The irises that were planted decades ago in the backyard do blossom most years, but haven't flowered yet. Should I fertilize them in the winter? Why don't they bloom? We did have a few days under 40 degrees over Christmas, and other places I've lived in L.A. have blossomed like mad! We're at 1800 feet here, with rather acidic soil and very alkaline water. Help?
Hardiness Zone: 10a
Brandy from Los Angeles, CA
Here are some ideas:
Because your daffodils are also failing to bloom, I suspect you may have a problem with your soil. You mentioned having slightly acidic soil and very alkaline water, but you didn't mentioned what type of irises you're growing. Although most irises are pH tolerant, some perform prefer slightly acidic soil, while others prefer a neutral to slightly alkaline soil. Just to be safe, I would recommend getting a soil test done. Your county extension agency can give you more information. Tests usually only cost around $15 and the results will tell you exactly what you need to add (if anything) to correct soil problems-including problems with pH and fertility. For example, test results may recommend adding sulfur to lower the pH of your soil, or recommend adding limestone to raise it.
Your soil may also lack sufficient organic nutrients. Irises can be heavy feeders, and they don't like to compete with nearby grass, weeds, or other iris rhizomes for nutrients. A light application of fertilizer in the spring and again 4 weeks after blooming is usually more than sufficient. It is also best to avoid fertilizers high in nitrogen. Again, a soil test will provide you with specific recommendations.
Planting Depth and Spacing
Iris rhizomes prefer to sit near the top of the soil with only their roots buried below the surface. They also like to have some breathing room, and should be divided every 3-4 years when they become overcrowded. Here is a great bulletin from the UC Davis Extension Agency on growing all types of Irises.
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As soon as leaves appear in the spring, add a 20-20-20 fertilizer. (05/31/2008)
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