Poplar trees are some of the fastest-growing, most popular shade trees in America. They encompass some 35 species, not including a massive number of hybrids, and can be found in zones 3 through 9. One question many homeowners want to know is, are poplar roots invasive? Poplar trees are invasive because they have an extensive, fast growing, and shallow root system (up to three times wider than the tree is tall) which can lift sidewalks, retaining walls, and clog pipes. Poplar roots send up suckers 100ft away from the base of the tree while spreading seeds rapidly.