Issue: November 16, 2002

Autumn leaf drop

Why don't the leaves on a tree all fall down at the same time rather than one by one during the autumn season? - Nikki

Leaves fall from a tree with different timing depending on the species of tree and weather. Here in New Mexico, leaves from a mulberry tree may fall all at once (all in only a few days time) after a hard freeze. The leaves from a cottonwood tree begin falling in June and may continue to drop through November, so it depends in part on the type of tree. The cottonwood drops leaves in the summer to conserve water by reducing the number of leaves using water. The mulberry drops leaves in the fall after frost has ended the growing season. Cottonwood leaves drop in the autumn also after the growing season ends.

Weather also has an influence. If the autumn is mild, cooling gradually, then leaves on the outside of the tree may protect the inside leaves like a blanket. The outer leaves will turn colors and drop first, inner leaves may turn and fall more slowly. If there is a sudden hard freeze, then all the leaves may be killed at once and all fall fairly quickly. In that situation there will probably be poor fall color but quick leaf drop.

The same mechanism is responsible for the leaf drop whether the leaf drops in the summer to conserve water or at the end of the growing season. As a result of stress (water or temperature stress), the leaf forms an abscission zone where the petiole attaches to the stem. This abscission zone is a layer of cells that separates the leaf from the rest of the plant so that it may drop without injuring the rest of the plant. Once the abscission zone forms, the leaf easily detaches from the stem without tearing the stem.

There are some trees whose leaves die in the fall, but those leaves are retained. Such leaves may benefit the tree and landscape by providing some wind protection. In nature, by blocking the wind, these persistent leaves trap moisture by forming snow drifts at the base of the tree and conserve moisture by shading the ground at the base of the tree. This is an excellent mechanism for trees in dry environments. Trees behave in several different ways as each variety responds to its environment. This makes gardening interesting.

Frost damaged rose petals

Can I take withered blossoms from my roses after frost to make potpourri?

Yes. Since you will be adding fragrant oils and perhaps other materials to the potpourri, dried rose petals (whether frosted or not) will serve just fine. If you are counting on the fragrance from the rose petals, you may be disappointed. If the flower cells have frozen, the essential oils that give roses their fragrance may have escaped. However, rose oil can be purchased to renew the fragrance in the potpourri.

Send gardening questions to Southwest Yard and Garden, Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu or at https://www.facebook.com/DesertBloomsNM/. Please include your county Extension Agent (aces.nmsu.edu/county) and your county of residence with your question!

For more gardening information, visit the NMSU Extension Horticulture page: desertblooms@nmsu.edu.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, office: 505-865-7340, ext. 113.