How do trees grow out of boulders?

story tease


OSU Extension master gardener volunteer


As we hiked through the Old Man Cave Gorge Trail at Hocking Hills near Logan, Ohio, I’m always amazed by the trees growing out of and around large boulders. It is simply amazing and makes me wonder how they can do that.

In addition to anchoring a tree in the soil, roots absorb minerals and water, conduct water and nutrients and store food.

Unlike the trunk that grows upward, roots tend to grow outward because most nutrients and oxygen are near the soil’s surface. Most of the tree’s roots are located within the top 6 to 24 inches of soil. The big woody tree roots that are often exposed around the base of the tree aren’t the roots absorbing water. Instead, they are anchoring the tree and storing food.

The absorption of water, oxygen and minerals happens through the smallest roots and root hairs.

As the roots grow and spread in the ground, they can encounter obstacles and need to find a detour. The roots find their way in a similar way as a person groping through the dark. If they encounter some obstacle, they feel their way around it until they come to a point where they can move again.

The roots do this through a self-reinforcing chemical cycle. A protein at the tip of the root hairs stimulate the uptake of calcium from the soil. The cycle stops when the root hairs hit an obstacle because the calcium can no longer be taken up; the root then starts to grow in another direction.

This system enables the plants the flexibility to explore a complex environment and to grow in even the most uncompromising soils.

When a tree is growing out of a boulder, if you look closely at the tree’s base, you’ll find a shallow gash into which the trunk and largest roots grow.

This is possible by biological weathering, which can be caused by the seedling’s root growing into the cracks of the boulder.

As they grow bigger, the roots push open the cracks and make them wider and deeper, allowing the tree to grow.

We know that trees need soil in order to grow because it contains nutrients, minerals and moisture.

Soil is a mixture of inorganic and organic materials.

The inorganic component is made of mineral rich rock that has been broken down into tiny particles such as sand, silt or clay. The organic component is the decayed remains of plants and animals.

How do these remains get up there in the first place? Well, tiny amounts of fine organic matter could be blown into the boulder cracks and ridges by the wind.

What happens next is called colonization, which is the process in which waves of steadily more complex, larger, more long-lived organisms colonize into the enlarging space in the boulder, allowing the tree to grow.

It is simply amazing.

I recommend taking a hike and enjoying the wonders of nature.

To learn more about how tree roots grow, visit

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