Fall(ing) – Why Many Leaves Aren’t

Nature is seldom predictable. This fall’s interesting weather has left many trees still holding their lifeless brown, apparently freeze-dried leaves much later than normal. Oaks, beeches, baldcypress and hornbeam are always among the last to shed completely. This fall’s weird weather wackiness has caused other species, maples in particular, to also be modest about fully disrobing.

A number of customers have posed the understandable question – "What has caused our trees to still be retaining so many of their leaves this late?" To speculate on this answer, I should share some of the science of fall leaf drop.

Let’s bypass a discussion of fall color and get right to it. As fall days get cooler and daylight hours decrease, changes take place in the cells at the base of a leaf’s attachment to the branch. A “wall” of specialized cells develop there that weaken the attachment to the plant. This abscission layer acts almost like a razor and “cuts off” the leaf, allowing it to drop under the stress of age, wind or precipitation. So, yes,  the oaks, beeches, etc. that don’t drop leaves aren’t forming abscission layers. The term for Fall/Winter leaf retention is marcescence and you’re right, that term won’t be showing up on Jeopardy… ever.

Here are two current theories on why some trees are aberrant and hanging on to their leaves for dear life:

1) We had lots of rain this fall so drought stress is a nonissue. Plants simply weren’t in a hurry to consider fall dormancy. October snow and well-below freezing temps likely caught the maples (and other literal hangers-on) with their abscission layers not fully formed. The leaves literally froze (with no fall color) still “stuck” on the tree.

2) Climate change- Increasing average daily temps and the lengthening of fall may trick trees into NOT producing an abscission layer, or not producing it early enough to drop their leaves before cold weather strikes.   

Will this late leaf retention phenomenon have consequences for the future growth and health of the tree? Perhaps not directly, but I can see several potential lateral consequences:

1) While admittedly an OCG (obsessive-compulsive gardener) the gentle raining down of leaves over the next few weeks or months transcends a neat-and-tidy appearance. Leaves matted and frozen into your turf for the winter generally become bare spots when the leaves thaw and are removed in the spring. Aesthetics of a spotty lawn aside, large bare areas (4-6” in diameter or larger) in full sun hold the greatest potential for crabgrass infestations in the new year. I’m personally going to continue raking/removing those leaves from my turf as long as the weather allows me to do so.

2) The surface area of larger leaves (magnolias, oaks, etc.) catching and holding wet, icy snow creates a weight issue that can cause limb breakage. I’ve already seen large branches snapped off my neighbor’s Saucer Magnolia this Fall. Unfortunately, when nature does your pruning the raggedy results don’t lend themselves to proper healing. A torn, open wound is a welcome mat for insects, as well as secondary bacterial and fungal infections that spell future trouble. So, assess storm damage and make horticulturally correct pruning cuts ASAP.

I know, aren’t you sorry you even asked about those maple leaves?  

Tony Fulmer

Chief Horticulture Officer