I’m sure you have noticed that many more trees than usual have kept their withered leaves this year. Yes, there is a botanical term for this: marcescent! We had a most beautiful fall but before some trees turned fall color and dropped their leaves we had a deep freeze. In fact, we recorded the coldest November temperature in 20 years!
Virtually all locally native trees (except some oaks that normally hold marcescent leaves) had already dropped their leaves before our deep freeze and were unaffected. Most of the trees that were affected were strains from more southerly climates and their marcescent foliage is a signature that the plant is not fully adapted to our climate. Sweetgums and oaks from Southern sources were the main trees showing the abnormal “freeze-dried” green foliage that was prematurely killed. (Shumard Oaks in the Perennial Garden shown above, image from Dec. 3, 2013.)
Many of our native oaks normally hold marcescent leaves but they never do so while still green. They can be beautiful shades of brown from two-toned tan on Swamp White Oaks (right above) to rich pinkish browns on White Oak and medium brown on Shingle Oaks (left above). I rather like these leaves in the winter landscape and they can be a great place for birds and wildlife to find some shelter. Marcescent oaks can even make good windbreaks! The above image taken Dec. 3, 2013 near the Fountain Garden.
So why do oaks hold their leaves? Since a tree can’t speak we have to make scientific observations to learn why. It appears many oaks hold their leaves into winter to ward off browsing by animals like deer and elk. Hard to imagine that elk once roamed over the entire Midwest! The unpalatable and nutrient poor leaves make it difficult to get to the rich, nutritious buds for next season’s growth at their base. That is also why marcescent foliage is more often observed on younger oak trees or at the base of larger oak trees. The image above is of the lower branch of a rare native hybrid Shingle x Northern Red Oak (Quercus x runcinata) in the Rock & Waterfall Garden. Very few leaves can be found on the higher branches.
One of the downsides of the marcescent fall, especially on Sweetgums is that those trees will now be more likely to catch ice or wet snow and be damaged by a winter storm. None of Powell Gardens’ Sweetgums have this trait and I recommend to buy them in fall and only trees with timely and beautiful fall color. The same can be said for most oaks. The above shows a Sweetgum in normal fall color but shows a glimpse of a tree to its right that was still mostly green — trees that didn’t turn are now covered in marcescent foliage around town.
There is one Missouri Oak that displays a unique trait that is beyond marcescent: it is a fine line between tardily deciduous and semi-evergreen! It’s the Water Oak (Quercus nigra) native across much of the American South and reaching the Missouri boot heel (see the above image of one growing in Blue Springs). Water Oak’s leaves were not killed by this latest freeze and in fact the leaves are hardy down to zero. They gradually color and drop through fall and winter, a few staying alive and green on the tree all winter. Yes, the leaves and the whole tree for that matter can be killed by a severe winter here. It is one oak we don’t yet have in our Powell Gardens collection but we plan to grow a good hardy one grown from the local champion tree which has shown extraordinary hardiness. If you know of a Water Oak in Greater Kansas City let us know — we know of only a handful of trees and they are easy to spot this time of year.