Yard & Garden Line News Volume 6 Number 18                                                               December 1, 2004

Features this issue: When to Prune Deciduous Trees Using Shredded Tires as Garden or Landscape Mulch Itch Mite Found in Midwest Editorial Notes

When to Prune Deciduous Trees
Patrick Weicherding, Regional Extension Educator, Andover Regional Center

Pruning  chart Image credit: Patrick Weicherding

A common question around this time of year is "When is the best time to prune trees?" Mike Zins, former Extension Horticulturist with the University of Minnesota Extension Service, used to say "When your pruners are sharp." We know this "tongue-in-cheek" adage can be applied any time of the year for light pruning - just removing small, damaged, or diseased branches - but more extensive pruning should be done in late winter to early spring, in March or April once the threat of severe cold weather has passed.

Many of you probably recognize that pruning in late winter to early spring is the standard "dormant season" pruning recommendation, but how many of you know "the science" behind this standard?

Throughout a 1-year cycle, a tree has five major phenological periods as illustrated in the accompanying illustration: The tree's stored energy fluctuates as a result of what occurs during these periods. During period one from February through March, the tree is coming out of dormancy. Growth is beginning but very little photosynthesis is taking place. At this point, the tree is relying on reserve energy from the previous year.

In period two (April) bud break and leaf formation are the major growth activities. Because photosynthesis is still minimal at this time, the tree must rely on the energy from its energy reserves. The tree begins to enter its lowest energy level during the growing season. Insects and pathogens are also becoming active at this time.

During period three from May through July, photosynthesis increases dramatically. The new leaves begin to produce more energy. Most of this energy is being used up in the process of leaf formation in the early part of the period. All of the tree's terminal and radial growth occurs within 6-8 weeks following full leaf expansion.

In period four (August through September) the tree begins its annual storage of energy for the following year. Much of the seasonal growth is complete and the tree enters period five (October through January). This is the dormant period when the tree is at rest as it idles through winter.

From the above discussion it should be obvious that pruning during the active part of the growing season (when the tree has little or no energy reserve to expend on wound closure) has the potential to change the growth of the tree. Removing living foliage, by pruning, affects the tree's physiology and future growth. The reduction in leaf area that results from pruning will reduce the tree's overall photosynthetic capacity and may reduce overall growth on the pruned section, or the entire tree. Pruning during the growing season can therefore have a lasting impact on the viability of the tree and can directly influence it's longevity.

So, what's the best time during the dormant season to be pruning trees? The late dormant season is best for most pruning on deciduous trees. The advantages of late dormant season pruning include:

ºFirst, wound closure is most rapid if it's done just prior to when new shoots emerge. Pruning too early in the dormant season allows the pruning wounds to crack and dry out, delaying wound closure. (Most research on the timing of pruning indicates that pruning cuts close more slowly in the fall than in any of the other seasons.)

ºSecond, there are few insects and disease spores active to infest pruning cuts. (This is the reason we don't recommend pruning oaks and elms during the growing season - the risk of Dutch elm disease and oak wilt gaining a foothold in fresh pruning wounds is too great.) The chances of serious invasion of pruning wounds by decay-causing organisms are at their height during the fall coloration period.

ºThird, deciduous trees at this time of year have not yet produced new leaves making it easier to make pruning decisions - you can easily see the plant branch structure without leaves.

Some trees such as elm, maple, walnut and birch are "bleeders" when pruned in late winter to early spring. This oozing sap is annoying when it drips on cars and sidewalks. There is scientific disagreement as to whether the oozing is harmful to trees. Some researchers believe that the oozing might interfere with the closure of pruning cuts while others argue that it is a positive benefit in that it reduces the probability of colonization of the pruning wound by decay organisms. If you are concerned, prune these species in the winter but keep in mind that this is not the best time of year to prune most trees.

Finally, what about the wound dressing question? Wound dressing or pruning paint is not essential; recent research has shown that pruning paint or wound dressing is not as advantageous as previously thought. Dressings may actually harbor disease organisms rather than exclude them. It has also been determined that wound dressing slows the formation of callous tissue over the wound. Wound dressings and pruning paints are strictly cosmetic.

The only reason for painting a pruning wound is to aid in the prevention of oak wilt during the growing season. Latex paint can be used to discourage sap beetles from visiting the wounds. Sap beetles are thought to be an overland vector of the oak wilt fungus.

If you're interested, Bonnie Appleton and Susan French from the Virginia Cooperative Extension Service have developed a nice "Deciduous Tree Pruning Calendar" that lists the best (and worst) times to prune specific trees. Temper their recommendations with the fact Virginia's species diversity and climate are not the same as ours in Minnesota. The Adobe Acrobat pdf-file can be found at:
http://www.ext.vt.edu/pubs/nursery/430-460/430-460.pdf

Using Shredded Tires as Garden or Landscape Mulch
Deborah Brown, Extension Horticulturist

Colored chips. Photo credit: Close the Loop

Here's some information to tuck away for next spring, when you go shopping for mulch to use around trees and shrubs or in your garden:

Bags of shredded rubber tires are appearing in "big box" building supply stores, discount centers, and garden centers for use as mulch in gardens, landscapes, and play areas. There's some controversy about the safety of those different applications, particularly when the tire chips are used to mulch edible crops. Not all of these products are marketed for use in gardens, however.

