Tony Dick­er­son an­swers your ques­tions

Garden News (UK) - - News -

Q Why does my liq­uidambar tree fail to colour up in au­tumn?

John Spi al, Wy­boston, Bed­ford­shire

A Some in­di­vid­ual trees never have much au­tumn colour, while oth­ers of the same species can be brilliant, but not nec­es­sar­ily ev­ery year. The rea­son for this vari­abil­ity is due to the in­ter­ac­tion of lots of fac­tors, from ge­netic through to en­vi­ron­men­tal.

The bright leaf colour of de­cid­u­ous trees in the au­tumn is due to the break­down of chloro­phyll, which is the pig­ment that makes most leaves ap­pear green. As this hap­pens, other pig­ments present in the leaves are re­vealed: carotenoids give yel­lows and or­anges and an­tho­cyanins give reds and pur­ples. The chang­ing con­cen­tra­tions of these pig­ments ex­plain the range of au­tumn colours, but these vary in in­ten­sity due to weather con­di­tions and ge­netic dif­fer­ences be­tween tree species, and be­tween in­di­vid­u­als of the same species. The pro­duc­tion of chloro­phyll slows down in au­tumn. A corky layer of cells (the ab­scis­sion layer), which even­tu­ally causes the leaf to drop, also be­gins to form at the base of the leaf stalk. As the re­main­ing chloro­phyll breaks down it re­veals first the yel­low and or­ange of carotenoids. As au­tumn pro­gresses, carotenoids are also bro­ken down while red and pur­ple an­tho­cyanins ac­cu­mu­late from the con­ver­sion of sur­plus sug­ars trapped in the leaf by the ab­scis­sion layer.

The ge­netic char­ac­ter­is­tics of trees in­flu­ences the de­gree and in­ten­sity of au­tumn colour. Liq­uidambars grown from seed may be very vari­able in au­tumn colour and some­times quite dis­ap­point­ing. Se­lected clones that dis­play good au­tumn colour, such as ‘Lane Roberts’ (crim­son- red) and ‘Wor­ples­don’ (or­angeyel­low), are prop­a­gated from cut­tings so they’re ge­net­i­cally iden­ti­cal. But, even with these, the in­ten­sity of colour may vary from year to year de­pend­ing on en­vi­ron­men­tal con­di­tions.

Au­tumn colour tends to be spec­tac­u­lar in places such as New Eng­land in the USA as they usu­ally have bright, dry, sunny days, com­bined with cool nights. The more muted au­tumn colours in Bri­tain are due to gen­er­ally cooler, damper, more over­cast con­di­tions, which have been the order of the day this year.

Au­tumn colour is af­fected by tem­per­a­ture, light lev­els and ge­netic char­ac­ter­is­tics

Liq­uidambar ‘Lane Roberts’ gen­er­ally gives re­li­able au­tumn colour

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