Why not weed mat?

Weeds are a pain, right? A weed is a plant that is growing where it shouldn’t – or more to the point, where you don’t want it to. In a garden, that could be any number of overenthusiastic “volunteer” garden plants, introduced exotic weeds, or natives making their way in.

Instead of spending your days pulling them out, hoeing, and mulching to suppress them, why not try something more permanent? After you rule out poison (because it’s poisonous! – more on that in another blog post) weed matting is a popular option.

Weed mat comes in many forms, one of the most popular being the old black plastic sheeting. Unfortunately, it does great damage to your nice garden soil. Really? It’s pretty inert isn’t it?

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My place had a layer of black plastic sheet under most of the garden beds, and about 50 square metres of the back yard under the River Redgum by the back fence. It must have been put there some time ago – I’d say up to 30 years ago, which was when the house was built.

On top of the old weed matting is a thick layer of fairly dry leaves and twigs. Weedy grasses were growing on top in places (including a plague of couch, Cynodon dactylon), and a few pepper trees (Schinus molle) and desert ash (Fraxinus angustifolia). In fact, these tough weeds seemed to have fairly easily punched a few root holes through the weed mat, and spread or grown up on top.

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As you can see here, the couch grass has thick rhizomes spreading beneath the plastic. With more attention, these weeds could have been kept out. But the other problem would remain: the soil below the weed mat is dry, cracked even, compacted, and low in organic matter. The leaf litter on top is rich in humus-forming material, but it hasn’t properly broken down, and in fact was building up and up. image

The weed mat is basically an impermeable membrane that has separated off the humus from the substrate. This means that moisture can’t get down to the lower soil (where a lot of the tree roots are). Nor can earthworms and fungi, all the small organisms that break down organic matter into humus and nutrients. Even the water can’t carry nutrients down there.

Plastic weed mat slowly breaks up, especially if exposed to sunlight, but it doesn’t break down – and will release various chemical pollutants, like synthetic estrogen, into the environment, in addition to strangling your soil.

Unfortunately, too many people still buy the cheap and nasty plastic weed mat. Woven plastic may be slightly less strangling for the soil, but breaks up into tiny fibres which you’ll never get out. Greg’s Indigenous Landscapes has blogged on the topic.

If you really need a layer of weed mat to suppress some particularly vigorous weeds, fair enough. You can try wet layered newspaper, and/or heavy mulching, but that isn’t always enough. Unfortunately, biodegradable weed mat used by responsible landscapers and revegetation works isn’t as easy to find, but it is worth the effort if you really need it.

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