It's really not possible to generalize about the safety of shredded rubber products as they vary in composition, even though they're all made from tires. Since about 1970, many automobile tires have been manufactured with steel belted reinforcements. Those steel wires are coated with zinc, to prevent rust.

Some shredded tires are just that: tires that have been shredded and bagged, "as is." They may even have been dyed a more attractive color, but they contain shredded steel belt reinforcement wires. Though there's been little research, some people – particularly those who read or write for "organic" publications – are concerned that as the tire chips wear, these metals will leach out and contaminate the soil.

Leaching is reportedly worse when the mulch is on acidic soils. It's thought to be less a problem on neutral or alkaline soils, like we have in most of the state. (The primary exception is the Iron Range and Arrowhead region in northeast Minnesota.) Additionally, soils that are high in organic content are able to quickly bind any zinc, manganese, iron, or copper that leaches into them, rendering it unavailable to plants.

Some manufacturers claim to have removed a very high percentage – or even all – of the steel wire and threads from the shredded mulch. Naturally, these products are more costly. If want to try shredded tire mulch in a garden and you're concerned about metals leaching from the rubber, it's probably worth paying the extra money. Or, you might try one of the cheaper mulches around trees and shrubs, where potential leaching would be less concern.

Shredded tire mulch should last for 10 – 15 years. No doubt, we'll be hearing more about them.

***** Here's the website for one supplier. No endorse intended or implied.
http://www.homestead.com/prosites-closetheloop/index.html

Itch Mite Found in Midwest
Jeffrey Hahn, Assist. Extension Entomologist

Close-up of itch mites. Photo credit: Jim Kalisch Univ. of Neb.

(Note: Much of the information for this article was found from Kansas State University, http://www.oznet.ksu.edu/news/sty/2004/itch_mite102204.htm and University of Nebraska, http://lancaster.unl.edu/nebline/2004/novdec04/page07.pdf)

A series of mysterious bites in eastern Kansas and Nebraska during early September sent state entomologists and health officials scrambling in search of a cause. Although people were being attacked, no one ever saw anything biting. As this situation was publicized, it was discovered that many people were experiencing unknown bites. Some people received only a few bites but in other cases people found dozens or even hundreds of bites.

As the problem was investigated, the biting problems were discovered to be associated with neighborhoods where oaks, specifically pin, red, and black oaks, were present. A number of people provided information about ‘small worms' occasionally being seen falling from these trees which prompted entomologists to examine the oak leaves. These oaks were found to be infested with several species of galls. Galls are abnormal plant growths resulting from attacks by parasitic organisms, in this case fly larvae (gall midges). The specific galls involved were leaf marginal galls and smooth vein pocket galls. These galls were reported in very high numbers.

Galls were opened and examined. Most contained fly larvae, including many that were dead. In addition, many galls were found to contain very tiny mites that were preying on the fly larvae. These mites were eventually identified as a type of itch mite, Pyemotes herfsi. This is a European species and it has not been previously identified in the United States before now. Despite this being the first record in the U.S., there is speculation that this mite species may have been in this country as long as twelve years ago when a rash of unexplained bites also occurred in Kansas and Nebraska.

These itch mites use needle like mouthparts to bite humans. There isn't any pain when people are first bitten. In fact they usually don't realize something has happened until the next day when quarter-sized red welts appear with a pimple in the center. These bites occur most commonly around the neck and shoulders and on the upper torso where clothing hangs loosely. The best bet for someone suffering from the bites of these itch mites, is to apply calamine lotion or benadryl to help relieve the itching.

The Extension Service in these areas counseled homeowners that they could protect themselves from these bites by using the repellent DEET (the same product used in mosquito protection). Although it was tempting for homeowners to treat areas adjacent to infested trees, insecticides have not proven to be effective. Also, insecticide treatments of the galls themselves have no affect on the gall-making fly larvae or the mites. These bites continued throughout the fall, well after it was expected that the mites would be active. It was common for people raking oak leaves in infested areas during the fall to be bitten.

These mites were not widespread throughout the regions of the states where they were discovered. But where they were present locally, they were found in very large numbers. The question now is what can be expected next year. Because these mites are predaceous on the gall-making fly larvae, they are dependent on high gall numbers to become numerous themselves. If these galls return to normal populations next year, the odds are that these itch mites will not be in high enough numbers to be seen very often or they may not even be noticed at all.

This mite species has not been identified in Minnesota so far. Pin, red, and black oaks are present here as well as the oaks galls that these mites like to infest so it is possible that they could be discovered in Minnesota (assuming that they are not already here). If in the future these specific oak galls become abundant, we may encounter these mites. Keep in mind that this mite is only active during warm or mild weather and would be dormant during the winter months.

Editorial Notes

Brunnera macrophylla. Photo credit: Beth Jarvis

Most Minnesota gardeners are probably looking forward to a bit of a break by now. The Thanksgiving weekend snow pretty much signaled the end for me. It won't be long before seed orders are sent in and fresh seed packs fill garden center racks and it starts all over.

Siberian bugloass, at left, is a versatile perennials that is shade tolerant, though grows well in sun or shade and a variety of soils. The flowers looks like forget-me-nots and so it's also known as the perennial forget-me-not.

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Back issues Yard & Garden Line News are on the Yard & Garden Line home page at www.extension.umn.edu/yardandgarden/.

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Happy gardening!

Beth Jarvis
Yard & Garden Line Project Coordinator


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