On average, peak tree leaf color in the Twin Cities is from mid-September to mid-October, but this year the color seems somewhat delayed. With the exception of some brightly colored maples, the colors appear a bit dull and some leaves just turn brown and drop off. Outside my window in mid-October were several trees that were totally bare of leaves, a few that were a drab yellow and many that were still green! Some of this may be due to the drought, but some may also have to do with warm temperatures we had in September. The normal changing of leaf colors is dependent on two things — decreasing daylight hours and cooler temperatures. There are four groups of biochemicals that are responsible for the yellows, oranges, reds and purples we see in the fall.
Chlorophyll is what is responsible for the normal green color of leaves; chlorophyll absorbs the sun’s red and blue wavelengths, and reflects the green. It gathers energy from sunlight during photosynthesis. During the shorter days and cooler temperatures of fall chlorophyll is broken down faster than it can be replaced and then the carotenoids are revealed.
Carotenoids are responsible for the yellows and oranges in fall leaves. The pigment is present in the leaves at all times but plays only a small part in the process of photosynthesis. It is only when the chlorophyll breaks down that the yellows and oranges are revealed. There are two types of these pigments, those that contain oxygen are yellow and those without oxygen – carotenes – are orange. They are what make carrots orange. You will see brilliant colors most often when days are sunny and the nights are cool, between 32° and 45°.
The red and purple pigments that show in some tree leaves are anthocyanin pigments and they are not present in leaves during the growing season but rather are formed in the fall. The formation of these pigments is dependent on sunlight, which is why the leaves on the tops and south are the first to change color. Once those leaves start to fall and the sunlight can reach the interior leaves they will change color. The color is affected by the PH level of the soil in which the tree is growing. Tree leaves growing with high acidity will turn red and those with higher alkalinity will turn purple or even blue. Various combinations of anthocyanins and carotenoids can produce yellow, orange and red leaves on the same tree. Many popular fruits contain anthocyanin pigments including blueberries, raspberries, cranberries, cherries and apples. Often you will see an apple that is very red on one side and paler or even yellow on the other. The red side is the one that was exposed to the sun and the pale side was probably shaded by leaves. And if part of the tree is shaded by other structures that part will often be duller than the sunny side.
A group of compounds called tannins are produced when both chlorophyll and carotenoids breakdown and cause the leaves to turn brown. Most oak trees do not add to the color of the fall because the amount of tannins in their leaves cause the leaves to turn brown.
The intensity of the fall tree color is a result of the weather and the condition of the tree. A healthy, pest-free tree that has plenty of nutrients and water, and which receives plenty of bright sunshine and experiences cool autumn days and cool (but not freezing) nights, will put on a splendid show of color. Frost does not add to leaf color, in fact frost kills leaf cells; an early autumn frost can put quite a damper on the autumn show.
Plants In – Bugs Out
If you have not already brought in your houseplants that have spent the summer outside, you will want to do so soon; generally temperatures of 45°- 50° will kill houseplants. When you do bring them in be sure to check them for aphids, mealybugs, white flies and other pests. Spray them with water to remove the outside dust and soft bodied bugs. Wash the top and bottom of the leaves with cotton ball soaked in soapy water and rinse. Wash the outside of the pots and add soapy water to the soil to kill the pests there. Check the plants for pests each time you water.
Chilly mornings and shorter days signal bugs to prepare for winter. Some will burrow into the ground or bury themselves in leaves. Some will simply die but others will look for ways to overwinter in our houses. There are three species in Minnesota that are especially aggressive at finding their way into our houses; Box Elder bugs, Multicolored Asian Lady Beetles, and Brown Marmorated Stink Bugs. Fortunately these bugs are really more of annoyance or nuisance than anything else. They don’t do harm — they don’t feed on wood or building materials, although Lady Beetles and Stink Bugs smell bad when crushed. To keep them out, seal up your home; patch window screen holes, repair broken seals around doors and windows and seal around pipes, vents, and wires that enter your home. Using pesticides is probably not a good idea. They would be ineffective on the bugs as they are not actively feeding and the pesticides may be harmful to people and pets.
So “batten down the hatches” and prepare for the next season.