Issue: July 31, 2004

Irises didn't bloom well

My irises have healthy leaves, but out of 30 plants I am lucky if there are 2 or 3 blooms. What do I need to do to increase the blooms? Many thanks.

Frances M.
Las Cruces, NM

There are several things that can cause this problem. The plants may be too crowded. As the irises grow, the clump begins to crowd itself and must be divided. Another possibility is that the surrounding landscape (trees and shrubs) may have grown and begun shading the irises. Fertilization with a fertilizer high in nitrogen will stimulate vegetative (non-flowering) growth rather than flowers. Lack of water in late winter and early spring may also prevent proper growth.

To remedy the problem, determine which of the scenarios described above is most likely to fit your situation. If none seem likely, contact your local Cooperative Extension Service agent who can make a diagnosis after seeing the problem on-site.

It may be a good idea to divide and replant your irises. Find a sunny location receiving at least 6 hours of direct sunlight each day. Prepare the planting site by digging the soil deeply to loosen compacted soil. As you are turning the soil, add amendments such as compost and a phosphate fertilizer. If you use a complete fertilizer that contains nitrogen, phosphorus and potassium, be sure to select one that has a high phosphorus content (the second number of three large numbers on the bag). You can also use superphosphate or colloidal phosphate. Both of these are sources of only phosphorus (no nitrogen of potassium) and will not "burn" the plants. If the site chosen is the existing site, remove the iris plants and store them in a shady location until you are ready to replant.

After you dig your iris clumps, divide the irises by cutting them back to retain just a few inches of rhizome (the thick, horizontal stem) and a "fan" of leaves. You can trim the leaf fan back to 3 or 4 inches in length, then plant the rhizomes so that they are at the surface or only half covered. Water them at planting, and thereafter twice per week. As winter arrives, reduce watering to once per month. In late winter, as the irises begin to grow (for Las Cruces probably in January), increase watering to twice per month. Irises are well adapted to dry conditions during the summer, and watering twice per month should be sufficient. Some gardeners water more often, but this is a time of relatively little growth, so don't overwater.

When the clump becomes dense (after two to three years), you will need to divide them again. In the meantime, enjoy them as very well adapted xeriscape plants.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!