According to the National Weather Service, July 2019 was the hottest month on record and here in Minnesota we had above average precipitation as well. However mid-August offered a little hint of fall with cooler temperatures and lower humidity. Whether the cooler weather continues or we experience a longer and warmer fall as we have in the past few years, there is no denying that fall is on the way. The hours and intensity of daylight is dwindling and in response plants and trees are preparing to go into dormancy. Before we know it we will begin to see the change in the tree canopy.

The normal changing of leaf colors is dependent on two things; decreasing daylight hours and cooler temperatures. On average peak tree leaf color in the Twin Cities is from mid-September to mid-October, but that could be changing as we are experiencing warmer temperatures in September and October.

Chlorophyll is the pigment in leaves that give them their green color; it absorbs the red and blue wavelengths in sunlight and reflects the green. As temperatures cool and daylight is reduced chlorophyll is broken down faster than it is produced and gradually previously hidden (but always present) carotenoid pigments become visible. There are two types of these pigments; those that contain oxygen are yellow and those without oxygen – carotenes – are orange. These carotenes are what make carrots orange!  (A carotene you may have heard of is β carotene a precursor to Vitamin A which in humans is a pigment essential for good vision. Carotenes = orange = carrots, thus the old ‘wive’s tale’ that eating carrots gives you good vision). You will see the most beautiful displays of yellow and orange leaves when we have warm sunny days and nights between 32° and 45°.

The red and purple pigments that show in some leaves are anthocyanin pigments and they are not present in the leaves during the growing season but 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. Another factor that affects the production of anthocyanin pigments is the pH level of the cell sap in the tree. Acidic cell sap will produce red hues and alkaline will produce the purples and blues. The exact color of a tree’s leaves can vary from year to year because of the presence of carotenoids and the anthocyanin pigments, and in fact a single tree can show a combination of red, yellow and orange leaves.

Weather is probably the most critical factor in determining the intensity of the fall colors. The intensity of the fall tree color is result of the weather and the condition of the tree. A healthy, pest-free tree that has plenty of nutrients and water, 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. Physiological stresses during the growing season, particularly drought, can impede the trees ability show good color. In years of severe drought instead of yellow, orange and red we have seen tan, toast and russet. Hopefully this year’s abundance of rainfall will provide us with a wonderful display.

Whether it is drought, frost or the natural cycle of the trees the leaves will come down. And then we have to rake them up. When you are attending to this autumnal chore, do not rake your leaves into the street or allow them to go down the storm sewer. Leaves, grass clippings and other organic matter are just as much of a pollutant to our creeks, lakes and river as the trash that ends up in the storm sewer. The organics add phosphorus to the waterways which contributes to algae growth which has many disastrous effects on the health of the river and wildlife.

Bag up your leaves in compostable bags and leave them out for pick up on your garbage day. The City will pick up and compost yard waste until probably early November. The City will also be sweeping the streets this fall. Do not rake your leaves into the street. Be a good environmental steward of our city and dispose of the leaves properly!