It’s Simple to plant a Lombardy Poplar Tree
Keep in mind when selecting your planting site that the Lombardy Poplar tree grows best in full sun and in wet or well drained soils. Dig each hole slightly shallower than the root ball and about twice the size of the root system. For a privacy screen, plant the trees five to eight feet apart. Try using stakes with twine wrapped around them to create a visual reference. Then just dig your holes where the stakes are. It is also suggested that you roughen-up the soil on the sides of each planting hole with your shovel. This will allow the roots to easily penetrate the soil as they grow.
Next, gently separate the roots with and place them downward in the hole. Make sure each tree is vertical in the hole. Use a level to be exact. The top of the root ball should be about an inch above the bordering soil. Turn each tree so that the preferred side of your Lombardy Poplar tree is where you want it.
In stages, fill the hole back in with soil and add water to remove air pockets. Water your Lombardy Poplar tree again after you’re finished backfilling. A good sign that you've gotten the tree wet enough is that air bubbles stop coming up through the water. You should place mulch around the tree to a depth of 2"-3" and up to but not touching the trunk. This will help keep the soil moist and slow the growth of weeds and grass.