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This is the time of year when Minnesota receives frequent thunder and lightning storms. The annual frequency of thunderstorm days is about 45 days in southern Minnesota decreasing to about 30 days along the Canadian border. Generally 80 percent or more of these storms occur during the heavier months, from May through September and particularly in June, which is our rainiest month.

A lightning strike to a tree in one's landscape can be a traumatic experience for both the tree and the caretaker. Three weeks ago lightning struck one of our two remaining elm trees. Both are about 65 feet high and close to the house. While we are grateful that nobody was injured or that no property was damaged, it did a number on the elm tree. It put a crack in the tree all the way down and at the trunk level it blew chips about 30 feet away.

I think one of the reasons those two trees have been spared Dutch elm disease is because there are no other elm trees in the immediate area. I also thought that because they are red elm, they may be less susceptible to Dutch elm. The arborist that looked at the tree said that all elm are prone to Dutch Elm but now that there is a wound on the tree, it is a good candidate. He suggested that we put latex white paint on as much of the area as possible.

Generally, two classes of damage can and often do occur in a wide variety of combinations when lightning strikes a tree. The mechanical and structural damage to a tree may be so slight that one doesn't even notice it. It also could be damaged as our tree was. It is believed that an electrical charge of up to 100 million volts which vaporizes the water inside the tree creates superheated steam which explodes when it exceeds the structural strength of the wood.

A common indicator of a lightning event is the classic vertical stripping where bark and sometimes the wood beneath, is torn from the trunk or major scaffold limbs. This stripping physically interrupts the vascular tissues that conduct fluids up and down in the tree's living cambial structures under the bark.

The second kind of lightning damage is systemic and it may not be immediately observable. This kind of damage burns and traumatizes the root hairs and conductive tissues. This damage may be slight or extensive to the extreme of complete vascular shutdown. Trees experiencing vascular shutdown would quickly brown out and die.

Although no species of tree is totally immune to lightning, some are definitely more resistant to lightning strikes than others. Birch are rarely struck, whereas elm, maple, oak and mot conifers are commonly hit. Birch and beech are poor conductors of electricity whereas oak, maple, ash and poplar have high starch content which are good conductors. Conifers like pine, spruce, hemlock and fir have high resin content. They conduct more electricity than trees with low resin.

In most cases it is best to wait six months or more before doing major corrective work on a lightning struck tree. In many cases it will become obvious at some point during the waiting period that the tree will need professional help or will not recover and need to be removed.

Here are some facts about lightning from Rita McKenzie of Purdue University:

1. Lightning kills more people than tornadoes and hurricanes.

2. Most deaths occur in open fields near or under trees or around water.

3. Lightning strikes the earth somewhere 100 times a second.

4. The temperature of a lightning flash can be 30,000 degrees celsius. or five times hotter than the sun

5. Peak currents can be 200,000 amps or 1,000 times the typical household wiring capacity.

6. The energy released by a strike of lightning can be more than the energy in 1000 gallons of gasoline or more energy than some bombs.

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Upcoming Classes

The last two sessions of our successful Summer Gardening Series, will be

Aug. 11––"Butterfly Gardening"

Aug. 25––"Attracting Beneficial Wildlife"

The classes meet at the Fairview Lakes Regional Health Care Center in Wyoming from 6:30-8 p.m.

For more information, call 651-674-4417, stop by the Chisago County Extension Office, 38780 8th Ave, No.rth Branch, or visit our website to download a ticket order form: www.extension.umn.edu/county/chisago.

In Your Yard and Garden

The summer heat and lack of adequate rainfall can really be a challenge for gardeners this time of year! Make sure any trees or shrubs that were planted this year are receiving a good soaking at least once a week. They put a lot of energy into developing their root system the first year after planting. Set the end of a barely dripping hose at the base of the plant for several hours (until the soil is wet at least 4"deep). The future health of trees and shrubs depends on their ability to develop a good root system.

Many annuals start looking tired this time of year. Deadheading (cutting off spent blooms) or in some cases, cutting the plants back a few inches, will extend the bloom season of most annuals. If the plant is not allowed to set seed, the normal response is for it to try again by sending out new blooms.

Ways to Access Information

Master Gardeners will be offering free plant clinics every Monday night from 4:30-8:00 at the Extension office in North Branch. Stop by 38780 Eighth Ave. (the Anderson Chiropractic Building) or call 651-674-4417 to speak with a Master Gardener. During the rest of the week, you can leave a message and a Master Gardener will return your call.

www.extension.umn.edu/county/chisago Check out the 'Hot Topics' box in the middle of the page for current Chisago County Master Gardener news and events.

You can also click on 'Ask a Master Gardener' next to the cute little flower on the right hand side of the page. Here you can search 1000's of answers from Master Gardeners around the state. If you don't find your answer you can submit a question online or search for University publications.

Bell Museum of Natural History

For information about snakes, skunks, raccoons or other wildlife around your yard, call the wildlife information line at (612) 624-1374 or www.bellmuseum.org

To see the latest Yard and Garden newsletter, go to: http://www.exten sion.umn.edu/yardandgarden/YGLNews/YGLNews.html

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