In Colorado and many other states in the US, we enjoy fabulous fall color in our gardens, parks and wilder landscapes. We notice it most on trees, but many shrubs and other plants change color in the fall too. Have you ever wondered where all that color comes from? Why do the leaves drop off the trees? And what use are all those huge drifts of dead leaves to us?
Color
Most plants have green leaves. This is because chlorophyll in the leaves absorbs the red and blue parts of the light spectrum but reflects green light-waves so we see “green”. Chlorophyll is used in photosynthesis. This is the chemical process by which plants convert light, carbon dioxide and water into oxygen and carbohydrates – i.e. food for the plant. Chlorophyll is an unstable compound and the plant continuously replenishes it throughout summer when good sunlight and high temperatures prevail.
When temperatures cool and nights lengthen, chlorophyll production stops and so does photosynthesis (the plant’s food production system). As the green-reflecting chlorophyll disappears, other colors “appear.” In fact, these colors were always present in the leaves but now they are no longer masked by the green light-waves reflecting from the chlorophyll.
Carotenoids absorb blue-green and blue light and reflect yellow light waves. This is perceived by the human eye as yellow or greenish-yellow. This is why the fall color of birches and aspens is yellow.
Anthocyanins absorb blue-green, blue and green light and reflect red light-waves. This is perceived by the human eye as red through to purple. This is why the fall color of red oaks, sumacs and some maples is red.
Acer tartaricum ‘Hot Wings’ still green
Acer tartaricum ‘Hot Wings’ turning red
The depth and shades of fall color depend not only on the presence (or absence) of these pigments, but also the temperature and sunlight available. Low temperatures and bright sunlight destroy chlorophyll. So, falls with dry, sunny days and dry, cool nights tend to produce the best fall color.
In severely dry falls, the lack of moisture available to the plant may mean that leaves simply die without producing their usual fall colors of yellow, red or purple. The leaves lose so much moisture that the normal chemical processes cannot continue and the leaves dry, turn brown and drop early.
The ‘Fall’
Once the spectacular show of fall color is over, deciduous (i.e. leaf-losing) trees and shrubs drop their leaves. Cooling temperatures and lengthening nights trigger plants into sealing off the point on their stems where leaves are attached so that no more exchanges of moisture and nutrients between the leaves and the rest of the plants are possible. This is called the “abscission layer”. When this layer is complete, the leaves drop (or “fall”).
What use are those dead leaves?
Think about how nature deals with this “problem”. Leaves fall from trees to the ground of the woodland, forest, mountainside, meadow – wherever they are situated. Rain, frost, snow, the trampling of animal feet all help to break the leaves down into smaller and smaller pieces. A huge drift of fallen leaves decreases significantly in size as air spaces between the leaves diminish. An army of creatures from the soil surface, and from beneath it, break down the leaves further through eating and excreting them (think: earthworms, beetles) or chemically decomposing them (think: fungi, bacteria). In this way, the dead leaves are decomposed into the tiny elements that create soil. It’s a mixture of humus and minerals. The humus is the last vestiges of the leaves that are hard to break down like cellulose and the minerals are the chemical components of the leaf tissue, e.g. nitrogen and carbon.
The humus and minerals help to form new soil structure in which new plants can seed, germinate, develop and continue the cycle. The new fertile or replenished soil provides the moisture and nutrients that the now-leafless trees will need to survive winter and re-start photosynthesis and growth in spring.
How to deal with those huge piles of leaves
We’ve seen above that the dead leaves have an important part to play in the garden’s eco-system. So, what can you do?
- Just let the leaves remain where they drop on garden beds. They provide great mulch to maintain soil temperatures and protect plant roots and will rot down over winter, improving your soil as they go.
- Leave a thin layer of leaves on lawns. Rake or blow them off (if you must) but a thin layer of leaves (especially if you run over them once or twice with the lawnmower) will break down quickly and help re-vitalize your lawn.
- Rake or blow leaves off walkways, drives and sidewalks on to adjacent garden beds, so that these hard landscape areas are visible and don’t become slippery. Do not sweep or blow leaves into the street, as they can cause serious blockages in street drainage systems.
- For a neater look, you can blow the front edge of borders clear, letting the leaves accumulate at the backs of borders and behind and below larger plants.
- Put layers of leaves in your compost bin (even better if you can run the lawnmower over them first) between your layers of green garden/kitchen waste.
- Save the leaves in plastic trash sacks (stored in an unobtrusive part of the yard) and let them rot down over winter, to be returned to the garden when they have decomposed. This leaf mold (the lovely dark brown material you get from decomposed leaves) is like “gold-dust” to the soil.
- Save the leaves in an open cage made of upright posts and chicken wire to decompose – more “gold-dust”. If you have room, let your neighbors drop their leaves in the cage too.
- BUT if leaves come from a diseased plant e.g. one with powdery mildew, black spot (roses), apple scab, anthracnose, they should be collected up and disposed of as garbage to help prevent re-infection in the next year.
Leaf cage made from old timber and chicken wire
Fallen leaves make great mulch (and are warm and cosy for the dog!)
If none of these options work for you, you can dispose of the leaves in degradable paper sacks which are usually available from your local hardware store at this time of year. The sacks can be collected with your normal weekly trash service. Some towns and cities will have leaf drop-off points where you can take the bags for the city to collect. The city will then use the leaves to make leaf mold for local parks or otherwise dispose of them. If you can’t do these things yourself, look for a local lawn service company that can, or hire a local teenager to help.
But, whatever you do, remember that the leaves really belong on the ground. That’s nature’s way, after all.
Acer saccharinum (silver maple) turned yellow
Quercus rubra (young red oak) turned red
Anne Hughes/Denver County Master Gardener
Ailanthus altissima or Tree of Heaven, is a notorious, fast growing, invasive plant which left unchecked can quickly become a large shrub or substantial tree. An urban nuisance, you’ve likely seen it jumping out of cracks in sidewalks, pushing through established shrubs and lining alleys where little else will flourish. I recently found a seedling growing in the corner of my pebble-lined basement window well, a testament to its ability to survive in poor soil with little moisture. It will grow most anywhere except very dense shade and swamps and is capable of choking out desirable plants, forming thickets and damaging sewer lines or building foundations. The vigorous lateral root system sends up numerous suckers and the plant will also easily establish by seed. Remember the novel A Tree Grows in Brooklyn? The title’s tree is Tree of Heaven – aptly used as a metaphor for determination and survival.
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