Have you noticed that our state tree, yellow poplar (Liriodendron tulipifera), is looking less than stately right now? All around the bluegrass, yellow poplar leaves are turning yellow and dropping. Leaves on inside seem most affected but this yellowing occurs relatively evenly distributed top to bottom in canopy.
While there are several insects and diseases that can affect yellow poplar, the yellowing we are currently seeing is a physiological issue and demonstrates yellow poplar’s response to stressful conditions. We’ve recently enjoyed rain and cooler temperatures but, prior to that, it was very hot with no rain for several weeks. Yellow poplar’s response to this “mini-drought?” Drop leaves and conserve resources.
You may be wondering why yellow poplar is stressed by normal summer conditions if it is a native tree. While yellow poplar is well adapted to the summer, it prefers to grow in deep, rich, moist soil that is well-drained, pretty different from the lawns and parking lots where it is commonly planted. Trees in the forest aren’t showing these symptoms (although they may have leaf problems from other issues like the yellow-poplar weevil) while, trees growing “off-site” are likely to show these symptoms first since they are less tolerant of additional environmental stresses. Yellow poplar in the bluegrass is at a disadvantage because it does not thrive in our soils or in the landscaped settings where it is typically planted. While it may still grow well in general, it will be more susceptible to environmental, disease and insect stress than if it was growing its ideal location.
The good news is that yellow poplar is resilient and trees with these symptoms will likely recover well. Yellow poplar will put out leaves to replace those lost and it is not likely that this mid-summer leaf-drop alone will hurt long-term health of trees. On the other hand, if drought is prolonged or if other stresses are severe it could be a more significant issue for the tree over time. Either way, if you (or your neighbor) have a yellow poplar it’s probably time for some summer raking!
Fun fact: Fall defoliation of our temperate species is an adaption from the response of tropical plants to drought. Yellow poplars are in the Magnoliaceae family, a relatively primitive family that evolved when the earth was wetter and warmer. Summer yellowing in yellow poplar is likely an extension of this ancient response to drought in the tropical rainforest.
Submitted by Dr. Ellen Crocker, Department of Forestry and Natural Resources Dr. Bill Fountain, Department of Horticulture University of Kentucky