Where’d the Red Go?

Sugar maples (Acer saccharum) are now doing just what I expected of them. But not exactly what I want them to do. Here in New York’s mid-Hudson Valley, at least, this autumn’s leaf show is not quite up to snuff. And it’s also later than in the past. It used to peak here in the middle of October; nowadays, with climate change, the peak has been pushed forward to about now.Autmn view of the 'Gunks mountains

Back to the color: This year the local sugar maples are mostly only yellow, lacking the oranges and the reds that, along with some yellow, really ramp up the blaze of landscapes and forests. Let’s blame that more subdued show on the weather. To know why, let’s backtrack to summer when, quoting from a section in my recent book The Ever Curious Gardener, Using a Little Natural Science for a Much Better Garden:

Green is from chlorophyll, most welcome in spring and through summer, but not what interests me in fall. Chlorophyll must be continually synthesized for a leaf to stay green. The shorter days and lowering sun of waning summer are what trigger leaves to stop producing it, unmasking other pigments lurking there.

Leaves’ yellow and orange colors are aways present, thanks to carotenoid pigments, which help chlorophyll do its job of harvesting sunlight to convert into plant energy. I offer thanks to carotenoids for the warm, yellow glow they give to gingko, aspen, hickory, and birch leaves.

Sugar maple

Sugar maple (not this year)

Tannins are another pigment, actually metabolic wastes, that, all summer, are hidden by chlorophyll. Their contribution to the fall palette are the season’s subdued browns, notable in some oaks and enriching the yellow of beeches.

Because leaves harbor carotenoids and tannins all summer long, nothing particular about autumn weather should either intensify or subdue their autumn show. The only glitch could be an early, hard freeze that occurs while leaves are still chock full of chlorophyll. In that case, cell workings come abruptly to a halt and all we’re left with is frozen, green leaves that eventually drop without any fanfare.

Autumn color also spills out reds and purples, most evident in red maples and some sugar maples, scarlet oak, sourwood, blueberry, and winged euonymous. Those reds and purples come from yet another pigment, anthocyanins. Except for trees like ‘Purple Fountain’ beech and ‘Royal Purple’ smokebush, whose leaves unfold dusky red right from the get go in spring, and remain so all season long, in most leaves anthocyanins do not begin to develop until autumn.Japanese maple

Anthocyanin formation requires sugars so anything that I or the weather does to promote sugar accumulation in autumn will increase anthocyanin levels in leaves. The weather’s role is to offer warm, sunny days to maximize photosynthesis, and cool, but not frigid, nights to minimize nighttime burning up of accumulated sugars. A cloudy, rainy autumn means less red because less anthocyanin is formed, and any that does form is diluted.

As I write this, it’s cloudy and rainy, as it has been so many days this autumn.

Other Reds

I was recently visiting my daughter in Pennsylvania. As we looked around her neighborhood I admired the rich, red, autumn color of the trees lining the streets. Red?! How can that be? What about what I wrote about this autumn’s weather and anthocyanin and red leaves in autumn?

Ah, but science takes care of that, too. Those trees in the Pennsylvania landscape weren’t sugar maples; they were a variety of Freeman maple (Acer × freemanii). Freeman maples are natural and deliberate hybrids of silver maples (Acer saccharinum) and red maples (Acer rubrum), the silver maple contributing fast growth to the hybrids and the red maples contributing strong branches and red leaf color in autumn. Freeman maple

Trees vary, both as to species and locations, in the amounts of pigments found at various times in their leaves. Red maples, as the name implies, have significant amounts of anthocyanins. In these trees, this anthocyanin is present not only in autumn as the chlorophyll fades, but throughout the growing season. Depending on the amount and kind of anthocyanin, its redness would be more or less masked by chlorophyll during the growing season. Some varieties of red maple, and many varieties of Japanese maple, are red or purplish all summer long.

The million dollar question is why a tree would have anthocyanin in its leaves all season long. Various theories have been floated. Anthocyanins do offer protection against excess sunlight, which is why young leaves emerge reddish on some plants.

Seedling red maples

Seedling red maples

(This characteristic does not correlate with the degree of red in leaves in autumn.) It’s also possible that anthocyanins could ward off pests.

