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September leaf fall

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Picture: hawthorn. Click here for more September leaf tint photos.

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 immediately 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. In 2017 there was also quite extensive tint on a wide range of species by mid month, but brisk winds and milder nights in the last ten days significantly reduced the amount of colour by 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. Some crack willows can be nearly bare by the month's end, as was the case in 2017, 2019 and 2021. More unobtrusively, some beech, oak, sycamore, sweet chestnut, wild cherry, hazel and field maple may also show some tint.

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. Whitebeam leaves may turn a dull yellow or gold, but otherwise just shrivel, something that you sometimes also see on sycamore. The black spots you sometimes see on sycamore leaves are due to a fungus - surprisingly, a sign of clean air.

Ash sheds green leaves - particularly in the second half the month, though you may see some on the ground in the first half. Occasionally it shows some yellow - 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.

Another tree that sheds green leaves is alder and the same is largely true of poplars (mainly hybrid black poplars, but sometimes also Lombardy poplars), though they may show slightly yellowing on a few leaves (quite a bit in 2018 and 2020), or turn a bright yellow once on the ground. Weeping willow and white willow can also tint very unobtrusively, but mainly shed green. In some years there is a small amount of green leaf fall from Norway maple too, while with London plane, the classic street and park tree, you mainly notice the shedding when brown leaves pile up on the ground, though there may well be a very few leaves with dull yellow tints on the branches if you look closely.

Since 2006 horse chestnut has been affected by a leaf mining moth whose caterpillars cause its leaves to become blotched from late June or July and then shrivel completely by the end of August or during September. In 2019 and 2021 the shrivelling was very muted, however, with most trees still showing a substantial amount of greenery throughout September, while in 2009, 2012, 2015 and 2020 some trees managed this. In all these years the unaffected trees started to show a bit of yellow or gold tint at the end of September.

Shrubs tinting

Shrubs reliably tinting and shedding in September include hawthorn, blackthorn, dog rose and cherry plum. Dog rose can be nearly or entirely bare by the end of the month. The same can be true of elder, which loses leaves without particularly turning colour or while only turning a pale yellowy-green (though unusually in 2017 some turned quite bright yellow). It is a very variable plant, however, and some may be bare while others are still fully covered.

Brambles can also tint a bit (some leaves turning yellow) or lose foliage while they still sport blackberries (or even the occasional flower). Dogwood, especially on downland, can go an attractive shade of maroon even while it still has berries. Towards the end of the month guelder rose leaves may take on a maroon-red tint too, attractively setting off its bright red berries. Forsythia and spindle can have attractive maroon tints at the same time, while wayfaring tree leaves can go a bright red.

Buddleia tints yellow and sheds leaves right from the start of the month, even before it is has entirely finished flowering: towards the end of the month you may even see new leaf shoots on it, which will stay on the shrub all winter before expanding to their full size in March and April. Both garden and wild privet may just show a tiny bit of yellow, though usually do not, while cherry laurel, though evergreen, sees a few leaves turn yellow and fall as they have been doing all summer.

Other sources of autumn colour in September include Virginia creeper, a climber found on garden walls and wasteground which can a display a little bit of its glorious red tint in the first half of the month but generally starts to turn widely towards its end. Bracken can start to turn yellow and then golden in the second half, though this is very variable from place to place and year to year.

The plants of some flowers can also produce colourful foliage as they die back - for example rosebay willowherb, which turns a bright orange colour, or stinging nettles whose leaves turn yellow. The leaves of black bryony (particularly, though not exclusively, earlier in the month) and large or hedge bindweed can produce some nice yellows too, as can the vines in commercial vineyards very late in the month.

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