Black poplar trees (Populus nigra) like the widely-available Lombardy poplar (Populus nigra 'Italica'), a deciduous tree found in U.S. Department of Agriculture plant hardiness zones 3 to 9, feature rapid growth rates. However, these trees are also short-lived and prone to devastating diseases, and their roots can become invasive, so you may opt to kill a black poplar growing in your yard. Cutting a black poplar down and removing much of the root system is the fastest way to kill the tree. Where this is not possible, you can use another technique--with or without herbicide--to kill an undesirable black poplar. Keep small children and pets away from the space around the black poplar as you are cutting into it and store, handle and apply herbicides carefully, keeping these materials away from children and pets.
Put on thick work gloves and safety glasses and make sure that the space around the black poplar is clear, with ample space to work and move around, and that children and pets are not in the area.
Cut through the black poplar's bark around the entire circumference of the trunk using a series of downward cuts with an ax or hatchet, or cut through the bark with a saw. Make sure the cuts penetrate at least 1/2 inch deep into smaller trees and 1 to 1/12 inches deep into larger black poplars. If you are using a saw, make two parallel cuts around the entire trunk circumference spaced 2 to 4 inches apart. If you are cutting a girdle or frill into the trunk using a hatchet or ax, make the cut at least 1 to 2 inches wide for a small black poplar and 6 inches wide for a larger poplar. This alone sufficiently disrupts the tree's sap flow to kill the aboveground portion of the tree. Skip to step 5 if you are not using herbicide.
Put on chemical resistant gloves, goggles, long sleeves and any other safety gear recommended by the herbicide manufacturer. For girdle or frill applications, you can use undiluted (concentrated) water-based herbicide or mix the herbicide with water so it contains no less than 25 percent herbicide unless otherwise directed by the herbicide manufacturer for girdle, frill or hack-and-squirt application. Use a non-selective herbicide like glyphosate or triclopyr on the black poplar.
Squirt or brush the herbicide solution--which contains at least 25 percent non-selective herbicide--onto the cut surface as soon as possible after making the cuts. Cover the entire cut surface completely, though not to the point of runoff. The herbicide will move throughout the tree, killing tissue and helping to prevent resprouting from the root system.
Monitor the girdle or frill you cut into the black poplar trunk for several weeks following the initial cutting and make sure the wound is not starting to heal over. Re-cut the trunk if needed.
Cut any black poplar suckers or sprouts down to ground level as soon as they appear. Without herbicide, it may take several years of vigilant monitoring and prompt removal of new sprouts to deplete the black poplar's reserves and kill the tree completely.