October 10, 2015

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Disease-Tolerant Elms in Minnesota: The Search Continues

The story of planting American elm trees has become much more complicated since Dutch elm disease (DED) arrived in Minnesota. Prior to the arrival of the disease, elm saplings were often dug up from the wild and transplanted into cities and communities. Now we have to go through a rigorous process to find disease resistant elm cultivars, because anything we plant from the wild has a high likelihood of becoming infected with DED. There are some disease-tolerant elm cultivars being planted in Minnesota, but they are not from the state. Team Tree is trying to find resistant elm trees in each quadrant of the state so that we can have Minnesota grown cultivars adapted to our unique climate. We are specifically looking for mature elms that have lived since DED was found in Minneapolis in the 1960’s and likely have natural tolerance to the disease. After identifying a specific mature elm that we would like to replicate, we collect fast growing branch tissue during the winter dormancy period.

Collecting in Marshall

Collecting elm cuttings in Mankato, MN

Scion Material

Branch material for experimentation


When we are ready to use the dormant branch material, last year’s new growth is grafted or processed as stem cuttings to make a clone genetically identical to the tree it came from. To make a rooted cutting, we cut a section of branch material several inches in length and plant it in a rooting medium. To encourage root establishment, we dip the cutting in a concentrated hormone solution that originates from a growth regulator that plants produce to initiate the growth of new roots.

Rooting cuttings (of cottonwood, not elm)

Rooting cuttings (of cottonwood, not elm)


Modified cleft grafting, however, is best cloning technique if we want quick growth, because the tree can allocate all its energy into one small grafted branch, instead of having to focus on developing new roots. When grafting, a wild elm seedling (rootstock) is used for the base of the new tree and a few inches of potentially resistant branch material (scion) is grafted to it. We do not allow the rootstock material to form leaves and branches, so the resulting tree will be completely from the scion wood. The purpose of the rootstock, as you could likely guess from its name, is solely to be the root system of the tree.

Fresh Graft

Grafted elm clone


Because we usually graft in the wintertime, we keep the newly grafted trees in a greenhouse so they have a head start on the growing season. We then move them out to our nursery for the summer.

Successfully leafed out elm in its first season post-graft

Successfully leafed out elm in its first season post-graft


From this time until the tree reaches 3-5 years of age, the main focus is on growing a large healthy clone. Once the tree is large enough, it is inoculated with Dutch elm disease to test for tolerance. Any trees that have shown that they can survive exposure to DED after several rounds of testing will be used for restoring American, red and rock elm to Minnesota’s landscape.

We hope that someday the fruits of our labor will pay off and result in a specimen such as this:

Residential Elm

Mature, disease resistant elm


Here’s the collection crew:

Team Tree

Team Tree in SW Minnesota

Stephan, Ryan, Chad

Team Tree in Lake City, MN

American Elm Forms and C.C. Laney in 1908

WillowElmsOne hundred years ago, well before Dutch elm disease (DED) made its way to our shores, we had the time to ponder the many wonderful qualities of America’s favorite shade tree, the American elm (Ulmus americana). In April 1908, C.C. Laney published this wonderful article in The Garden Magazine:

“The Types of the American Elm”

One might dismiss this article as just a bit of horticultural antiquity, but there is much to be learned from it. These elm forms can still be found on the streets and in the parks throughout America. Look around and you’ll likely see a “willow-type” or “pitcher-type” elm. In the Twin Cities area we’re apparently quite lucky to have many of the reportedly “rare oak-type” American elms.

Take a few moments out of your busy day and travel back in time when we could encourage our children to:

“know [these elms] by name … to make pilgrimages to them, to hold picnics and little parties under them. And so a love for the trees would be cultivated by association and the next generation, being friends to the trees, there would be no need of enacting laws to compel their preservation.”  – C.C. Laney, 1908

Shade Tree Short Course Talks

We all had a great time at Shade Tree Short Course this year.  Here are a couple of presentations done by University of Minnesota Forest Resources staff and students at the STSC.

Sugar Maple Decline – By Dustin Ellis
Effects of Street Reconstruction on Boulevard Trees: 20 years of Information – By Ryan Murphy


Aronia melonocarpa


Our research nursery here on the University of Minnesota St. Paul campus has a variety of species including a new favorite of mine – Aronia melonocarpa (black chokeberry).  “Black chokeberry is an open, upright, spreading, somewhat rounded but leggy, deciduous shrub which typically grows 3-6′ (infrequently to 9′) tall; features clusters of 5-petaled, white flowers in spring which are followed in early autumn by blackish purple, blueberry-sized fruits; and has lustrous, dark green foliage that turns an attractive purplish red in autumn” (chokeberry page).  A. melonocarpa is found naturally in swampy wooded areas in North America.  It can survive in both full sun and partial shade.  The plant prefers a slightly acidic soil (pH 6.0 to 6.5), however, is tolerant of a wide pH range (pH 5.0 to 8.5).  Aronia is a good substitute for those growers who would love to grow blueberries but do not want to deal with dropping their soil’s pH into blueberry’s preferred range (pH 4.0 to 5.0).  The other awesome thing about this plant is the highly nutritious berry.  The fall berries are super-rich in anthocyanins and proanthocyanidins as well as a variety of other minerals and vitamins.  The visual appeal of this plant’s white flowers in the spring, attractive color in autumn, and nutritious berries make this a very exciting plant indeed.  (Photos published under GNU Free Documentation License and found at commons.wikimedia.org – left photo by: BotBln and right photo by: Pawvic)

