Archive for February, 2011

Gosh, nothing provokes more discussion than the topic of mulching. We had a big round table discussion last week and we reached a consensus on a few things. But firstly lets define what we mean by a mulch and what we expect to get out of it.

Some degree of temperation moderation in the soil and with it some degree of water conservation. Weed suppression.

So where does that take us?

Mulches MUST be coarse. In excess of 25 mm particles. Big and chunky. Because if they aren’t, they use more water than they save. They don’t allow water to go past them in into the soil where the water needs to be which is around the plant roots.

Mulches that permit roots to grow in them are bad. In the average home garden, those roots will be exposed to wild fluctuations in moisture and that’s bad.

You don’t need to reapply mulches year after year. You might need to top them up with a few mm of extra but to apply 25-50 mm of mulch year after year is stupid. All you do is raise the level of your garden beds to ridiculous heights. The height of a mulch layer must be relative to the plants you’re putting it round. 25 mm is fine for most things and only big trees will take 50-75 mm.

Composts are NOT mulches. Composts are finer. They need to be incorporated into the soil. They won’t discourage weed growth for very long when placed on top of soils and they may become non-wetting if they dry out. And with Perth’s current silly watering regime or 2-3 days a week this is highly likely. They supply nutrients which must be taken into account when you apply them. But they seldom contain enough nitrogen in relation to phosphorus and for many plants eg veges the rate of supply of nutrients is too slow to promote good plant growth without supplementation. If it is you can bet a good slice of the phosphorus in it is leaching through into the aquifer below and polluting the waterways.

A few other things that you need to take note of. That free council supply of green waste is the best way of spreading weeds we know of. If you have a pocket handkerchief size garden that probably isn’t so important but if you have ANYTHING to do with bush regeneration eg as a member of a Friends group then it does matter, and it is important because you are likely going to end up with all manner of weeds in your prized piece of bush. Some real life examples cited recently were Cocos palms and Cape lilac trees coming up in bushland due to the use of green waste.

All green waste and indeed all mulch and compost should be composted or at least pasteurised to reduce the risk of spreading weeds and diseases.

I think its fairly well recognised that cypress canker was spread around Perth in the early stages by mulch. Cypress canker is that disease that decimates pencil pines and many other conifers. It’s a foliar (leaf) disease that makes whole patches of leaves go brown, it kills them branch by branch. I’m sure you will have seen it around.

There endeth the lesson for today. Short and sweet.


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