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.)
Normal leaf fall
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.)
When
marcescent leaves spell trouble
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.)
Three
concerns
When we see a tree or shrub fail to drop leaves normally:
- We investigate to see if there's trouble we should try to
correct.
- We keep an eye on the tree during winter storms. Foliage
holding wet snow and ice can greatly increase branch weight, and we
may need to prop iced branches or remove snow. (Right and
below: The kousa has to hold extra weight when it's not only twigs
holding ice but leaves, too.)
- We watch the next year for signs the plant needs extra water,
and to catch any pest trouble and nip it in the bud. All those
holes that didn't seal over are open ports where moisture can be
lost until new bark forms, or they can become entry points for
pathogens.
Closer look
We looked more closely at the oak
in question and found problems. We think it's
probably no
big deal but it should be watched next year to see if the
problem continues or is on a natural wane.
