We're worried about our oak. Why would an oak tree turn brown early? This fall it went from green to brown a lot quicker than other oaks around it, and hung onto its leaves. - R.S. -
It may be totally okay. Or it may be saying "I could use some help, here." Look for signs of stress now and you can decide whether to ignore it or provide the tree some help next year. (We help you with that closer look.)
Leaf fall is a living process. In fall natural cues tell the tree or shrub to grow an abscission layer -- material that physically separates leaf stalk from twig. Whatever starch and nutrients are still in the leaf dissolve and ship out to the wood during the stoppage, before the abscission layer seals over the holes where wood and leaf exchange sap and starch.
Sometimes marcescent leaves are just natural
However, some leaves do not separate in autumn but hang on naturally through winter even though they are not evergreen. They're called marcescent leaves, and we see them most often on members of the beech family. (This includes beeches, true chestnuts and oaks, winning oaks the vote as worst tree to have if you don't like raking). Most often, these are leaves produced on juvenile wood -- branches without flowers. Juvenile wood may occur anywhere on a tree and although there may be more of it on a young tree it can occur anytime in the tree's life. The leaves on these branches hang on until spring.
Right: Oak flowers dangle from a mature branch.
The leaves on this branch will abscise in fall.
Since very young oak trees (under 25 years) may not have any flowering wood at all, they're often nearly covered in brown foliage all winter. Mature oaks almost always have some juvenile wood.
Once in a while an older oak will skip flowering for a year (even in its prime at 150 or 200 years old) and then it hangs onto most of its leaves until spring. This
might follow a year of very heavy nut production, almost as if the tree is resting.
Below: The young oak that gave us this Yule Branch is mature -- nut producing. It shed the leaves from all its mature branches. Although it's 50 years old, it still has as much or more juvenile wood with marcescent foliage as mature wood.
Right: One comforting sign is that the leaves on the oak
in question didn't shrivel and die, then hang on.
They remained firm until the end.
(Photo ©2013 R.S. c/o GardenAtoZ.)
If a leaf is killed before the abscission layer finishes growing, it may hang there until it's rattled lose by wind or until spring growth dislodges it. Japanese maples in building-warmed courtyards often fail to get the necessary cues that tell them to drop their leaves in fall. Then one night when it suddenly becomes very cold, all the leaves die while still attached to the tree.
Any stressed tree or shrub might do this. Our own kousa dogwood struggles because it was planted too deep. Every three or four years it still has leaves when winter comes. The first time it did it, it was an alert and we investigated. Now, it's a reminder, "I still need help." (That we've not gotten around to rescuing it is a long story involving not only scientific curiosity but own-garden procrastination.)
When we see a tree or shrub fail to drop leaves normally: