Sunny days, cool nights bring out the best colour in leaves

Medicine Hat News - - HOME & GARDEN - LEE RE­ICH

Sugar maples, na­tive to the forests of east­ern Canada and north­ern parts of the Cen­tral and East­ern United States, paint the land­scape each au­tumn in fiery shades of yel­low, or­ange and red.

What puts colour into the leaves of the sugar maple, or of any tree?

Green, of course, is from chloro­phyll, most wel­come in spring and through­out sum­mer, but not our con­cern now. A leaf has to keep mak­ing new chloro­phyll in or­der to stay green, and shorter days, with the sun hang­ing lower in the sky, trig­ger leaves to stop pro­duc­ing it, un­mask­ing other pig­ments lurk­ing there. ONCE-HID­DEN COLOURS The yel­lows and oranges were there, hid­den by the green of chloro­phyll. They come from carotenoid pig­ments, which help chloro­phyll do its job of har­vest­ing sun­light to con­vert into plant en­ergy. We can thank carotenoids for the warm, yel­low glow they give to gingko, aspen, hick­ory and birch leaves.

Tan­nins are an­other pig­ment, ac­tu­ally meta­bolic wastes, that are hid­den ear­lier in the sea­son by chloro­phyll. They give us the sub­dued browns of fall, no­table in some oaks but also en­rich­ing the yel­low of beeches.

Be­cause leaves har­bour 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 while leaves are still chock full of chloro­phyll. In that case, cell work­ings come to a halt and you’re left with frozen green leaves that even­tu­ally drop with­out any colour change. AND NOW FOR SOME RED AND PUR­PLE Au­tumn colour also has its reds and pur­ples, most ev­i­dent in red and some sugar maples, Ja­pa­nese maples, scar­let oak, sour­wood and winged eu­ony­mous. Those reds and pur­ples come from yet an­other pig­ment, an­tho­cyanins.

An­tho­cyanins do not be­gin to be formed in leaves un­til au­tumn. Ex­cep­tions would in­clude trees like Pur­ple Foun­tain beech and Royal Pur­ple smoke­bush, whose leaves stay red right from the get-go in spring and re­main so all sum­mer.

An­tho­cyanin for­ma­tion re­quires sug­ars, so any­thing that you or the weather do to pro­mote sugar ac­cu­mu­la­tion in au­tumn in­creases an­tho­cyanin lev­els in leaves. Ideal weather for an­tho­cyanin for­ma­tion is warm, sunny days to max­i­mize pho­to­syn­the­sis, and cool, but not frigid, nights to min­i­mize the burn­ing up of ac­cu­mu­lated sugar dur­ing dark­ness.

Cloudy, rainy au­tumn weather re­sults in less red in au­tumn leaves be­cause less an­tho­cyanin is formed, and any that does form is di­luted. MAXING THE COLOUR We can ratchet up the reds and pur­ples by mak­ing sure that leaves bask in light. Plant a tree where light is ad­e­quate and, if nec­es­sary, prune it so the branches do not shade each other.

Street lights don’t count as light, and ac­tu­ally have a neg­a­tive ef­fect by dis­rupt­ing the sig­nal that days are get­ting shorter and it’s time to slow chloro­phyll pro­duc­tion.

We also can play a role in the au­tumn show by plant­ing trees ge­net­i­cally pro­grammed for good au­tumn colour. Among those most col­or­ful trees and shrubs — which be­sides those pre­vi­ously men­tioned in­clude gold­en­rain tree, hick­ory, ironwood, black tu­pelo and fothergilla — in­di­vid­u­als within each species might pack a big­ger wow than oth­ers. Ex­am­ples of es­pe­cially col­or­ful va­ri­eties in­clude Rubra spice­bush, Cheyenne lilac, Fall Fiesta sugar maple, Sumi na­gashi Ja­pa­nese maple, Moraine sweet gum and Au­tumn Gold gingko.


This un­dated photo shows a Ja­pa­nese maple tree in Till­son, N.Y. The bold red of this Ja­pa­nese maple re­flects not only the tree's ge­net­ics but also au­tumn weather, with sunny days and cool nights bring­ing out the best in the leaves.

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