Published on 06/10/99

Don't Let 'Summer Droops' Damage Your Trees

The "droops of summer" have come early this year. As water deficits become greater in the landscape, remember that both old and young trees need your help. A few deep waterings can help minimize damage and may save a tree's life.

Water is the stuff of which tree life is made. Trees use water for transporting materials, chemical reactions and supporting growth. Water solutions allow all of the tree's living processes to function properly.

As water becomes less available, trees begin to have problems. One of the first noticeable signs that trees are short of water is leaf wilting. On sunny, hot days, leaves lose water faster than roots can absorb it. As leaves lose more water, they begin to droop.

Leaves Hanging Limp

If you go out in the afternoon and look at your trees, leaves across whole crowns or on individual branches may be hanging limp. By early the next morning, the leaves will be back to normal. Refilling with water overnight means the tree can still extract water from the soil.

Wilted leaves that don't recover overnight signify serious water shortages in the soil. Leaves that stay wilted for three days or more are being severely damaged.

Usually, permanently wilted leaves yellow and die quickly. Sometimes, small amounts of water may keep these leaves hanging on to life (and the tree) even though they're sustaining long-term damage.

Woody cells don't change shape much, even when dry. Whole trees don't change shape or wilt. Succulent new twigs and annual leaves supported by their thin petioles can droop severely. Water expands thin-walled cells. Without water to maintain the cells' size, they shrink or collapse.

Receding Cell Walls

As some leaf cells shrink, their walls pull inward. Each cell shares a wall with a neighboring cell. As one leaf cell shrinks, it pulls other cells closer. As all cells shrink, leaf tissues may appear deformed and shriveled. The leaf stalk, or petiole, on many deciduous trees can quickly lose water, which allows the leaf blade to droop.

Many evergreen trees, like pine, have strong fibers and heavy veins which prevent noticeable wilting. Needles may curl but stay in position.

Many deciduous trees hold their leaves erect in sunlight, but fold and droop under water shortages. The act of wilting can help minimize further water loss from the leaves.

For leaf expansion in spring, many new cells were outfitted for life. Each new cell's wall was thin and flexible, like a balloon. In order to grow, the cell must expand its wall.

Trees' 'hydraulic fluid'

Water is the hydraulic fluid a tree uses to "blow up the balloons" of its cells. Each cell uses nutrients shipped from other cells to help it collect water.

Cells draw water to surround many kinds of materials with a shell or layer of water molecules. The more surface area in individual pieces of nutrients (like sugar) a cell has, the more water it attracts.

The more water a cell can contain, the higher the water pressure becomes, and the thin cell walls expand outward. The cell begins to grow. Finally, the cellulose strands in the cell's wall stop further expansion, and a "glue" is deposited around the wall to hold everything in its new, expanded shape.

If water isn't available, tree cells can't grow. If the cells dry too much, the new walls may collapse inward. One symptom of severe drought damage is small, stunted and deformed leaves.

Keep an eye on your trees. Don't let the "droops of summer" cause permanent damage.

For more information on trees, droughts and watering, visit the University of Georgia School of Forest Resources Web site at: / .

Kim Coder is a forester with the University of Georgia Warnell School of Forestry and Natural Resources.