The summer has been a wild ride with 10 days of 90°+ temperatures and several torrential rainstorms. I love gardening in May, June, July, September and October. Not so much in August when the combined heat and humidity make for tropical conditions, not to mention the mosquitos. We are now headed to fall, which is my favorite season this time of year. It may still feel like summer to us but the perennials, lawns and trees know from the decreased hours of sunlight and the reduced intensity of the sun that it is time to wind down the growing season and prepare for the dormancy of the winter ahead.
Although a lot of plants are past their bloom time, fall can still be a riot of color in your yard and garden. The blues, whites and pinks of asters will soon be emerging and garden chrysanthemums bloom in an assortment of colors including, white, yellow, gold, maroon, pink and red, and a variety of flower types such as button, pompom and quill are decorative. Tall sedum such as ‘Autumn Joy’ with blossoms that look like large pink heads of broccoli bloom in the fall, and the golden yellow heads of many varieties of rudbeckia (black-eyed Susans) continue to bloom throughout September and into October.
If you don’t have any of these flowering plants to brighten your fall landscape it is not too late to add some now for next year. Late summer and early fall is an excellent time to plant perennials. Plant them at least 4-6 weeks prior to when you expect the ground to freeze to allow for good root development. Keep them well watered until the ground freezes and then mulch them for the winter.
Of course the thing that comes most to mind when we mention fall colors is the changing of our canopy. On average peak tree leaf color in the Twin Cities is from mid-September to mid-October. The normal changing of leaf colors is dependent on two things — decreasing daylight hours and cooler temperatures. With decreasing daylight, trees develop a layer of cells at the base of each leaf which slowly cuts off the water and minerals to the leaf and reduces the production of chlorophyll. Chlorophyll is what gives leaves their green color; without it previously hidden carotenoid pigments become visible. There are two types of these pigments, those that contain oxygen are yellow and those without oxygen – carotenes – are 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. Cartenes = orange = carrots, thus the old wives tale (?) that eating carrots gives you good vision).
The red and purple pigments that show in some tree 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. Once those leaves start to fall and the sunlight can reach the interior and northern leaves they will change color. 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.
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, 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.
Enjoy the autumn colors before it becomes time to truly put the garden snugly to bed for a long winters nap.