What causes autumn leaves to stain concrete?

Each year, in early autumn, I notice tree leaf silhouettes staining the sidewalk. Do all trees do this? I mainly see maples and occasionally oak leaf shapes. What substance in the leaves causes this staining effect?

Tannin in the leaves is responsible for leaving behind those silhouettes. Rain rinses the tannins from the fallen foliage, and leaves a trace. The prints are most visible on lighter colored surfaces. This is similar to the way strong black tea leaves a tannin stain on porcelain cups and tooth enamel.

The very word ‘tannin’ has its origins in the Latin, tannum, meaning the bark of an oak tree (which was used to tan animal hides to make leather). Oak has especially high levels of tannins, but because its leaves are thick and take longer to decompose, you may notice fewer leaf prints from oak trees.

Tannins are widespread in many different plants, and may be present in many parts of a plant. They are especially common in leaf tissues, “particularly in the cells of the upper epidermis (on the top surface of a leaf).”

The substance may also be found in the bark and wood of trees, as well as the buds, stems, fruits, seeds, roots, and plant galls, where it may provide protection to the plant. For example, cottonwood trees can adjust their level of tannins to defend against beavers harvesting their wood.

Autumn, Leaves, Tannins

Leaf-out date for deciduous trees

Can you direct me to a list of deciduous trees whose leaves generally emerge in early spring, or a list of trees ranked in order of their leaf emergence? I know this will vary from year to year and from individual tree to individual tree due to climate and genes, but if there is a list out there with a general sequenced time schedule, it would be a great tool for design.

An early American observer of the varying timing of leaf emergence was Henry David Thoreau, whose journals list leaf-out dates for the trees and shrubs he saw in Massachusetts in 1854. In fact, his data is now being used in climate change research. Though it’s a subject that hasn’t been much approached from a garden design standpoint, the increased interest in climate change means that more research on phenology and the leafing out sequence is becoming available. There are several articles by Richard Primack in the Arnold Arboretum newsletter, Arnoldia. Primack (of Boston University) is a specialist in this topic. Primack and Caroline Polgar co-authored “Leaf-out phenology of temperate woody plants: from trees to ecosystems” (Arnoldia, Volume 68, Issue 4,2011) which states that “maples (Acer spp.), birches, alders (Alnus spp.), and poplars” tend to leaf out earlier, while “oaks, ashes (Fraxinus spp.), and hickories (Carya spp.)” are among the later-leafing trees.

The article “Why Do Temperate Deciduous Trees Leaf Out at Different Times? Adaptation and Ecology of Forest Communities,” (The American Naturalist December 1984, Martin J. Lechowicz) has a chart (p. 825) showing the tree species the author studied leafing out in this order:

Acer rubrum
Populus tremuloides
Betula papyrifera
Sorbus americana
Acer saccharinum
Betula alleghaniensis
Ulmus americana
Tilia americana
Quercus macrocarpa
Fraxinus pennsylvanica
Populus grandidentata
Fraxinus nigra

Another chart on the same page compares 1980 and 1981 leafout dates, with Populus tremuloides and P. balsamifera and Betula species consistently leafing early, followed by Acer and Prunus, then Fagus and Populus grandidentata, then Fraxinus and Tilia, and finally Carya and Juglans.

You may want to read a short article in the March 11, 2015 online version of Conservation Magazine on predicting the future of forests, based on two centuries of data from citizen scientists in England. Here is an excerpt:
“It is likely that the variation in each species’ sensitivities to both spring forcing and winter chilling will mean that forests will look quite different in the future. Those species for whom spring forcing is most important will grow leaves earlier in the year; those for whom the autumn and winter chill is more critical could leaf later in the year. Eventually, a late-leafing species like oak might wind up growing its leaves earlier than an early-leafing species like birch.”

Another resource that may help you determine the leaf-out date of specific trees is The Botanical Garden: volume 1: Trees & Shrubs by Roger Phillips and Martyn Rix (Firefly, 2002). The book presents photos of branch samples from many tree species, often showing the young leaves associated with a date (though not with the geographical location; bear in mind that the authors reside in England). While it doesn’t have such a photo for every tree, it might have enough trees for you to get useful data.

Leaves, Plant phenology, Trees

raking leaves in the Fall

I have heard that raking to clean up leaves in the fall is no longer a recommended practice, and that it is more sustainable to leave the leaves alone, to decompose in place. Can you provide information supporting this?

This is a complex issue without a single simple answer. Letting leaves lie in place is fine in some situations and not in others. Undisturbed leaves can be a beneficial mulch in the garden, but not all leaves, and not in all circumstances.

Here are a few suggestions:

Don’t leave diseased or insect-affected leaves on the ground. (As a precaution, I always rake up and remove leaves of fruit trees.)

Be aware that a thick mat of leaves can make a great hiding place for slugs and snails, and early stages of stink bugs. Learn to recognize their pearly little eggs!

If you have thick or leathery leaves (such as oak and magnolia), a mulching mower might be a good way of breaking them down to use as mulch.

If leaves are slippery or are going to obscure uneven terrain that might cause people to trip or fall, rake them.

If you do decide to collect leaves into a pile, raking is a better method than using a leaf blower (and if you must blow, use an electric or battery-operated blower, which is still noise pollution but at least not air pollution).

In their 2014 book The Living Landscape: Designing for Beauty and Biodiversity in the Home Garden (page 146), Rick Darke and Doug Tallamy do advocate letting leaves lie in the garden, but they recommend running over them with a mower first if you want to have finer material for mulching (so as not to smother fragile plants)—which means they would need to be raked into a mowable pile first. Another way to make leaves more useful as mulch is to compost them for a year—which would also require raking them up and putting them in a pile. Generally, though, the authors favor the let-it-lie philosophy: “In many places, the most conserving, functional, low-maintenance approach to autumn leaves is to let them lie where they fall. This is exactly what happens in unmanaged forest ecosystems. Though a too-heavy leaf layer can have a negative effect on delicate herbaceous plants, most trees and shrubs and a wide range of sturdy perennial herbs grow well under natural leaf fall.”

I will point out that a tiny urban garden is quite different from an unmanaged forest ecosystem, so sometimes we need to rake leaves where they impede access to paths, steps, sidewalks, etc. And again, consideration should be given to preventive measures. Don’t let lie the leaves of any plant you know to be struggling with pests and diseases.

Acer palmatum and leaf scorch

I planted an Acer palmatum a year ago spring. It has healthy leaves in spring then they all curl and turn brown in summer: not a pretty sight. I feed and water regularly. It gets late-morning and afternoon sun. It doesn’t get very hot, maybe in the 70s to low 80s on average in summer with an evening fog rolling in from San Francisco Bay. Any ideas?

Acer palmatum sometimes displays this foliage problem. According to Japanese Maples by J.D. Vertrees and Peter Gregory (Timber Press, 2009), root weevils can cause this type of damage. Also, Verticillium wilt as well as other fungi can cause leaves (and often branches) to die. Leaf scorch seems to me the most likely cause, and your fertilizer may be contributing to it: Vertrees says excess nitrogen in the soil can lead to leaf scorch, as can watering so that the leaves become wet in the hottest part of the day.

I recommend that you contact the Alameda County Master Gardeners for more information and a more specific diagnosis.