Here’s how fall is supposed to work: Tree leaves turn red or gold in late October, then they turn brown, then they drop to the ground, then suburbanites grouse for the next two weeks about all of the dratted raking the whole thing caused.
Something weird happened this year, though.
A prolonged warm spell into early November faked our trees and shrubs into “thinking” the growing season was still going on. That short-circuited the usual, gradual, leaf-turn-and-drop, and when a sudden nosedive down to 19 degrees happened in the early-morning hours of Nov. 11, leaves went straight from green to flash-frozen brown.
Leaves went straight from green to brown this fall — and are still hanging on — instead of turning color first.
Curiously, many of those leaves are still hanging around – enough so that even non-plant-people are noticing and wondering why the dead leaves aren’t dropping.
The explanation has to do with a little-known process called “abscission.”
Annette MaCoy, the Penn State Extension educator and Master Gardener coordinator in Cumberland County, says this is a tree’s way of pulling nutrients from the leaves and then sealing them off from the twigs and branches when they’re no longer needed.
Leaf-dropping trees like maples and dogwoods don’t manufacture nutrients during the cold and low-light conditions of our winters and so leaves become useless appendages that otherwise would just be a way for the trees to waste water through evaporation.
Those no-longer-needed leaves don’t just fall off, however. They’re actually pushed off as woody plants create scar-like tissue at the base of the leaf stems. This is called “abscission” tissue.
MaCoy says the abscission process is supposed to be “a gradual process as the weather cools, ending with the final ‘ungluing’ that allows the leaf to drop. The sudden cold interrupted the process by freezing the leaf tissue overnight. The trees could not complete abscission. They kind of got ‘stuck’ partway through the process.”
MaCoy doesn’t foresee this short-circuiting as causing any significant plant damage, although those brown leaves could hang around for awhile.
“There’s really nothing to do about it,” she says. “I think the leaves will either gradually fall off with winter winds, or some may hang on until spring arrives and new foliage pushes them off as the buds expand.”
While species such as oak, beech and hornbeam usually hang onto their brown leaves through much of winter, it’s unusual to see our other leaf-dropping species brown instead of bare at this point in the year.
Besides the somewhat “dead” look it’s giving to the landscape and woods, the wacky fall weather ended what could have been a spectacular year for fall foliage – given the plentiful rain and good growing season we had up until then.
The widespread browning was particularly noticeable last week at Hershey Gardens, where the 4,000 rose bushes (not to mention scores of trees) were clothed in saggy brown.
Jamie Shiffer, senior manager at the Gardens, said he doesn’t expect the browning will cause any plant damage. Staff went ahead and pruned the roses as usual after the Nov. 11 hard freeze sent them into winter dormancy.
His only concern is that if a heavy snow happens before the leaves come off, snow could stick on the leaf surface and create extra weight that could then snap branches.
What to do if you don’t like the brown look of this year’s late-fall landscape?
If it really bugs you, you could pull off brown leaves that are reachable from the ground and/or use a blower to create your own mini-windstorm for leaves a bit higher up.
Otherwise, let nature take its course. Better that than falling off a ladder while trying to “neaten” your trees.
Read George’s PennLive column on winter-prepping jobs in the garden.
Read George’s PennLive column on why there’s no need to rake every last leaf out of the yard.
Read George’s PennLive column on what we learned from the 2017 growing season.
Why Are Leyland Cypress Trees Turning Brown
Warm falls followed by cold winters can take its toll on Leyland cypress trees.
Leyland cypress (x Cupressocyparis leylandii) has a lot going for it. It’s fast-growing with evergreen, feathery foliage and a pleasing, slender profile. It makes an excellent specimen tree or screening plant. A cross between two Pacific coast species, the Leyland cypress thrives best in moist, cool climates with moderate temperatures. These trees are hardy to zone 6, however, they do not tolerate sudden temperature fluctuations. We indeed experienced some of these very cold sudden temperature fluctuations this past December and January. The first winter damage symptoms will begin showing up as browning and dieback this spring as temperatures begin to warm and stimulate new growth.
Since Leyland’s are often used for screening and windbreaks they are frequently exposed to temperature extremes and windy conditions that lead to drying out and cold damage. One of the weaknesses of Leyland’s is their shallow root systems which makes them susceptible to stress through desiccation. Another weakness is the dieback and death of the water-conducting tissue and cambium layer just under the bark during extreme winter temperature fluctuations.
There is no actual treatment for winter damage on Leyland cypress. Before pruning, allow the damaged tree to begin new spring growth. Often, if the damage did not injure the branch, new growth will emerge and the browned needles will drop off naturally. If new growth does not emerge, the branch was severely damaged and should be pruned above where green color is still visible. Leyland cypress trees are tolerant of heavy pruning, but if more than one-third of the tree is damaged, the tree may need to be replaced.
Winter cold injury can lead to greater infections from fungal diseases, such as Seiridium canker. Photo: Dave Clement, University of Maryland Extension
Winter cold injury can often lead to greater infections from a few common fungal diseases that affect foliage, stems, and branches such as Seiridium and Botryosphaeria cankers, as well as Cercospora needle blight. Symptoms of canker diseases include branches that start to turn yellow to reddish-brown. Closer examination will reveal slightly sunken cankers, with resin exuding profusely several feet down on the infected branch, usually closer to the main trunk.
The fungal cankers spread primarily by releasing spores during rainy spring weather. The rainwater will carry the fungal spores to other branches. Infection on multiple branches throughout the tree or on the main trunk can kill the entire tree. The only known cure for Seiridium or Botryosphaeria cankers is pruning the infected branch below the infected area.
In the future, it would be best to diversify evergreen borders with a variety of evergreens and deciduous plant selections rather than just one species.
By Dr. Dave Clement, Principal Agent, University of Maryland Extension, Home & Garden Information Center. For plant disease and pest updates, follow Dr. Dave on Facebook.
Resources from the Home & Garden Information Center
- Winter Damage to Trees and Shrubs
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