For Some Irises, Divide to Conquer

Q I have irises that are planted in a bed that receives full sun half of the time. Every year there is profuse foliage but very few flowers. What can I do to get more?

ABearded irises need full sun all day long to bloom well. Sometimes, dividing them and replanting will result in more flowers if the clumps have begun to crowd themselves. If that doesn't work, consider planting other irises that do well with a bit of shade.

Siberian iris will tolerate some light shade, and they do not need to be divided as often as bearded iris. The flowers are smaller, but each clump tends to produce more flower stalks, so the impact of the flowers in the garden can surpass that of bearded iris. Unlike bearded iris, Siberian irises tolerate heavy, wet soil.

If you have a woodland garden, the petite woodland native crested iris is a very nice addition. It needs moist soil based on leaf mold to grow well. If happy, it may form a tight ground cover. Another iris worth looking into is the Japanese roof iris. Given good, loose, well-drained soil, it will tolerate considerable shade, giving hostas a run for their money.

You recently attributed a reader's problems with dying oaks to a series of possible problems, from saturated soils, irrigation systems, lawn herbicide and soil disturbance from construction. But haven't you heard of sudden oak death, oak wilt, oak decline or the red oak borer? I live in Clarksville, where we are losing the beautiful old oaks.

While insects and disease can be factors in the death of oaks, the insect pests have evolved to finish off failing trees that are in decline for other reasons.

Oak wilt disease is strongly associated with tree wounds, a common problem in the vicinity of construction work. Sudden oak death, more properly known as ramorum blight, picks on plants that are drought-stressed and kills individual trees, not entire forests. Both oak wilt and ramorum blight have yet to be reported on oaks in your area. Bacterial leaf scorch is a far more common problem where you live, and some entomologists are also beginning to wonder whether the soil-borne larvae of the 17-year cicada, which emerged as adults in 2004, did far more damage to roots than previously understood.

And I have to reiterate that construction can have serious direct and indirect effects on oak trees, which generally are upland species that don't tolerate as much injury as bottomland trees such as sycamore and maple.

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