Because of its graceful, umbrella like form, the stately elm has long been a favorite shade tree across much of the country. Unfortunately, this favorite of shade trees is subject to a number of damaging pests and diseases. Most of the agents that cause serious damage to the American elm and other elms have been accidentally introduced into North America from Europe and Asia.

One of the most common insect pests of elms along the Colorado Front Range is the European elm scale, known to entomologists by its Latin name, Gossyparia spuria. This is a sucking insect that colonizes the branches of both American and rock elms in large numbers and sucks plant juices from the stems. Heavy infestations, over long periods, can weaken branches, causing premature yellowing and dropping of leaves and dieback of twigs and branches. An indirect effect of this insect's feeding is the production of honeydew, a sweet sticky material. If you've ever parked your car underneath a tree infested by aphids, scales or mealybugs, you have undoubtedly found it covered with droplets of sticky honeydew the following morning. Honeydew is a medium for the growth of fungi, especially the sooty molds. Elms infested by European elm scale typically have branches in their crowns that are black from sooty mold growing on honeydew deposits.

Unfortunately, this insect generally does not cause severe damage, although the sooty mold covered branches are conspicuous and unsightly, especially in winter. It is also a difficult insect to control. An interesting aspect of European elm scales is that their "blood" is an intense orange brown color and could be used as a natural dye. Adults of a related insect, the cochineal, which infests prickly pair cacti, has a vivid red "blood" and has been widely used as a commercial dye.

Another common insect pest of elms is another European import, the elm leaf beetle (Pyrrhalta luteola). Both the larval and adult stages of this beetle feed on elm leaves. The larvae skeletonize the undersides of leaves, leaving only the veins and upper surface of the leaf tissue. Adult feeding can chew holes in the leaves. Damaged foliage turns a dull, ugly brown color. Elm leaf beetle can have from one to three generations a year and damage is most conspicuous in late summer. The adults have another pesky habit. They like to spend the winter months in warm dry places like attics, barns and sheds. During periods of warm weather, the beetles become active and crawl into living quarters or onto windows.

Adult elm leaf beetles are fairly easy to recognize. They are yellowish to dull green and have a black stripe down the sides of the wing covers. Infestations on elm trees are easily controlled with applications of insecticides such as carbaryl (Sevin) or malathion.

The most devastating pest of elm, without a doubt, is Dutch elm disease. This disease is caused by a fungus, which has the scientific name Ophiostoma ulmi, which invades the woody tissue and plugs the vessels that carry nutrients from the tree's root systems to the crown. The first evidence of Dutch elm disease infection is the occurrence of branches with yellow wilted leaves in the tree's crown. Tree death is usually rapid. The fungus can spread from an infected tree to neighboring elm trees by root grafts.

Dutch elm disease has a symbiotic (beneficial) relationship with several species of bark beetles. The most common of these is the smaller European elm bark beetle (Scolytus multistriatus), a native of Europe. These beetles breed in the inner bark of trees dying from Dutch elm disease. Emerging beetles, carrying spores of the Dutch elm disease fungus feed in the branch crotches of healthy elms. The feeding creates small wounds, which are inoculated with fungus spores thus spreading the disease.

Dutch elm disease is native to Asia and arrived in North America via elm veneer logs infested by beetles carrying the Dutch elm disease fungus. This disease was devastating to many communities in the East and Midwest where most of the shade trees were elms.

The most effective control for Dutch elm disease is sanitation. This involves rapid detection and destruction of infected trees by chipping. In addition, breeding sites for elm bark beetles, such as elm firewood containing bark must be eliminated. Forestry officials with the City of Fort Collins have done an exceptionally good job of managing our community's elms. As a result, our streets are still graced with many stately elms. An excellent example is the CSU oval, which has many large umbrella shaped American elms. Beautiful trees such as these have long disappeared from many other communities.

Homeowners in Larimer County, who need advice on pests affecting elms or other trees, can contact either the Larimer County Cooperative Extension Service, or the Colorado State Forest Service, to help identify the problem and the most appropriate course of treatment.

Q: I have a daylily that is about five years old. It is very big and has always had beautiful blooms. This year, it does not have as many blooms as usual. What is wrong?

A. Many perennials must be divided after a number of years growing. Daylilies should be divided every three to five years. This can be done when the plant is through blooming, preferably in the fall. If the plant is really big, it will take two people with digging forks or shovels to get it out of the ground. Divide the clump any way that seems to work. You can put two digging forks back-to-back and pry the clump apart or you can use a sharp knife to cut the fleshy roots apart. Divide it into pieces, each with a nice-sized section of roots and sword-like foliage. Don't divide the clump into too many pieces as it will take them a long time before they bloom. Just get some good pieces and you will be able to have a few new daylilies to plant or to give away.

Q: I would like to fill in a perennial area with a few annuals for color. Is it too late to plant them?

A: Temperatures are usually high in July and planting is risky. However, plants purchased from garden centers have roots which can do well if the plants are properly watered and mulched.

Q: My columbine grew tall and flowered early this year. Is it okay to cut them back?

A: Cutting foliage and removing spent flowers is routine maintenance for perennials. Foliage will grow and you may enjoy a second flowering in the fall.

Mulch not only helps keep your soil cool and moist, but also prevents soil compaction and improves the soil's texture. To help prevent weeds, fine mulches should be applied 1 to 2 inches deep, while coarse or fluffy mulches should be 3 to 4 inches deep.

The benefits of core aeration and efficient watering begin to pay off during July's warmer days. If you did not aerate your lawn this spring, consider doing so this fall to improve the soil and root-growing environment. Core aeration is also a great first step for planting new grass seed in the fall.

Grass clippings that are allowed to fall back onto your lawn decompose quickly and provide a good source of nutrients for the soil. Mulching mowers slice the clippings into tiny pieces and blow them back into the turf. You can achieve the same benefits with a side-discharge mower by mowing frequently enough to remove only 1/3 of the turf's total height.

The authors have received training through Colorado State University Cooperative Extension's Master Gardener program and is a Master Gardener volunteer for Larimer County.

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This page updated:  July 8, 2004