Tree Conditions in the Twin Cities

August 2007

We are now in our second season of drought exacerbated by a lack of snow cover during last winter. According to the NOAA we are considered to be in the severe stages of  drought with soil moisture conditions in the Twin City Metro area 30% to 50% below average. In 2007, so far, we have had up to 50% less precipitation than in an average year. What do these statistics mean to the trees shading our homes and what can we do to minimize the effects this drought may have on our trees?

Trees require water to cool their foliage and to maintain a good heat balance by opening and closing their stomata, the minute pores located in the stem and leaves. Without adequate water trees lose their ability to photosynthesize and can’t discharge the heat they accumulate during the day. The visual effects are scorch and dieback. The heat damages the photosynthetic machinery of the leaves and causes irreparable damage. Leaves that are damaged will not regenerate and the tree will be forced to use stored energy reserves to try to replace foliage. As trees slowly weaken they become much more attractive to diseases such as oak wilt, Dutch elm disease (DED), cankers, wilts, and fungi to list just a few; and insects such as borers, scale, worms, and leaf miners. As trees become more stressed we will continue to see an increase in their decline due to these contributing factors.

Researchers at the UMN have identified there are on average 12 unprotected weed or scrub elms to serve as DED inoculum per city block throughout Minneapolis. With that many untreated elm trees available and capable of producing masses of elm bark beetles we anticipate many long summers of intense DED pressure. To protect your elm trees that may already be stressed by drought we recommend treatment by macro-infusions with the fungicide Arbotect injected into the root flares of your tree. This will protect your elm from beetle-transmitted infection and provides up to three growing seasons of protection. This treatment is best accompanied by weekly deep root watering. Your elm may also be at risk through root graphing with these untreated scrub trees. Call your Rainbow Arborist if you have concerns about your elm.

Another big concern is borer control for Oaks, Birch and flowering trees. The state wide drought conditions have set us up for banner borer problems next year regardless of watering regime. This type of insect has the potential to decimate entire stands of native and ornamental trees.  As drought weakened trees start to die, the beetle populations have the potential to explode and the pressure will likely cause the loss of many otherwise healthy trees. Rainbow Treecare’s recommendations are to water and to apply the insecticide Xytect (imidacloprid) this fall and again next spring. Our arborists can prescribe the best treatment schedule for your type of tree.

During this drought period deep root watering will benefit your tree regardless of other recommended treatments. The optimal watering program is a slow soaking of the roots out to the drip line of the tree canopy, one inch per week should be sufficient. This translates to approximately 100 gallons of water for an average size tree or about 3 hours watering with a soaker hose. As fall approaches it will greatly reduce your trees’ chance of mortality to insure that adequate moisture is in the soil prior to freezing ground temperatures. This is especially important for conifers as transpiration through the needles make them sensitive to the drying winds of winter. Taking time and care now will help your trees withstand the stresses of drought and winter and make it possible to for you to again enjoy the benefits of your trees next year.

As always, if you have any tree questions or concerns please contact your Rainbow Arborist. They are available to visit your site and can give you answers for your specific tree issues.

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