A Forsythia colouring reversal

Press, 10 May 1979, Page 13

A Forsythia colouring reversal

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As autumn passes into official winter, so should deciduous trees and shrubs be passing from summer green to the peak of their autumn display. But this year, while the autumn colour has been quite good in coastal areas, the display in many inland areas has been disappointing, in some cases even nonexistent.

Forsythias, for example, have coloured beautifully in many Christchurch gardens where more often they simply turn brown and drop off; but in our garden, where conditions are usually more favourable for the development of full leaf colour, the forsythias are still green, despite heavy frosts.

We of course blame the weather for these aberrations, but in a sense the weather is abnormal every year. No two seasons are alike.

And the weather is only one of a complex of factors whose interaction leads to the disintegration of the green pigments and the development of yellow and red leaf colours.

The symptom of autumn colouring is an excessive accumulation of growth sugars in the leaves. In autumn, the ambient temperature drops, and the relative gap between day and night temperatures widens, and so con-

ditions become less favourable for growth. But while the leaves survive they continue to accumulate sugars which the tree can no longer use; so the leaves “choke” themselves, and the green pigments disappear, leaving two complex groups of chem-

icals called xanthophylls (yellow) and anthocyanins (red), from which the colours develop.

It follows from this htat sunny days and cold nights are the ideal combination to encourage the development of autumn colours. The first promotes the formation of sugars, the second slows their rate of removal, and prolongs the leaf display.

If the autumn is mild and damp, with a lot of overcast weather, or even sunny with mild nights, growth continues, though slowly, and the leaf colours fail to develop properly. Aspect is another factor. As the depth of colour depends in large part on the rapidity of the disintegration of chlorophyll

by sunlight, it follows that a plant fully exposed to sunlight will develop better colour than one in the shade, and the north side of a tree or shrub will be more colourful than the south side.

Colours can develop in summer, too, but when they do appear out of time it is an indication that all is not well with the plant. A plant that develops .autumn leaf colouring in midsummer is doing so to indicate that it is dying of indigestion —■ its rate of manufacture of food is exceeding the rate at which the food is being used.

Numerous factors or combinations of factors can bring this about, but the most frequent are also the most obvious ones: lack of water, or lack of nitrogen. Sometimes a plant becomes adapted to such a condition, and the anthocyanin becomes a fixture, causing the plant to become red leaved. Plants so affected, which include several maples, berberis, and orach, develop their most intense colour when summer is hot and dry. A wet season, or heavy feeding with nitrogen, pushes them towards the green end of the spectrum.

All this is perhaps an oversimplification of the process, for while the factors I have mentioned are

major ones, they are only a few of many. Altitude, wind, humidity, and various factors connected with soil moisture and nutrition, and their interaction, also play their parts in helping or hindering the development of autumn colour.

And the process of converting sugar to anthocyanin itself goes in a series of complex steps each of which depends on the presence of a particular enzyme — if one is missing, the whole thing stops. Plants vary in their requirements, too, and a combination of seasonal factors that suits one may upset another. We have had a glorious display of reds and oranges this autumn from various berberis, which failed last year; but the lilacs, which unexpectedly gave a prolonged and spectacular display of butter yellow last year, are still green. In the next week, I expect, the lilac leaves will just go brown and drop off, as the leaves of our walnut and fig did weeks ago. I think the weather is directly responsible for the latter’s failure to colour up.

The fig tree almost always can be relied upon for a fine flash of yellow, but its leaves were killed off prematurely last month when heavy frost occurred immediately after a long spell of moist, mild weather. The plants had got too soft and sappy for their own good. Happily, some of our shrubs and trees — Japanese maples, cherries, silver birches, and deciduous azaleas — can be relied on to supply a respectable display every year. One of the'best this autumn is the winged spindleberry, Euonymus alatus, so called because of the corky, wing-like encrustations on its stems.

The “wings” are prominent in winter, when the

branches are bare, and in combination with the dense, twiggy growth make the bush look like some enormous, primitive, one-legged hedgehog. Euonymus alatus can be tatty in spring when the nor’westers burn off its young shoots, but it recovers in summer. In autumn few shrubs are brighter. In my garden it positively glows scarlet

among some dark greens, a glaucous young snowgum, a silvery grevillea, and a photinia or two whose tips are still bright red.

A young plant of Chamaecyparis lawsoniana “Lane” (surely the best of the golden cypresses) supplies the necessary highlight of bronze and yellow.