Don’t Fence Me In

There is a newly completed subdivision near where we live, and the other day while driving past one of the houses, I noticed something interesting.  The back yard of this house abuts a busy street, and the homeowners had planted a row of young trees just inside an elegant iron fence that encircles the yard, most likely in the hope that the trees will eventually fill out and provide some privacy from the outside world.  A great idea.  We used the same approach with a row of Black Hills spruce in our yard.

Only we don’t have a fence next to them.  Fences and trees don’t mix well together, at least not in close proximity.  The reason is that as trees grow in height they also grow in girth.  This should be obvious, but ofttimes when a young tree is planted, people don’t think about circumstances 10-15 years or more down the road.  Another example I’ve seen is a Colorado blue spruce planted within 5-6 feet of a house.  These trees can grow up to 80 feet tall and over 20 feet wide.  I’ll let you do the math.

When a growing tree meets resistance (fence, house) the tree invariably wins.  I’ve seen a house overhang crushed by the lateral growth of trees planted close by.  And trees will literally grow through or engulf a metal fence (see pics) or destroy a wooden fence.  This is because a new ring of growth forms each year from a layer of stem cells, called the cambium, that encircles the tree trunk on its periphery.  Each growing season new xylem and phloem is produced that becomes active in moving life-sustaining water and nutrients, respectively, within the trunk, while the year’s previous growth becomes inactive and adds to the inner accumulating “wood.”

Many campers who have foraged for kindling materials to build a campfire probably know it can be deadly for a birch tree to use its bark for this purpose.  Actually, removing the bark from any tree has similar consequences.  This is because when bark is torn from a tree, inevitably the cambium is removed along with it.  Such loss can be a death sentence, but only if the bark is removed from around the entire circumference of the tree, for then no water from the roots can make it to the top of the tree and no food produced in the leaves can reach the roots.  A small patch of intact tissue will allow the tree to survive.

New upward growth in a tree only occurs at the top in its upper meristem tissues.  Thus, once formed, the trunk of the tree never grows upward, only laterally.  This can be shown when a person carves their initials in the trunk of a tree.  Returning 20 years later, the trunk will be bigger, but those initials will be the exact same distance from the ground that they were at the time of the carving.