Tony Dickerson answers your questions
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