Landscape and Nursery - Leaf Retention can be a Sign of Problems or Can Be Normal

Leaf retention in woody deciduous plants can be a sign that the plant has problems that you need to look for. However, in some species it is normal. The following is some information on the subject.

Leaves emerge in the spring, function during the growing season, turn colors and drop in the fall. Leaf drop occurs whether the plant is deciduous or evergreen; it just happens with different aged leaves. There are times when leaf drop or retention is normal and times when it may be related to a plant problem. Leaf abscission or drop is triggered in the fall by seasonal changes in daylength. Chlorophyll degrades, and the plant hormones abscisic acid and ethylene cause an abscission layer to be formed at the base of the petiole. In evergreen species, needles usually function for 2 to 3 years, depending on the species. Normal leaf yellowing and abscission occur on the insides of the stem on 2- or 3-year-old wood. Yellowing or browning of needles on the shoot tips is a sign of problems.

If a stem or leaf is injured or dies before seasonal changes occur, abscission layers will not form and leaves will remain on the stem. Leaves remaining on isolated stems in the fall or winter are signs of stem or leaf problems. Stems may be girdled by an insect borer or mechanical injury, causing a break in vascular connections resulting in stem death and leaf retention. An early killing frost prior to the formation of the abscission layer may cause dead leaves to be retained throughout the winter. In some deciduous plant species, leaf retention is a normal characteristic. Marcescent leaves are the dead leaves retained by some plant species. Typical examples of genera exhibiting marcescent leaves are oaks and beeches. Species within these genera do not complete the formation of the abscission layer until the following spring. Leaves will fall when bud break occurs and stems resume growth. If leaf retention is a concern, identify the plant species, check for evidence of stem injury, and review recent weather conditions.

Pin oak is often marcescent even as a mature tree. Photo by Nancy Rose, University of Minnesota.

Deciduous trees that hold onto their leaves through the winter are described as marcescent (mahr-CESS-ent). Some tree species are more likely to show marcescense than others. Oak species (Quercus) including pin oak, red oak, and white oak are the most noticeably marcescent trees. American hornbeam, also known as blue beech (Carpinus caroliniana), and ironwood, aka American hophornbeam (Ostrya virginiana) also tend to hold leaves. Other trees such as sugar maple are occasionally marcescent.

In autumn, the leaves of most deciduous trees develop an abscission layer where the petiole (leaf stalk) meets the branch. This allows the leaves to fall off without leaving an open wound on the stem. Dry leaves stay on marcescent trees because the leaves didn’t develop the normal abscission layer in autumn.

Marcescence is often a juvenile trait and may disappear as the tree matures. It also may not affect the entire tree; sometimes leaves persist only on scattered branches. Marcescence is typically based on a tree’s genetics, but sometimes weather plays a part. In years with early freezes tree leaves may be killed before developing an abscission layer, resulting in persistent brown leaves on many trees that aren’t usually marcescent.

Information from a section of "Abiotic Plant Disorders - Symptoms, Signs and Solutions A Diagnostic Guide to Problem Solving" by Robert E. Schutzki and Bert Cregg, Departments of Horticulture and Forestry, Michigan State University Michigan State University. Go to http://www.ipm.msu.edu/cat08land/pdf/9-19abiotic.pdf for the full factsheet with photos. Information and photo on marcescence is from and article in the March 1, 2007 edition of Yard and Garden News from University of Minnesota Extension.