Garden Q&A: Factors besides weather could have affected irises

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Q. I have a large bed of irises that also has touch-me-nots and coreopsis. This year, very few of the irises bloomed, although they grew to their normal size and looked healthy. They are not too crowded. Did the winter weather affect it as it did other plants, or do I need to fertilize them? What kind of fertilizer should I use? Due to the other plants in the bed, it would be difficult to work fertilizer into the soil. 

A. You do not say which kind of iris you are growing, but tall bearded and Siberian irises bloom best in full sun – at least six hours a day – and evenly moist, well-drained soil. Irises do not seem to have taken the hit that many other plants suffered this past winter. However, there are other factors that keep them from blooming well.

Are your irises not overgrown because you divided them recently? If so, they may take a growing season to re-establish and reach blooming size again. Also, irises prefer to be planted very shallowly, with the rhizomes just at the soil surface. If they are planted too deeply, they produce all leaves and few or no flowers. Do not mulch over the rhizomes, as mulch can hold too much moisture and cause them to rot.

Although the irises are not overgrown, are the other plants in the bed impinging on their space? Touch-me-nots (Impatiens spp.) can grow quite tall, and they may be casting shade on your irises, and competing with them for moisture and nutrients. If you do not have room to apply fertilizer because there are so many plants in the bed, it may be time thin out the touch-me-nots and coreopsis.

Fertilization may be helpful but should be based on a soil test. That is the only way to tell for sure what nutrients are lacking and how much of which fertilizer will correct the deficiency. Be careful with high-nitrogen fertilizers, because they push lush foliage growth and no flowers. Soil test kits are available from your local Penn State Extension office. In Allegheny County, soil test kits are $12 for the first kit and $9 for additional kits ordered at the same time. They come with instructions for taking a good soil sample and understanding your results. Make checks payable to Penn State Extension and send to Soil Test Kit, Penn State Extension, 400 N. Lexington St., third floor, Pittsburgh, PA 15208.

Q. I started a compost pile last year, but I got lazy about turning it. Everything still seems to be decomposing. Do you really have to worry about carbon to nitrogen ratios and turning the pile frequently?

A. Composting fanatics will say “yes,” but the lazy method of composting works as long as you are not in a hurry for the finished product. The instructions for strict carbon to nitrogen ratios (preferably 30 parts carbon to one part nitrogen) and frequent turning are intended for those who want to make a lot of usable compost in the shortest time possible. The lazy method takes six months to two years for the finished product, while intensively managed compost piles can be ready to use in 30 days.

Although it is not a good idea to compost diseased and heavily insect-infested plant material, you should definitely avoid doing so if you use the lazy method of composting. Even intensively managed compost piles may not heat up enough to destroy insect eggs and disease-causing organisms; the lazy composting method definitely will not.

Send questions to Sandy Feather by email at slf9@psu.edu or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.


Send questions to Sandy Feather by email at slf9@psu.edu or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.

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