Many people forget that trees--despite their size--are plants the same as flowers growing along the sidewalk or even the grass in the yard. Like lawn plants, trees are sensitive to certain chemical compounds that are used for herbicides and can be killed with an application of chemicals. One easy way to kill a tree is to apply copper sulphate to its roots. Copper sulphate, a blue crystal available at home improvement and hardware stores, is also used to kill tree roots in sewer pipes.
Check to see how shallow or deeply rooted the tree is. Some trees such as spruces are shallow-rooted, while others such as linden have deep roots.
Dig a series of 4-inch-diameter holes around the drip line of the tree with the posthole digger. The more deeply rooted the tree, the deeper the holes should be, with the bottom of the hole above the roots. Larger trees will require more holes. For example, a tree with a 30-foot canopy will have a drip line of about 90 to 100 feet in circumference.
Fill the holes halfway with copper sulphate crystals and fill in the rest of the hole with the soil. Leave the balance of the soil on the ground to use as backfill as the crystals dissolve.
Watch for signs that the tree is dying, such as the leaves turning yellow and falling off. Copper sulphate takes some time to work, so it will not happen overnight or even within a week.
Do not use copper sulphate around a pond, lake or drinking well. Purchase copper sulphate crystals at a garden or home improvement store.