By Sharon Ossenbruggen
Urban Forester-USDA Forest Service
Most people are aware that many trees were removed from the community this spring because of an infestation by the Asian long-horned beetle, a destructive pest introduced from Asia several years ago. This beetle kills trees or leaves them structurally unsound and dangerous to people and property. It is a very tragic thing to have lost so many trees in a community where they are precious.
Now the community is seeing that out of this adversity, there is new promise. New trees are being planted on the streets. The new trees are replacing those recently removed because of the beetle infestation but they are not just being replaced. To increase their chances for long term survival, they are being replanted with new specifications. One of these new specifications requires increasing the size of the tree planting pits wherever possible. Here are some reasons why.
In their natural forest environment, trees may live from forty years to hundreds of years. Trees planted in the harsh environment of the city streets have many stresses that can shorten this life span. Some examples are lack of water, poor soil conditions, too much winter time salt drying out the roots, tree pits being used as dog toilets, weed trimmers and lawn mowers that wound the trunk, cars running over the trees or car doors breaking them, bicycles chained to the trees. The list of abuses is very long.
One of the most serious problems for trees growing on streets is lack of space. Increasing the size of the planting pit gives the trees more space to grow.
The bigger the planting space, the better it is for the trees. With more space to grow, they can become established more easily and there may be less risk of uprooting sidewalks and causing hazards in the future. Another good reason to increase planting pits is that larger tree pits increase the volume of soil in which the trees can grow. Extremes of cold and heat can kill roots. More soil volume helps to keep the soil temperature more stable.
The young trees being planted were dug from the nursery a few weeks ago. The tree's roots were dug with a ball of soil to help keep the roots moist. Trees usually loose more than 70-90% of their root systems when they are transplanted. It takes transplanted trees an estimated one year for each caliper inch of trunk diameter to generate new roots for those lost. The new trees are three inch caliper, so it may take up to three years to generate new roots.
Trees have two kinds of roots: supporting roots and absorbing roots. Supporting roots are woody and strong. They support the trees in the ground. Absorbing roots are more tender and absorb water and 13 of the chemical elements from the soil that are essential for healthy growth, such as: nitrogen, phosphorous, calcium, and potassium, copper, and iron, etc. Tree root systems do not grow deep in the soil. They grow away from the trunk horizontally. Think of a water goblet standing on a large dinner plate. Most of the supporting roots of trees are in the top 18 inches of soil and most absorbing roots are in the top 3-6 inches of soil. This is another reason why the more space trees have the better they will survive.
Tree roots require two very important elements in the soil: water and oxygen. The expanded tree pits will help the roots have access to these. Soil should have air spaces. Roots grow best and are less likely to come to the surface in well-aerated soils. Walking around under the tree will compact the soil and squeeze out the air. It is much harder for the tree to grow well. Larger tree pits will make compacted soil less of a problem.
Trees require water in the proper amounts. With the new trees, the right amount of water can be the difference between living and dying. The larger planting pits will help to give a greater margin for error by having more soil to disburse the water in case of too much or too little water.
These are some of the reasons to expand the tree pits. Planting healthy trees and keeping them healthy is the goal. They are a valuable resource for our community that should be with us for many, many years.