Fast-growing poplars make ideal firewood trees and shade trees, springing up quick. Poplar trees reach a height of 40 to 50 feet and can grow up to 8 feet per year. While poplars require pruning to shape mature trees and train young trees, the wood's characteristic weakness makes annual pruning necessary.
There are a few good reasons to prune poplars. Pruning helps remove dead, diseased or structurally weak wood, which can occur on sick or weak poplars. Pruning out infected limbs on diseased trees helps prevent the disease from spreading, maintaining tree health. Mature poplar trees may be too tall for you to prune, even with a ladder. When your tree gets too tall for you to handle safely, call a landscape professional to cut it.
Poplars can be pruned in either the spring or fall. Avoid pruning in late winter or early spring, since poplars pruned at this time ooze a sap. While this is not harmful, it detracts from the tree's appearance. Fall pruning -- Virginia Cooperative Extension's Bonnie Lee Anderson recommends October to December -- is the best time to trim these trees. When pruned in autumn or in late spring, they won't ooze sap.
Poplars have naturally weak wood and poor structure. To prevent injury to someone in your family, inspect the tree each year and remove wood that is compressed by other limbs, since it's under pressure and could break. While removing visible weak wood helps, environmental damage can strike. A bad wind or snowstorm can tear branches off your tree -- due to the wood's weakness -- leaving both your tree and lawn a mess. When this happens, clean up the fallen wood from your yard and prune back broken and damaged branches on the tree.
Poplars grow in USDA hardiness zones 3 to 9. While growers in the southern end of the range can prune poplars in the fall without worrying that winter frosts will harm the tree, more northern gardeners can't. If you experience severe winter frosts, prune your poplar in late spring to prevent winter injury to the just-pruned tree and avoid the oozing sap of early spring.