Q: I understand there's been progress in developing Dutch elm disease-resistant American elms that are also relatively fast growing. Do you have any information and varieties you can recommend? - Dan Zink, Oxbow, N.D.
A: American Elm is making a comeback thanks to disease-resistant cultivars, and North Dakota State University's Prairie Expedition Elm tops the list. The parent tree was discovered along the Wild Rice River south of Fargo, growing healthy among a stand of elms dead from Dutch elm disease. Clones of the tree were injected with the disease and found to be resistant. The cultivar was named Prairie Expedition Elm and released in 2004 to commemorate the 200th anniversary of the Lewis and Clark Expedition.
Prairie Expedition is a beautiful, classic American elm umbrella-shape, with branches arching gracefully upward. It's a full-size shade tree, growing to 60 feet in height and 40 feet wide, so it's perfect for boulevards, parks or yards where a large shade tree is desired. It's moderately fast, averaging three to four feet of growth per year.
Prairie Expedition can be found at local garden centers, along with other resistant elm varieties like Cathedral, Triumph, St. Croix, Jefferson and Discovery.
Q: There are iris growing around the Moorhead, Minn., home in which I grew up, and they need dividing. Are they worth dividing considering they're about 60 years old? If so, is August the preferred time? - Mary Gnahn, Fargo.
A: August is the preferred time to dig and divide iris. Whether these older irises are worth saving depends on flower quality and personal preference. Some of the newer varieties are greatly improved over older types. For example, when I was growing up we had iris with small flowers in a washed-out lavender color and the foliage was always poor. We opted not to save them. In making the decision, the age isn't as important as whether you feel they are attractive or not.
Today's iris cultivars are much-improved. Many of the colors are more interesting, the blossoms are much larger, many have stronger flower stalks to prevent flopping, and the blue-green foliage is an attractive accent, even when the plants aren't blooming.
Q: The foliage of our rose bush has turned pale yellow the last two years. The plants are over 100 years old and have never done this before. We've applied an all-in-one rose and flower product. - Sandra Kurzweg, West Fargo.
A: The photo you sent shows rose leaves that have turned yellow-white in color, with the veins remaining green. These are classic symptoms of iron deficiency chlorosis, and roses are among the plants commonly affected. The product you applied is useful for insect and disease control, but usually doesn't contain enough iron to remedy this situation.
Iron deficiency can be caused by insufficient iron in the soil, or the existing iron is in a form not accessible by plants, or the plant's ability to use the iron has been compromised by injury, drought, excess moisture, etc. To remedy iron chlorosis, apply chelated iron, available at garden centers. For fastest absorption, use an iron product that can be applied to the foliage, but also apply chelated iron to the soil around the plant for longer-term results, following label directions.