Irises' Sunny Disposition

By Scott Aker
Special to The Washington Post
Thursday, October 30, 2008

Q I have 40 or more irises that I dug up last year, cleaned, trimmed and replanted, as I only got two blooms. This year I got three. The leaves are lush and vigorous, but why so few blooms? They receive about four to five hours of filtered sunlight.

A I assume you are talking of the common bearded or German iris. The amount of sunlight you have is not enough to support flowering. Move them to a position where they get at least eight hours of full sun, and they will bloom well for you.

If you still want irises in that bed, try growing a species that tolerates a bit of shade. If the area has ample moisture, you could grow blue flag ( Iris versicolor) or Southern blue flag ( I. virginica). If you have drier conditions, try the diminutive crested iris ( I. cristata). The Japanese roof iris ( I. tectorum) and the stinking iris ( I. foetidissima) also like shade. Although these species are not as large as bearded iris, they are lovely and are not prone to some of the pest and disease problems that plague other irises. You can find a good selection of plants available by mail order at Iris City Gardens (; 800-934-4747). The nursery doesn't take orders until January and ships only from mid-July to late September.

My roses have been badly affected this year by black spot. I tried spraying with a product that combines an insecticide with a fungicide, but with not much improvement. Curiously, one rose has not had the disease in the two years I have been growing it.

Along with rust and other fungal diseases that thrive in wet periods, this has been a banner year for black spot. The frequent rain and humid conditions have challenged even the most disease-resistant roses.

The key to controlling black spot is to apply fungicide regularly, beginning as soon as the leaves are fully expanded -- generally in late April or early May. Spray the plants weekly to prevent any problems.

It's also a good idea to alternate fungicides to prevent the fungus Diplocarpon rosae from developing a resistance to certain fungicides. Cleary's 3336, Immunox and Funginex are just a few of the many fungicides labeled for rose black spot control.

The rose that has not been afflicted with black spot is obviously resistant to the disease. Breeders have been working on this in recent years, particularly in new varieties of landscape roses. They have smaller flowers than hybrid teas but generally do not need to be sprayed with a fungicide. Some species, such as Rosa rugosa, are also resistant to the disease and need not be sprayed.

Scott Aker is a horticulturist at the U.S. National Arboretum.

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