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September leaf fall

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Leaves do not usually change colour to any great extent during September: by the end of the month, the treescape is mainly green. Quietly, however, leaf fall has already started. All sorts of trees and bushes are unobtrusively thinning out their leaves, but sometimes the only evidence for this is fallen leaves on the ground. In other cases you may notice a few tinted leaves - either individual ones or small patches.

Factors that drive this early tinting and shedding are varied. Dry summers can definitely sometimes produce it, or possibly how much moisture the tree has had during its entire growing season. Unexpectedly cold nights are almost certainly also a trigger, and possibly also colder than normal weather in general. But part of it does seem to be a general reduction of foliage in preparation for autumn.

Early tinting in September does not lead on to more widespread leaf colour, however: it does not mean an early autumn. Any tinted leaves fall and the remaining ones remain green. For this reason strong winds can paradoxically leave the treescape looking less, rather than more, autumnal. It is also quite possible for the month to start with many trees showing a bit of tint and finish with them looking completely green: this happened in 2016, when dry weather in August seems to have been a factor.

September 2017 was exceptional in that there was quite extensive tint on a wide range of species, probably due to nights of low single digit temperatures on the 1st, 2nd, 9th to 10th, 15th to 17th and 20th to 22nd of the month. But even here the combination of brisk winds and milder nights in the last ten days of the month significantly reduced the amount of colour by the end of the first week of October.

Trees which reliably seem to both tint and lose some leaves during the month include crack willow, goat willow, birch, lime, hornbeam and rowan. Alder also sheds foliage but without any obvious tinting and the same is true of poplars (mainly hybrid black poplars, but sometimes also Lombardy poplars), though they may show a very slight yellowing (quite a bit in 2018). Weeping willow and white willow can tint very unobtrusively, but mainly shed green leaves.

Beech, oak, sycamore, sweet chestnut, wild cherry, hazel and field maple can also produce some tint this month (all did so in September 2017 and 2018) and ash often starts to shed green leaves in the second half. Occasionally it also shows some yellowing - this was quite extensive in 2017 - which definitely does seem to be triggered by unexpectedly cold nights of low single digit temperatures. But if you see an ash-like tree aflame with yellow, it is more likely to be a robinia or honey locust, ornamental trees which are quite common in parks and suburban settings. You can also see ash trees with blackened shrivelled tips to their leaves, though I am informed that this is NOT an effect of ash die-back disease.

In some years there is also a small amount of shedding from London plane and Norway maple. Red maple, an American import that is found as a street or park tree in this country, can produce fine red tints from quite early in September and may even be getting quite thin by the month's end

Traditionally horse chestnut foliage used to produce wonderful golden tints in late September, but since 2006 its leaves have been affected by a leaf mining bug that causes they to become blotched and then shrivel. In 2009, 2012 and 2015 some managed to beat the bug and put on a reasonable display, but in other years their foliage have been shrivelled completely by the end of September or - in 2017 and 2018 - the end of August.

Shrubs reliably tinting and shedding in September include hawthorn, blackthorn, wild rose and cherry plum, while elder may lose quite a lot of leaves in September without particularly turning colour or while only turning pale green (though unusually in 2017 some turned quite yellow). Some elders and crack willows can be quite bare by the month's end (this was true in both 2017 and 2018 for elder, and in 2017 for crack willow).

Brambles also often tint or lose leaves even while they still sport blackberries (or even the occasional flower), and in the second half of the month both dogwood and guelder rose foliage can also go maroon even while they still have berries. Forsythia and spindle can have attractive maroon tints at the same time. Buddleia tints and shed leaves right from the start of the month, even before it is has entirely finished flowering. Both garden and wild privet may also yellow a bit.

Other sources of autumn colour in September include Virginia creeper, a climber found on garden walls and wasteground which turns a glorious red colour as the month goes on, and bracken, which can start to turn in the second half, though this is very variable from place to place. The plants of some flowers can also produce colourful foliage as they die back - for example rosebay willowherb, which turns a bright orange colour later in the month, and black bryony, large or hedge bindweed and even stinging nettles, which can produce some nice yellows.

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