When do trees loose their leaves?

8th November 2015 - 4 minutes read

Whilst there is an abundance written online about why trees shut down in autumn and the whole process of loosing their leaves and the factors that affect the display of colours (especially of course in maples), less is written about the things that affect the exact timing. So I thought I would add a little to the debate and hopefully with a bonsai slant.

Take a look at this photo which is centred on two of my Beech bonsai, one is a well developed cascadIMG_0276e (obviously the top one hanging down) and the other is a piece of raw material. You will notice that one is still green which the other has long since gone through the autumn transition and is now cloaked in dried leaves that have already started falling – Beech leaves can stay attached until the next spring so a tree looking full of autumn leaves all winter can be quite attractive.

As they are of the same species and in exactly the same amount of light and exposure logic would dictate that they should ‘feel’ autumn at the same time. However something is happening here, but what?

Firstly there are tree genetics. These trees were collected from different places , whilst one is a true UK native the other was from a plantation and may well come from imported seed. This is important because trees ‘learn’ to respond to certain temperature or day length that triggers both spring and autumn changes. Trees from latitudes further south or north are programmed for different triggers. This is used by foresters to adapt the amount of growth that can be achieved each year, if a tree starts in leaf earlier and stays in leaf longer it will grow faster and produce more timber. The flip side of this is that the UK is prone to late spring frosts which can catch out many trees, beech included, and seriously damage tender young growth.

However another more important factor is working on these trees.I will remind you briefly that the whole reason that trees loose their leaves is because winter can be more problematic to trees in terms of drought that in the height of summer. Think about it, winter would still provide sunlight for photosynthesis (you can get sun burnt when skiing so there must be enough sun in winter)  but a frozen soil will not have enough water.

It is the amount of soil moisture available that has caused one tree to turn before the other. The developed tree is in akadama whilst the raw material is in ordinary compost. The akadama will dry out much quicker. It was the long dry October we experienced down here, together with drying winds, that has caused the cascade to get slightly dry and triggered it to turn dormant earlier. The raw material will still be growing and laying down cells around the trunk, increasing the all desirable girth whilst the other tree has stopped growing for the year. So yes a mistake on my part, but fortunately not a serious one!

Now you know this you can always use it to your advantage if trying to get maples to show their amazing colour at a time of year to suit you – rather than having the cold winds of winter tear the leaves off just as they are beginning to look good…

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