[therodgers21]

therodgers21  asked the Naked Scientists:

Hi,

A young weed just cracked my concrete. Has a plant's "growth force" ever been measured?

Love the podcast,

Bruce Rodgers

What do you think?

[chris]

I think we need a serious plant scientist for the actual force exerted by a root, but what I can say is that when I was in Queensland, Australia, in 2004 I saw a massive rock that had been split in two by a tree growing through it!

There are a number of ways that this could happen. First, in frost-prone areas, the roots have a helping hand from the weather, because the roots find small holes in the rock and allow water to penetrate - perhaps by dislodging other covering material. This water then freezes and freeze-fractures the rock locally. This introduces a weak spot in the rock and leads to force concentration around the imperfection, making failure more likely.

Another way this can happen is seen in limestone rocks. If you look at the roof of a solution (limestone) cave you'll often see a host of perfectly round holes in the ceiling. These are sink holes where tree roots have penetrated. This has provided a route for ground water which flows along the hole made by the root, dissolving the rock on the way.

A combination of these factors probably apply to your patio!

Chris

[RD]

The maximum root pressure measured is about 0.6 megapascals

http://en.wikipedia.org/wiki/Root_pressure

[chris]

No no, this "root pressure" is something totally different. This is the effective upward pressure applied by the root to elevate salts and water in xylem (one of a plant's conducting systems). This is NOT the same as the lateral pressure being referred to in the question. Please take care not to confuse the two.

Chris