn more and see what it looks like, see https://www.portlandoregon.gov/bes/article/172630

Want to plant new iris? Here are some pointers: 

  • Plant irises July through September in a place with at least half a day of direct sun. They all like well-drained soil. Plant bearded iris shallowly, so you can still see the rhizome.
  • Avoid planting yellow flag if you live near water or have a water feature. 
  • Consider bulb irises as well. These are the Dutch iris, that bloom earlier in the spring than the other types. 

For more information on iris varieties and care, these local websites are helpful: 

Carol Savonen is a naturalist and writer. She is an associate professor emeritus at OSU and tends a large garden in the Coast Range Hills west of Philomath with her husband and dogs. She can be reached at Carol.Savonen@oregonstate.edu or c/o: EESC, 422 Kerr Admin. Bldg., OSU, Corvallis, OR 97331.