Killing any tree seems a shame, but pine trees can become diseased, overgrown or may be situated in an inconvenient area. Killing a mature pine tree can be difficult. Forest management professionals sometimes resort to burns, banding and chemical treatments and yet often the trees still come back. The most important thing to remember if using chemicals on a pine tree is to avoid poisoning anything else. Wind drift and soil contamination can cause problems for the plants you want to stay.
Forest managers use Round Up (active ingredient glysophate) as a chemical pine tree control. It enters the plant through its foliar structure, so the herbicide has to hit the needles to penetrate into the tree's system and kill it. The process will be slow and the pine could rally and survive the poisoning. You can also use a 1/2-inch drill and make holes around the trunk and pour in liquid glysophate. The holes must penetrate the cambium to be effective and you must be careful not to get any on vegetation surrounding the tree. This is a very effective control for pine trees.
Metsulfuron is the active ingredient in a few industrial herbicides such as Escort and Matrix. It is being used in the UK for pine tree management. It is similar in use to the glysophate but you only need to drill two or three holes and it kills the tree much faster. It is used in a highly concentrated form to achieve the quick-acting effect.
This method takes a long time but is less toxic. Again, drill holes around the trunk and fill them with nitrogen fertiliser. Keep the holes moist with a mister and eventually the compound will cause the tree to grow a fungus that decomposes the wood. This method is not recommended if you want to kill the tree quickly. It is appropriate for a pine tree or any other tree.
This chemical may require re-treatment but it can be used in a foliar spray, cut method or injection. It can cause problems with drift so use caution. If you want something stronger, 2,4-D is mixed with tichlophyr as Crossbow or Garlon. Use Crossbow in pasture and range land pine removal and the Garlon in forests to thin pine stands. Do not use near cropland.
Imazypyr is recommended by the University of Florida for pine plantation management. It can be used alone to kill pine trees or works well in combination with glysophate for a quicker kill. Pine plantation management can be part of silviculture, which is optimal management to grow healthy trees. This often means removal of taller pines to let in more light and other vegetative culling.