Friends of the Riverfront

The DeLaSalle stadium, if built, would require removal of many trees planted in 2000 as part of a University of Minnesota forestry research experiment, as described in this fall 2004 article: 

Greening Up the Brownfield: Tree Performance on Nicollet Island  

By Chad Giblin and Jeff Gillman  

Department of Horticultural Science, University of Minnesota  

What’s a Brownfield?  A brownfield is a site that has unusually harsh climate and soil conditions and is considered generally unsuitable for plant growth.  Many brownfields are the result of  contamination from old dumps, construction sites, and staging areas. In recent years  there has been a lot of interest in rejuvenating these sites for both public and private  use.  One of the major objectives is to identify landscape plant species that can tolerate these hostile conditions.  There are numerous local, state and federal organizations that specialize in reclaiming these areas.  Just do an internet search for  “brownfield”, you’ll be surprised at what you find.  

Brownfields are usually dominated by weeds and other invasive species.  Like most  others, the brownfield on Nicollet Island has extremely poor soil properties.  Texture  is quite variable but usually contains a fair amount of gravel, sand, clay, and other  “non-soil” components.  Soil pH is very high, ranging from 7.9 to 8.6 with organic  matter very low at less than 0.5%.  

Project History  

In 1999 a group of cooperators from the Minneapolis Park and Recreation Board, Tree Trust, and the University of Minnesota initiated  a plan for planting trees on Nicollet Island.  By planting time in  2000, they had selected 15 different species and cultivars with a  total of 183 trees to be planted.    

Data Collection  

Caliper and stem growth rate are measured annually.  Mortality, winter injury, stem  and canopy condition ratings are also measured every year.  Condition ratings are  based on a subjective quality assessment on a four-point scale (4=highest to 1= lowest rating).    

Tree Performance  

In most cases the trees either performed very well or quite poorly.  All of the ash  varieties did very well; overall mortality after four years was around 5%.  Other species at the top include: hackberry, Autumn Blaze Freeman maple, and Triumph elm.   These all had no mortality.  On the other hand, Fall Fiesta sugar maple and Green  Column black maple both had some issues.  After four years, both had mortality  around 50% with overall stem and canopy condition below other species.  Heritage  oak, Boulevard linden, Kentucky coffeetree, Amur maackia, and bur oak all fell into  the 15 to 25% mortality group after four years.            

The Planting Depth Issue  

After planting, many trees were found to have been planted too deep on-site.  After a  close examination the trees were found to be too deep in the container even before  planting!  Overall, 99 trees were lifted and corrected to get the first main order root  at or near the soil line.  This correction involved lifting the trees, removing excess  media and pruning off any adventitious roots.  You might expect this to really stress  these trees and increase mortality; in most cases the opposite was true.  Two out of  three times the mortality for corrected trees was lower than that of the overall species mortality.  This procedure, especially for container-grown or containerized  trees, shows a lot of promise for correcting deeply planted trees.  This spring at the  TRE Nursery, we planted out 50 containerized Cathedral elms after removing 6 to  8” of excess media from over the root system.  In many cases this left us with 50%  or less of the original soil ball.  Thus far, we have had no mortality in these trees.   This process, while time-consuming, can be of critical importance when planting out  container stock that is planted too deeply in the nursery can.  

Hardiness and Tough Sites  

There is a lot of concern about the winter hardiness of some of the white ash cultivars.  Autumn Purple, in particular, has had a number of problems with winter stem  injury like frost cracks and frost cankers, especially with smooth-barked, young  trees.  Interestingly enough, there has been no incidence of winter stem injury on  this or the other ash cultivars on Nicollet Island.  Dr. Pellett theorized that this might  actually be due to the tough conditions of the site!  In many cases we see the trees on  Nicollet Island going dormant in the fall prematurely.  This may allow them additional acclimation time before winter sets in.  Highly vigorous trees, growing in better conditions may have acclimation timing issues causing them to be more susceptible to winter injury.    

The Nursery Grower Connection  

These brownfield sites offer valuable insight into future demand for urban  trees.  Commercial and residential development is expanding at an incredible  rate generating numerous new customers for Minnesota’s nursery growers.   In these new developments soil pH is often high and drainage can be poor  due to construction pressures.  Furthermore, the climate is often harsh due to  open, windswept conditions.  By tracking tree vigor on Nicollet Island and  other brownfield sites we can make confident recommendations for trees to  use in new developments for both customer satisfaction and long-term tree  performance and health.  

A full presentation of the results is now posted on the TRE website at:  http://www.tre.umn.edu/growers/growers.htm  

Research Cooperators:  University of Minnesota – Department of Forest Resources  (Gary Johnson, Dave Hanson); University of Minnesota – Department of Horticultural Science  (Jeff Gillman, Harold Pellett, Chad Giblin); University of Minnesota – Minnesota Extension Service  (Patrick Weicherding); Minneapolis Park and Recreation  Board; DeLasalle High School;  Nicollet Island Community; Tree Trust 

Tree Species Used on  Nicollet Island

Acer nigrum 'Green Column'  

Acer rubrum x saccharinum  

Acer Saccharum  

Celtis occidentalis  

Fraxinus americana 'Autumn Blaze'  

Fraxinus americana 'Autumn Purple'  

Fraxinus nigra 'Fall Gold'  

Fraxinus nigra x mandshurica 'Northern Gem'  

Fraxinus nigra 'Northern Treasure'  

Gymnocladus dioicus  

Maackia amurensis (male only)  

Quercus macrocarpa x robur 'Heritage'  

Tilia americana 'Boulevard'  

Quercus macrocarpa  

Ulmus hybrid 'Accolade'   

(excerpted from www.mntca.org/Newsletter/newsletters/ 2004/TCA_news_Autumn_2004.pdf)