"Our sea-walled garden, the whole land, is full of weeds, her fairest flowers choked up, her fruit-trees all unpruned, her hedges ruined.  Her knots disordered, and her wholesome herbs swarming with caterpillars."

So William Shakespeare described England in Richard III.  His gardeners, like our Janesville gardeners and arborists, can see problems with the green realm around them.

The most important element of our green city, our trees, is in trouble.  We have not planted enough different varieties of trees to create a sustainable urban forest.  Trees that come with less baggage than our old favorites:  fewer insect and disease problems, less maintenance, slower growing and more dependable, with greater tolerances to environmental stresses, are necessary to protect our urban forest.

Planting a greater diversity of tree species is the best way to perpetuate our community forest for our present benefit as well as the benefit of the next generation. 

In 1971, 45 percent of all street trees in Chicago were American Elms (and Janesville likely had a similar percentage.)  Their death due to Dutch elm disease shows the danger of monoculture -- the over-planting of one tree species.  The extensive planting of a limited numer of tree species places our city at risk for a reenactment of the elm, or some new, disaster.

Lustig Park is a monoculture of white oak.  It is now in jeopardy of loss because of the gypsy moth.  In many of our cities up to 75 different tree species can be found.  This sounds impressive but is undercut by the fact that many tree surveys reveal that five or six species comprise 60 to 70 percent of the street tree population. 

In relying upon a limited number of tree species we are ignoring the principals of isolation and dilution that can reduce the harm from a virulent disease or pest.  Our culture has an incurable itch for the one sure thing which translates into a fixation upon the perfect urban tree.   This has led to the overplanting of such fashionable trees as honeylocust, green ash, little leaf linden, Callery pear, Norway maple, red maple, and silver maple.  We've already forgotten about the elm disaster.

Many of the above species have perfectly acceptable improved cultivars but the sheer number of individually identical trees planted has led to problems.

The differences in environmental conditions for trees from such factors as rooting space, soil compaction, drainage, soil fertility, soil acidity, (pH), salt exposure, and overhead power lines means that our street trees are exposed to widely different conditions.  A broader distribution of tree species has a better chance of surviving these hostile urban conditions.  Public safety, which is endangered by falling trees during summer or winter storms, can be improved with better, more durable trees.  Greater maintenance efficiency can be produced from a more balanced, diverse urban forest through the selection of trees that are most appropriate for their spaces and require a minimum of care.

We can best reinforce and enhance Janesville's visual character by planting a wide variety of trees.  We can add interest to our landscape through unique bark, textured foliage, and beautiful fall color displays.  Different trees can create layered canopies,  vertical structure and different outdoor rooms in each city block.  We can search for trees to provide seasonal interest all year long.  If you plant only Norway maples you are never going to see a Katsura tree with silver-frosted seedpods on a moonlit December night.

A general rule for urban tree planting includes:

• plant no more than 10 percent of any species,
  
• no more than 20 percent of any genus, and
  
• no more than 30 percent of any family.

Following these guidelines will help increase tree diversity in our community.  A random walk in Janesville will reveal that the widely planted maple species (Norway, red, silver, and sugar maples) easily make up more than the recommended 20 percent limit for members of a single genus.  The 30 percent rule can be challenged by the widespread usage of the legume and rose family in tree plantings.  Legumes such as loneylocust are very common.

Also present in janesville are legumes such as redbuds, yellowwood, Kentucky coffeetree, black locust, and pagodatree.  The rose family is widely represented in Janesville by serviceberries, hawthorns, crabapples, cherries, pears and mountain ashes.  All are important and useful ornamentals that are often susceptible to the same insect pests and diseases, particularly fire blight and leaf scab.  To minimize damage by pests you should strive to limit the quantity of any particular tree planted. 

This goal for diversity can be met by taking advantage of the many varities available at local garden centers.  A simple J-STAC diversity index was created to rate the garden centers by adding the number of genera to the number of species (including cultivars) available.  As might be expected, a larger garden center had an initial index total of 73, which, after deducting the overplanted maples (11) left it with a whopping index of 62.  Another nursery had an initial total of 60 from which 13 maples were deducted with a respectable diversity index of 47.