by Ann Davenport

Quaking Aspen (Populus tremuloides) has become a staple in many northern landscapes. We love its straight form, smooth gray-green bark, and the way its leaves quiver in the wind. Its strong presence is able to bring a little bit of the north woods to our urban landscapes. Sometimes despite our best efforts, pests and or disease can put a blemish on our otherwise well manicured space.
The U.S. Department of Agriculture Forest Service has categorized Hypoxylon Canker, caused by the fungus Entoleuca mammata, as one of the most important diseases affecting Aspen in our area (Anderson, R., Anderson, G., & Schipper, A., 1997). The fungus enters the tree through wounds in the bark. Wounds can occur through insect damage or mechanical damage (pruning, mowers, etc…). Initial infections often occur in small branches. With age, the infection moves into the main stem of the tree. Unfortunately, the prognosis for trees infected with Hypoxylon Canker is grim. Once the cankered area reaches the main trunk it will girdle the tree. Most trees that reach this stage will die within five years.

Here are some things to look for when monitoring Aspen for signs of infection:
• Leaves on the infected branch(s) appear smaller than normal, turn yellow, then brown
• Cankers usually begin near wounds, branch stubs, or at the base of branches
• New infections are marked by sunken bark which is initially yellowish-orange, turning black at the center with yellowish-orange margins as the infection ages
• During early infection, the bark of the canker has a blistered appearance
• With time, the blistered area breaks apart and falls off revealing blackened wood
Unfortunately, there is no chemical treatment for Hypoxylon Canker. Trees with main trunk infections should be removed to prevent further spread to nearby Aspen. Routine monitoring of trees in the landscape can help identify early signs of the disease. Removal of infected braches before the cankered area reaches the main trunk may delay infection. This should be done during dry periods to minimize spread of the fungus. In general, pruning cuts should only be made when necessary and sterilization of equipment between cuts is recommended to minimize risk of spread. The best way to protect your Aspen from this disease is to provide an environment that encourages tree vigor; appropriate fertilization, and adequate water to prevent drought stress.

Source:
Anderson, R., & Anderson, G., (1997). Hypoxylon Canker of Aspen. U.S. Department of Agriculture Forest Service. Forest Insect & Disease Leaflet 6. Retrieved August 1, 2010 from http://na.fs.fed.us/spfo/pubs/fidls/hypoxylon/hypoxylon.htm

Do you have that itch to get out in your garden yet?  I know your garden is filled with snow and that it is falling as I type, but there are some things you can do right now to get your garden off to a great start for the 2010 landscaping season.

Deciduous trees and shrubs need regular pruning than other plants, but fortunately, it is relatively easy.  Dormant pruning is done to produce strong, healthy, and attractive plants that are safe to be around.  Proper procedures should begin when the plants are young to produce a strong structure and to eliminate potential problems when plants are young and small.  Listed below are a few reasons to prune:

 Safety

• Eliminate dead or dying branches that are injured by disease
• Remove branches that cross and rub against each other

 Aesthetics

• Encourage flowers and fruit
• Shape limbs that could damage people or property
• Eradicate branches that have poor angles resulting in their susceptibility to snapping in high winds or ice
• Eliminate branches that obscure intersections or cross with power lines

 Plant Health

• Shape, i.e. a hedge
• Control size
• Remove suckers and water sprouts

Trees and shrubs that bloom later in the growing season should be pruned when they are leafless and dormant, just before the new growth begins for the season.  This is typically in February when new tissue develops rapidly in response to spring.  Another reason to dormant prune is to see the overall architecture of the plant you want to prune.  The leaves aren’t there to muddle your views.  Dormant pruning also prevents the spread of oak wilt disease.  It is a disease that is dispersed by spores between the months of April to June.  Do NOT prune an oak tree during these months.

One exception to dormant pruning involves trees such as maples and birches.  The tendency for them to bleed in the early spring can be unsightly.  It is best to prune those types of trees well after the trees have leafed out in the summer.  Below is a diagram depicting a proper pruning cut on a tree:

 

There are two primary pruning methods, renewal and rejuvenation. Renewal pruning cuts out older stems near the base.  This stimulates the growth of younger stems and keeps the shrub flowering and fruiting.  Subsequent pruning may need to occur to maintain the shape of the shrub.  Understanding the natural shape of shrubs will help you determine how to prune.  Shrubs that respond well to renewal pruning include lilacs, red twig and yellow twig dogwood, some species of viburnum, forsythia, mockorange, and weigela.  Most of these shrubs are pruned within two weeks after they flower in the spring. 

The second type of pruning, called rejuvenation pruning, involves cutting stems down to the ground. This method is useful when a shrub has become overgrown with many stems growing together. Some shrubs that respond well to rejuvenation pruning include Anthony Waterer spirea, honeysuckle, snowberry, and privet. Early spring is the best time to initiate this type of pruning.  There is a rejuvenation program that takes three years to complete but the overall attractiveness of the plant is maintained.

Year 1: Remove 1/3 of the oldest canes that are not productive anymore. 

Year 2: Remove one half of the old stems. 

Year 3: Remove the remaining old branches.  New stems will quickly grow to fill in where the old wood was thus creating a more lush plant.

 

A lot of cuts are made on suckers which are vigorous vertical stems that are unsightly. The best way to get rid of suckers is to rip them out of the ground early in the spring while the stems are still small.  Opposed to pruning, this method removes the bud and will discourage the growth of new suckers. 

Finally, recent studies have shown that applying a paint or wound dressing to the cut will not prevent decay like it was previously thought.  The tree or shrub will form a callus on its own that will close over the wound and protect itself.  You also need to disinfect cuts between each cut with a product such as Lysol.  This will prevent passing diseases on to the next plant.

With these pruning tips and methods you will be armed with the knowledge to make thoughtful and decisive pruning cuts in your landscape.  Remember that if you make a mistake, you can always try again next year.

Shannon and I were driving to a client site yesterday via 42nd St. in south Minneapolis. WOW! The entire street is lined with crab apples - and, in this high season, is literally draped in pink and white flowers. I think our crab apples must rival the Japanese cherries in this glorious time. The flowering doesn’t last long so if you have time head down 42nd St. between Hiawatha and the river, or the east side of Lake Nokomis, or the MN Landscape Arboretum or the Japanese Garden at Normandale College.

Sometimes it takes a grove of a particular tree to express its positive characteristics. If you are driving down Colfax somewhere around the 48 hundred block you will suddenly notice a grouping of trees that give the block an ethereal feel. These are tamaracks - a tree that can easily look out of place by its lonesome, but in natural clusters takes on a totally different hue.

Many Minnesotans share a fondness for tamarack trees built up from trips to the cabin where the trees bright green early season leaves and especially the trees brilliant gold fall color make it remarkable. But few take advantage of these characteristics in their urban homes. Take a drive by the trees on Colfax and see what you’re missing.