Aronia_melanocarpaAronia_melanocarpa berries

2014 Minnesota Elm Collection Begins!!!

 On Tuesday February 18, we began our 2014 Elm collection process. On Tuesday, we collected material to propagate new baby trees for screening from two American elm selections located on streets in the metro area. Throughout February and March, we will be rolling around the state of Minnesota to collect material from high preforming elms found in the state observed by foresters or home owners for propagation. Eventually, the baby trees propagated from cuttings and grafts will be inoculated with Dutch elm disease (DED) to see if the selection is a resistant to DED. Hopefully we will be able to find DED resistant elms that were found right in the great state of Minnesota!!

Bellow is a short photo essay of our 1st day of 2014 collections    

1st American Elm collected in 2014

P2183033P2183045 P2183038 P2183037 P2183042  P2183050


2nd American Elm collected in 2014

P2183053 P2183055  P2183062 P2183067 P2183070

More updates on Minnesota elm collection to come!

Time Travel along Olson Memorial Highway

Team Tree’s decade-long odyssey with Olson Memorial Highway began with a Minneapolis Park & Recreation Board Arbor Day planting in the early 2000’s.  This included many of the selections originally grown at the Urban Forestry Outreath & Research Extension Nursery.  Since then, the median down the center of Highway 55 has become a living laboratory to track performance of many of the Dutch elm disease resistant cultivars.

The benefits of timely developmental pruning are very evident.  These trees are well on their way to shading this busy metropolitan thoroughfare, without pruning at the right time and correct dose this wouldn’t be possible.\

Check out the embedded Condition Rating charts, these track overall tree quality based on a quantitative rating system meant to identify defects that detract from overall tree quality.


YouTube Preview Image

Tree of the Week: Metasequoia glyptostroboides (Dawn Redwood)

Heeled in Dawn Redwood

Metasequoia glyptostroboides (Dawn Redwood) is a tree that looks similar to the Redwoods that grow in the Redwood National Forest (Sequoia sempervirens, Coast Redwood) and the Sequoia National Forest (Sequoiadendron giganteum, Giant Sequoia). Metasequoia glyptostroboides has a much shorter mature height of 70 to 100 feet as opposed to the record Sequoia sempervirens of 379 feet!! The foliage has a soft fern like appearance with exfoliating bark and a pyramidal form making it a perfect specimen tree where adequate space is available. Metasequoia glyptostroboidesprefers likes moderately wet soil making it a good rain garden choice.


Dawn Redwood is also a fast growing tree. This past spring in the nursery, we planted ten Dawn Redwood trees into 15 gallon containers and were about less than 1” in caliper. At the end of the season, the Dawn Redwoods had an approximate caliper of 1.5” and a stunning 2 feet of new growth! While Metasequoia glyptostroboides is of the hardiest redwood/sequoia family, it still is a zone 5 making winter hardiness in Minnesota a potential problem. Our Dawn Redwoods are currently heeled in a cozy layer of mulch for the winter to keep the roots insulated (right image). Hopefully come next spring, all ten will have survived this cold winter and will be able to be planted out in the city!

By planting unique species like Dawn Redwood in Minnesota will help diversify the urban forest. The ten trees that we will plant out in the city will be a helpful trial on how well they perform in the harsh Minnesota winters. While Metasequoia glyptostroboides is a zone 5 tree, micro climates in the city should help the likelihood of survival when extreme cold hits.

Check out some of the awesome close up images in the gallery below!!




What Happens to Boulevard Trees After Construction?

Street-Tree-plan-picThe benefits of trees in the urban environment are both qualitative and quantitative.  Trees offer an aesthetic enhancement to the urban environment.  There is something magical about driving, biking, or walking down a street lined with mature trees.  There is a connection to the past – with the people who planted those trees and all the residents that have consciously or unconsciously enjoyed their presence.  But in today’s society money talks.  Lucky for trees – it turns out they have an economic voice – and its pretty loud. 

Researchers at the USDA Forest Service, Northern Research Station have developed a tool for discovering, among other things, the economic impact that trees have in the urban environment.  Its called the Urban Forest Effects (UFORE) model.  This model is able to estimate the economic value of various ecosystem services provided by urban trees.  For example, would you have guessed that the urban forest of Minneapolis removes around 384/tons of pollution each year?  And the value of this service?  $1.9 million/year.  How about the current ‘hot-topic’ – carbon.  Well, Minneapolis trees sequester around 8,900 tons/year – worth around $164,000/year according to the UFORE model.  Urban trees also happily store this sequestered carbon for us.  The Minneapolis urban forest is estimated to store 250,000 tons of carbon – a value of $4.9 million. (For more information visit  http://www.nrs.fs.fed.us/tools/ufore/).