Whatever the reason, red or orange or yellow leaves, I’m not complaining. Autumn is still a beautiful season here in the Hudson Valley. 




Bleeding Is Okay

Everyone wants to prune this time of year. And rightly so. It’s a good time to prune most trees, shrubs, and vines, as it was a couple of months ago and, looking forward, will be until about when these plants come into bloom.  Or, finished blooming, in the case of those plants whose pruning gets delayed until after we all get to enjoy their early blossoms.

A reader wrote me about her Japanese maple, which needed to have one of its multi-trunks cut off. Should she do it now or in autumn? If lopped back now, would the tree bleed to death? Would the gaping wound get infected, possibly leading to the demise of the whole tree?

Japanese maple in fall

Japanese maple in fall

Bleeding sap generally does more harm to gardeners’ psyches than to plants’ physiologies. My grape and hardy kiwi vines bleed when I prune them this time of year, with no harm done. So why worry about harm to a maple? 

(The bleeding of grapevines that climb the arbor over my patio does have one downside. It’s very pleasant to sit outdoors on that patio on warm, spring days; it’s very unpleasant to sit where sap drips on my head.)

Root pressure of water being forced up the vines is what makes grape and kiwi vines bleed. Once leaves unfold, they take up that pressure and bleeding ceases.Maple syrup buckets

Root pressure is not what forces sap (which sounds more benign than “bleeding”) out of wounds of maple trees. With maples, cooling temperatures cause gas bubbles in xylem cells (the inner ring of trees’ cells in which liquid is conducted upwards from the roots) to shrink and to dissolve. Something’s got to fill that newfound space, so more liquid is sucked up from the roots and into the cells. As temperatures drop further, ice forms and gases are locked within the developing ice. Come morning, pressure builds in the cells as rising temperatures melt the ice and release the gases. The expanding liquid is forced out any holes in the bark, whether from a maple sap spile or from a pruning wound.

Although maples bleed for a different reason than do most other plants, the bleeding itself causes no harm to the plants. The reason small maple trees, with trunks narrower than 6 inches in diameter, should not be tapped is because the wound left by the tap hole extends within the trunk beyond the hole; sap will never again travel past the wounded area. A tap hole is large in relation to the size of a small tree’s trunk, so significantly restricts liquid flow.

Of course, there’s no need to conduct sap up a trunk that’s been lopped off. So, Barbara, go ahead and prune, now, when the gaping wound can soon begin to heal. Autumn, which leaves a gaping wound exposed to the elements and pests until spring, would be a very bad time to prune.

A Wabbit!!!!

I looked out the window awhile ago to see a rabbit crouched against a backdrop of pure, white snow. How cute. NOT! It’s the same old story. Farmer McGregor and Peter Rabbit, and now farmdener me and some other rabbit.

A few days previously I had noticed that some bark had been nibbled off the pencil-thick “trunks” of some young, grafted trees — the handiwork of those awful furballs. That nibbling probably won’t kill the small plants but will set them back a year, or more, if the nibbling kills the scion down to the graft.Rabbit damage to branches

As for the rabbit and its probable kin, I’m setting traps. Unfortunately, my Peter Rabbit seems to enjoy my plants more than anything I put in the trap.

Rabbit, At Bay

I’ve kept my Peter Rabbit at bay from all my older trees this winter with diligence and hardware cloth and or commercially available plastic spiral tree guards. The protection goes 2 feet above ground, or higher, not that a rabbit could reach that high — except when there’s snow to give it “a leg up.” The light-colored spirals also protect the thin barks of young trees from sunscald, which results when sunny, cold winter days warm the bark, whose temperature then plummets as the sun drops below the western horizon.rabbit

I remove all the spirals in spring so insects can’t find shelter from birds beneath the spirals.Tree protected with plastic spiral

Monthly, throughout winter and into early spring, I also sprayed plants with Bobbex, a mix of “putrescent whole egg solids,” garlic, and cloves that is repellant to rabbits, as well as deer (and me).

And finally, there’s Sammy, my trusty dog who would chase away any rabbits if he happened to be awake, happened to be on the right side of the house, and happened to see them.