October 2, 2002
Woodbury Bulletin
Betsy Halden

St. Croix River valley fall color

Each year we are treated to a spectacular display of fall color in the landscape in and near the St. Croix River valley. It is one of the perks of living in our Zone 3 - 4 climate; the conditions favor the expression of the underlying pigments present in foliage. Some years the expression is more brilliant than other years. This is due primarily to local weather conditions during the early fall. In general, when there are bright sunny days followed by chilly nights, the fall color is at its best. The explanation for this fact is physiological; during warm, sunny fall days, leaves manufacture a lot of sugars. If the night temperatures are warm, the sugars are readily translocated out of the leaves into the stems and roots. If the night temperatures are cool, in the range of 45 F or less, the sugars are trapped in the foliage. The accumulation of sugars in leaves favors the production of the red pigment called anthocyanin. The brilliant reds and scarlets of the maple trees and other plants are the result of the build up of anthocyanin and tannins. Tannins are responsible for the garnet and burgundy color of oak leaves.

As fall progresses, the production of new chlorophyll ends and the remaining chlorophyll that is present in the foliage begins to break down. Yellow fall foliage is the expression of two pigments that are present all season but are masked by the chlorophyll. As chlorophyll breaks down, the underlying pigments, carotin and xanthophyll, are revealed.

Other factors that play a role in the intensity of fall color include the site conditions, orientation of the plant and the amount of shade that an individual plant may receive. Frequently plants that are situated near ponds, bogs, marshes, or other low areas are the first to show fall color. This makes sense because these areas are where cold air settles on quiet nights. Sometimes a particular plant my have part of its foliage growing in the shade. The leaves on this portion will not manufacture the quantities of sugars necessary for the development of anthocyanins. That particular plant may be vivid scarlet on the sunny side and yellow on the shaded side.

If you would like to learn more about fall foliage color go to www.extension.umn.edu and enter fall foliage color. You will have your choice of sites. The Minnesota Department of Natural Resources also has a web site where you can get an update on where the fall color in Minnesota is at its best. The address is www.dnr.state.mn.us/fall_colors/index.html.

Not only is it non-colorful, but buckthorn is a huge problem in our area. It is a non-native plant and it is terribly invasive. We need to help eliminate it from our urban landscapes. The University of Minnesota Extension Service has helpful information about what we can do to eradicate this undesirable large shrub. Go to www.extension.umn.edu, and, as I write this, the Extension Service Homepage has a feature article on buckthorn. If necessary, enter buckthorn in the search box.

Betsy Halden is a resident of Woodbury. University of Minnesota Extension Service and Iowa State University Extension Service materials were consulted in the preparation of this column. You can reach Betsy Halden at eahalden@comcast.net.