News
What do you see when you walk through our urban forest? Most people see a riot of colors.
That's a great start especially with all of the new cultivars of trees and shrubs that have been planted over the last 10 years that give our area the reds, purples, pinks and yellows we all love at this time of year. There is, however, a great deal more to notice than just the colors.
Have you ever noticed that the color change typically starts in the top of the tree and works its way down the tree? Have you noticed that the leaves on some branches turn color sooner than other branches even when the leaves from those branches are intertwined? Have you noticed that leaves in the shade have less intense colors? Have you ever considered the reason for these differences and what this tells you about the tree?
Trees tend to acclimate for winter at the tips of their branches based on the length of day and light intensity. The chemical signal that causes this change in cold tolerance works its way down the branch creating physical and chemical changes in the leaves as seen by the shift in color from green to whatever color the plant breeder has selected for.
In the case of a Tartarian maple (Acer tartaricum) that color could be a deep red as is seen in one of these maples at our office. Or it could be the pink in the top leaves of the Sensation boxelder in our collection of trees at the Mesa County Fairgrounds.
This same signal is received by all trees in our area but not always to the same degree or at the same time. The globe willows and peaches in our area are not as sensitive to these changes as many of our other trees. This is due to where these trees originated.
Many of our trees started to acclimate for winter back in July as the days got shorter. Peach and Globe willow trees often wait until the first frost to start this change. As a tree acclimates from the tip down, one of the last places the tissue will become accustom to the drying and cold conditions of winter is the branch crotch. With peach trees trained to an open-vase system or globe willows with all or most of their branches origination from the same point on the trunk this attachment point is the last to acclimate and thus is most susceptible to winter injury. That is part of the reason why many peach trees have infections of Cytospora canker develop at that point and why globe willows have problems with Frothy flux at those crotch angles.
These diseases enter these points through winter injury due to the non-hardiness of the tissue. If there was a way to encourage these trees to start to acclimate for winter earlier than the first frost we could reduce the incidents of these diseases in these trees. That's why we suggest avoiding any application of nitrogen fertilizer near these trees after July 15 through mid-October, and why we suggest reducing the frequency of watering around these plants in September and October. This helps encourage these trees to prepare for winter properly. A thorough soaking just before irrigation ditch water turns off is still a must-do task. With our native trees or trees from like latitudes, withholding water and fertilizer can cause the death of the tree.
On your walk you might notice the leaves on an ash (you know those trees with seven or more leaflets on one leaf) don't change color uniformly. These compound leaves tend to change color from the lower part of the leaf toward the tip of the leaf. This is due to the mobility of nitrogen in the leaf. Since nitrogen is what keeps the leaf green, when the source of nitrogen is shut down by the development of the abscission layer, the nitrogen in the leaf is shuttled from the lower leaflets to the upper leaflets. As this occurs the portion of the leaf losing nitrogen takes on its fall color.
Now for a question. Why do herbaceous plants tend to change color in the fall from the ground up instead of from the top down? These are the non-woody plants that die down to the ground each year. There is a simple explanation. Do you have the answer? Let me know your thoughts at Curtis.Swift@colostate.edu or leave your answers on my Facebook page at http://www.facebook.com/#!/curtis.swift1
That's a great start especially with all of the new cultivars of trees and shrubs that have been planted over the last 10 years that give our area the reds, purples, pinks and yellows we all love at this time of year. There is, however, a great deal more to notice than just the colors.
Have you ever noticed that the color change typically starts in the top of the tree and works its way down the tree? Have you noticed that the leaves on some branches turn color sooner than other branches even when the leaves from those branches are intertwined? Have you noticed that leaves in the shade have less intense colors? Have you ever considered the reason for these differences and what this tells you about the tree?
Trees tend to acclimate for winter at the tips of their branches based on the length of day and light intensity. The chemical signal that causes this change in cold tolerance works its way down the branch creating physical and chemical changes in the leaves as seen by the shift in color from green to whatever color the plant breeder has selected for.
In the case of a Tartarian maple (Acer tartaricum) that color could be a deep red as is seen in one of these maples at our office. Or it could be the pink in the top leaves of the Sensation boxelder in our collection of trees at the Mesa County Fairgrounds.
This same signal is received by all trees in our area but not always to the same degree or at the same time. The globe willows and peaches in our area are not as sensitive to these changes as many of our other trees. This is due to where these trees originated.
Many of our trees started to acclimate for winter back in July as the days got shorter. Peach and Globe willow trees often wait until the first frost to start this change. As a tree acclimates from the tip down, one of the last places the tissue will become accustom to the drying and cold conditions of winter is the branch crotch. With peach trees trained to an open-vase system or globe willows with all or most of their branches origination from the same point on the trunk this attachment point is the last to acclimate and thus is most susceptible to winter injury. That is part of the reason why many peach trees have infections of Cytospora canker develop at that point and why globe willows have problems with Frothy flux at those crotch angles.
These diseases enter these points through winter injury due to the non-hardiness of the tissue. If there was a way to encourage these trees to start to acclimate for winter earlier than the first frost we could reduce the incidents of these diseases in these trees. That's why we suggest avoiding any application of nitrogen fertilizer near these trees after July 15 through mid-October, and why we suggest reducing the frequency of watering around these plants in September and October. This helps encourage these trees to prepare for winter properly. A thorough soaking just before irrigation ditch water turns off is still a must-do task. With our native trees or trees from like latitudes, withholding water and fertilizer can cause the death of the tree.
On your walk you might notice the leaves on an ash (you know those trees with seven or more leaflets on one leaf) don't change color uniformly. These compound leaves tend to change color from the lower part of the leaf toward the tip of the leaf. This is due to the mobility of nitrogen in the leaf. Since nitrogen is what keeps the leaf green, when the source of nitrogen is shut down by the development of the abscission layer, the nitrogen in the leaf is shuttled from the lower leaflets to the upper leaflets. As this occurs the portion of the leaf losing nitrogen takes on its fall color.
Now for a question. Why do herbaceous plants tend to change color in the fall from the ground up instead of from the top down? These are the non-woody plants that die down to the ground each year. There is a simple explanation. Do you have the answer? Let me know your thoughts at Curtis.Swift@colostate.edu or leave your answers on my Facebook page at http://www.facebook.com/#!/curtis.swift1
