Abstract: A plant injection device is disclosed which comprises a narrow injection needle having a smooth outer surface, a variable high pressure system having a one-way output valve in the body of the device, and a fluid reservoir. The needle portion of the device may be inserted into the plant either by applying force to the body of the device or by drilling hole sized relative to the width of the needle in the plant and inserting the needle therein. The device allows a fluid to be injected into the tree within minutes at the optimum fluid pressure for the particular plant, thereby providing a single visit plant injection method. The high pressure acts upon the interior of the plant body to force the sides of the hole to contract about the outer surface of the needle thereby forming a pressure seal which prevents fluid from escaping around the needle.