Why don't weeds and grass grow on tarmac roads?

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Offline thedoc

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Why don't weeds and grass grow on tarmac roads?
« on: 19/09/2012 19:30:01 »
Roger Jones  asked the Naked Scientists:
Hi Chris and Team,

Love the podcast, and always listen in.

I have a question for you:

How come weeds and grass don't grow on tarmac roads?

Are they treated with weed killer, have special properties meaning weeds won't propogate in them, or is it just the traffic which keeps them at bay?

Not sure if this is appropriate for the podcast, but finding it difficult to find an answer on the net.

Many thanks,
Roger Jones, Brighton

What do you think?
« Last Edit: 19/09/2012 19:30:01 by _system »


Offline chris

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Re: Why don't weeds and grass grow on tarmac roads?
« Reply #1 on: 19/09/2012 19:59:04 »
Hi Roger

I think there are several factors at work here.

One is that the environment on the road is pretty extreme. When it's dry there's very little water reserve because the soil on the road, if any, is invariably very shallow. Likewise, when it's wet it's easy to find yourself washed away. In winter half a kilo of salt is spread on each square metre of road surface producing hypersaline soils similar to what you'd find at the beach. Add to that the traffic, which flattens things fairly effectively, and makes winds capable of up tearing up young plants.

So, all in all, it's not the ideal nursery...

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Offline Mazurka

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Re: Why don't weeds and grass grow on tarmac roads?
« Reply #2 on: 24/09/2012 11:05:55 »
They do!

OK, this is generally in the middle of single track very quiet roads in rural areas.  Often these roads have soils tracked  or washed onto them (from agricultual activities etc.), getting over the biggest problem of the (practical) impermeabillity of tarmac / concrete - leading to the drought/ drowning that Chris describes.

Furthermore, these roads don't tend to get gritted as they are very minor parts of the highway network and because they are single track, the central part is not driven over, so grass & weeds can grow.   Where vehicles turn (into gates/ passing places) across what would be the central grassy strip the strip tends to not be there.

« Last Edit: 24/09/2012 12:32:22 by Mazurka »


Offline Don_1

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Re: Why don't weeds and grass grow on tarmac roads?
« Reply #3 on: 26/09/2012 17:22:50 »
There are some plants which are as tough as old boots. A seed, creeping root or runner finds a tiny crack and before you know it, you have a plant growing. Just take a look at garden walls, the cracks in patios etc.

But on the roads, at least those frequently used, they are up against a real killer: Traffic.

The constant passing of tyres is just too much for a plant to stand. But, as Mazurka has pointed out, on those roads which are used once in a blue moon, plants can, and do take a hold.

The salt Chris mentioned would not do plants a whole lot of good, but their seeds could withstand such a problem, since the salt is used in the depths of winter. Also, there are some plants which can withstand the constant coating of sea salt they are subjected to in your neck of the woods.

Just take a walk along the prom at Rottingdean, a few miles east of Brighton, and you will see plants growing in the rock face. Being in Brighton, you will know only too well how rough the sea can be along that stretch of the south coast and how much sea salt those plants are smothered in on a daily basis.

While you are there, take a look in the car park just off the coast road. This has a tarmac surface, but you will find plants taking a hold where car tyres do not tread.

Two of the most succesful plants in this battle against tarmac are the Buddliea (Butterfly Bush) and the dreaded Japanese Knotweed.
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