Moss grows on the North side of trees – Fact or Fiction?Moss grows on the North side of trees – Fact or Fiction?

Watching Scooby Doo was the first time I came across the old adage that Moss grows on the North side of trees. So obviously the villain picked it up and moved it and everyone was lost for the rest of the episode.  

Is it true? Is it this simple? Can we really rely on this?

In a word 

Is it true? Yes, 

Is it this simple? No, 

Can we really rely on this? Yes

In a sentence

Is it true?

It holds true as a general rule of thumb, however there are lots of variations that can make it false, especially in isolated circumstances.

Is it this simple?

No; mosses like damp places, therefore the North sides, which in the northern hemisphere gets less light, are generally damper and therefore will have more moss growth, however for example a strongly leaning tree, is more likely to have more moss on its lower side not its Northerly side.

Can we really rely on this?

Yes; however it has to be used with caution, you cannot take a bearing of North with this information alone, but you can tell that that direction it is a Northerly one and this can be used to confirm that you are still walking in roughly the same direction as you set out on using a more accurate (natural) navigation form, or used to hit a large linear feature like a river or road. (And breathe, sorry that’s a really long sentence!)

Explained in detail:

Mosses tend to grow in damp places, damp places tend to be darker, the heat from the sun dries out bright sunny places sooner than dark shady ones, therefore the North side of the tree which gets less sunlight should have more moss.

In the Northern hemisphere the sun rises in the Eastern aspect of the sky, travels through the Southern sky and sets in the Western aspect of the sky. Note it doesn’t rise in the East and set in the West; it only does this on two very specific days of the year, the spring and autumn equinoxes. The rest of the year it will rise somewhere on the Eastern skyline, moving more Southerly in the winter and more Northerly in the summer. We will look more at sun movements in another blog, but for now the key bit is we understand that the North facing side of a structure be that a wall or a tree trunk is often in shadow, and therefore darker, cooler and hence damper, especially when compared to the South facing side.

Therefore if no other factors are involved the Northern side of a structure is likely to be damper and therefore encourage more moss growth on it than the other sides.

Which other factors could stop moss growing on the North side of trees?

Wind direction, 

Shape of the object, 

Surface of the object,

Shape of the land around the object,

Surface of the land around the object,

Overhanging features,

Sources of dampness.

Wind direction

Winds can bring moisture in the form of fogs, rain & other precipitation. They can also be warm and drying. If an area has a constant supply of damp air blowing through that deposits moisture on one side, this might have a greater impact than the sun, especially in an area that gets little sunshine. Likewise, for an area that has a dry arid wind, this could dry out an aspect and similarly override the variation caused by the sun.

Shape of the Object 

Still thinking about a tree for the time being; it could be that it is leaning heavily to one side. This will shade that side, as it effectively becomes the underside. This shading could override the variation caused by the sun, or compound it, or confuse it, because a much wider aspect of the tree has more moss on. It could also be that the tree is strongly leaning because it’s a very windy exposed spot, compounding the effects of the shape of the object and wind. A strong continuous wind such as that which will shape a tree is likely to dry out the exposed limbs more, and the strong leaning aspect of the tree will shelter the undersides from the sun’s drying warmth. 

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Surface of the object 

Is the surface gnarled and fissured, or smooth and clear? Obviously one will collect and hold damp, nutrients and spores much more easily than the other. Is there a variation across the tree or surface you are looking at? 

Thinking of surfaces such as rocks and walls now; is the variation in surface also due to dampness? The damper side of a boulder or wall will have more frost damage than the drier side, as the increased water in the fissures of the rock or other surface when frozen will expand and likely cause damage to the surface. 

Back to trees; why is there surface variation? Is it due to aspect or is it due to the tree’s growth? Does one side have compression wood causing fissures and channels whilst the other is smooth wood not struggling to hold up a heavy limb?

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Shape of the land around the object

If our imaginary tree is growing in a steep sided valley, then the sun’s rays aren’t going to hit it as evenly as we predicted at the start. The damp sides of the trunk are potentially pointing at the steep sides of the valley, that the sun rarely/never reaches over, rather than towards the Northern aspect. This is the same and easier to mistake for your natural navigation sign if you think about a steep sided hill, where only one side of the tree is masked from the drying of the sun’s rays.

Surface of the land around the object

Thinking about the shape of the land as we did in the paragraph earlier we also need to think about the properties of that land, is it rock does it absorb heat and reflect it back at the shaded side of the tree, drying it anyway?  In an urban context is it a glass or mirrored surface that will reflect the sun’s rays back directly? 

Overhanging features

We mentioned that the landscape can have an effect, so can nearby features. We have a South facing roof on our house, that every few years is recovered with carpets of moss. Why? It has a large yew tree shading out almost all the light to the roof’s surface, and preventing much of the wind from drying it too, meaning it is almost permanently damp and a perfect location for growing moss.

Sources of dampness

Is there a waterfall nearby splashing water at one side of the tree, or a road that often puddles and the spray from the cars is having the same effect? Are there any other sources nearby that could have created an increased initial level of dampness on one side of our tree to the other? 

Conclusion: Is it true that moss grows on the North side of trees, and can I use this to navigate?

If we’ve taken all of the above factors into account and there are still considerably more mosses and damp loving plants growing on one aspect of a group of trees or structures then we can conclude that this is most likely to be a Northerly facing aspect.

A really nice example of this working, where I live there is a flint wall that runs East-West for part of its boundary and North-South for its other part, both sides have a similar typography and things growing around them. There is little difference in the growth of mosses and small plants on the wall facing East-West. There are more and healthier mosses growing on the North facing wall than the South facing wall. The ratios of the species of plants growing on these two walls is also quite different. 

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