Urban citizens live in highly developed and altered landscapes.  We have houses connected to gas lines, sewer lines, boulevard abuse resizeelectric lines, cable lines, phone lines, paved roads, sidewalks, and parking lots.  In many cases our urban trees must coexist with these structures in close proximity.  Furthermore, trees need to deal with all the construction and renovations that must be done to keep these urban systems going.  Now, no one is suggesting we get rid of our modern conveniences to make room for growing bigger and better trees…well, some probably do!  So we need to understand how our construction activities effect the nearby trees. 

This is the goal of a 20 year study undertaken by University of Minnesota Urban and Community Forestry Professor, Gary R. Johnson.  In 1993-1994 the Kenwood neighborhood of Minneapolis underwent a complete utility and infrastructure renovation.  New utility lines (sewer, electric, water and gas) were run, streets were widened, curbs were lowered, new sidewalks installed, and the boulevard was regraded.  The goal of the study was to see how trees planted in the area between the sidewalk and street – also known as the boulevard – performed when subjected to intensive construction activities.  For comparison, a control group of boulevard trees that did not experience any construction activity was also established nearby.  Five different genera of trees were looked at including maple (Acer spp.), American elm (Ulmus americana), ash (Fraxinus spp.), hackberry (Celtis occidentalis), and linden (Tilia spp.).  Stem diameter (DBH) and condition ratings were taken for each tree in the study three times over the course of 20 years.  Survival was also noted. 

What is left of ash rooting space resizeA complete statistical analysis is still forth coming but several trends are appearing from the data.  For example – elm and maple trees experienced a greater percentage of loss in the construction group compared to the control group.  Ash and linden on the other hand had a smaller percentage of loss in the construction group compared to the control group.  Causes of tree loss are unknown.  Elms by far had the greatest percentage of loss over all genera with 57% loss in the construction group and 45% loss in the control group.  Average increase in DBH was not significantly different between construction and control groups among the various genera.

This represents a case study of boulevard trees in one neighborhood of Minneapolis.  More studies are needed to form any definitive answer on the effect of construction on our boulevard trees.  A complete report of this study is forthcoming.  Stay tuned. 


Research commences in Minneapolis storm damage project


Antoine uses the penetrometer to measure soil compaction



Carl uses a Biltmore Stick to measure DBH


Armed with Shovels, Sharpies and scientific-grade Ziploc bags, Team Tree continued its intrepid adventures in urban forestry this week.  Charged with the mission of soil data collection, the team took soil density ratings at depths of six and twelve inches, and collected a pint of soil for analysis from a series of random sites located within the study site. 

“Can I help you?”  Is probably the most common question you can expect to get from homeowners if you are using a penetrometer in a residential area.  A penetrometer is a tool used to measure soil density, useful in determining compaction.  It is similar in shape and execution of a soil probe, except no soil core is removed, and a pressure gauge on the top of the unit indicates the level of force required to penetrate the soil to a desired depth.  The process of taking penetrometer readings and removing a soil sample from each location looks just unusual enough to raise concern from vigilant homeowners, leading the conscientious scientists of Team Tree to recommend that similar expeditions be carried out in classic lab coat attire using vehicles clearly marked with the University of Minnesota Department of Forest Resources logo. 


Summer tree root data collection



The team collects soil samples

While some may argue that nothing is more exciting than collecting soil data, the summer data collection portion of this project proved to be among the highlights of Team Tree’s year.  Procuring data during visits to each of over 1800 trees that failed during the storms of June 20th and 21st, 2013 provided opportunities for close examination of some extremely large trees and their root systems.  Included among those trees were those which failed due to tip over and those that broke off above the ground line.  Experience and teamwork aided the crew as they assiduously compiled information on boulevard, sidewalk and street conditions amidst what evidence remained of the dendronomic destruction dealt by the astonishing summer storms.


reNewell Wrap Up

As the growing season of 2013 was drawing to a close, we teamed up with Scott Alsleben and his Forestry Occupation students at Great River School (GRS) to wrap up the reNewell project and prepare it for the long winter.

During our first meeting we introduced the site and project to the 6th and 7th grade students from GRS and discussed some of the inspiration and challenges faced when maintaining urban tree canopy.  During the next few sessions students helped mulch all the new trees and shrubs on the site had a chance to get to know these native species a little better.

We wrapped up the 12-week session with a few weeks of survey and mapping training.  Students worked in teams to survey and map all of the new oaks planted on the reNewell site.  On our last session together I joined Scott and his class at GRS and created a web map at ArcGIS.com. 


We also teamed up with the students to plant a new crop of oak seeds that they collected at Newell Park in hopes that they will become part of ongoing efforts to regenerate oak canopy throughout the City of Saint Paul.


As we get ready for the long winter – thanks again to everyone who made reNewell 2013 a great success!

  • Saint Paul Garden Club
  • Hamline-Midway Coalition
  • City of Saint Paul Parks & Recreation
  • Great River School