The validity of bark scratching
So.. One of the questions often asked when growing bonsai is “Is my bonsai dead?”. Invariably, someone will suggest the bark scratch test. The reasoning being: You scratch off the bark. Below the bark is (often) a light green layer which people think is the cambium (It is not). This layer of green is not present when a tree dies and dries out. Then it slowly turns brown. I wanted to know what exactly to look for.
Is my spruce dead?
So I took a spruce that by early summer (June 2019) had not grown any new buds and was losing some needles. With a sharp knife, I cut into the bark and exposed a bit of the layers below it and asked what the chances of survival were.
Interestingly, the first answer was that I took too much bark off (Which I did, see above). And second: This tree looks healthy enough. Just be careful with watering, spray with a fungicide and it will be right as rain. I was a little confused, but accepted the advice.
Downside of the bonsai bark scratch test
Plants that are weak, have a hard time defending itself against infections. In a way, it is no different from you, laying in a hospital bed, and the doctor passing by, cutting your skin to ensure you still have blood. You would not appreciate to have more things to fix on your body than the infection already roaming around. So from a pure infection risk perspective I would not recommend it. But.. Even worse…:
Why the bonsai scratch test does not tell you much
Let’s take a wider perspective on the issue. Or rather, on the tree I showed before. If I zoom out.. You will see that the tree is the Christmas tree from last year. Without roots, this tree was already dead in early December. It had however stored so much liquid in the trunk, that the tree stayed green for a very long time. In other words: Scratching the bark can confirm something is dead (And in some cases, long dead) but it cannot confirm a plant is still alive: Finding green means there is sap. Not that is is alive.
How to determine whether a tree is alive?
I find that most bonsai where a scratch test is recommended have thinner bark and are in fact young trees. There you can actually see narrow ridges form along the length of the trunk and branches when the tree starts drying out after dying. Furthermore, buds start to dry out, and turn completely darkbrown to black, instead of green. And young twigs lose their shine and may appear porous. Those are all clear sign of dying trees. No need to scratch bark.