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Sawdust mulch

GrannybeeGrannybee Posts: 332
in Plants
I have a heather bed which has been mulched with sawdust. It seems to work at keeping the weeds down. However, on GQT recently, one of the experts said that sawdust was not a good mulch but did not explain why. Is it to do with chemicals in wood? Does anyone know? My mulch is from chopped up trees as opposed to planks.  Thanks!
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  • Hostafan1Hostafan1 Posts: 34,889
    edited January 2019
    do you mean woodchip? from a shredder? or sawdust from a sawmill? 
    It's often said that woodchip " depletes nitrogen " , but as hard as I've looked, I've found absolutely no evidence to support the claim. If someone out there has found such evidence,I'd love to see it. 
    I've seen evidence that it "can" reduce nitrogen, if it's dug , fresh ,into the soil, but if it's left for about 6 months, or if laid on surface, it's not a problem . Even where "nitrogen depletion" occurs, I've seen no evidence that the "depletion" is at a level that it impairs plant growth.
    IMHO it's one of those things which , if repeated often enough, becomes accepted as fact.
    Devon.
  • JoeXJoeX Posts: 1,783
    Grannybee said:
    I have a heather bed which has been mulched with sawdust. It seems to work at keeping the weeds down. However, on GQT recently, one of the experts said that sawdust was not a good mulch but did not explain why. Is it to do with chemicals in wood? Does anyone know? My mulch is from chopped up trees as opposed to planks.  Thanks!
    If I recall correctly GQT said that sawdust absorbed nitrogen from the soil, and also mentioned that there was “not enough bark” to make the questioners sawdust a good mulch, which was output from stump grinding.

    If your mulch is chopped up relatively thin branches like mine,then  I don’t think the advice applies.

     
  • Hostafan1Hostafan1 Posts: 34,889
    if you repeat the same myth often enough, it's still a myth. 
    Where is the evidence?
    Devon.
  • GrannybeeGrannybee Posts: 332
    Thank you for your thoughts. I'll leave it then! The heathers are in full bloom and the sawdust has been there on top of the soil around the plants for a year now! It comes from the felled / windblown over trees scavenged by my husband for our log fire. As he uses his chainsaw to cut, it creates quite a lot of sawdust. Depending on the tree it can be large or small dust! Thanks again.
  • NollieNollie Posts: 7,529
    I cant quote any sources offhand, but there is a fair bit about this on the internet.  Woodchip/sawdust mulches do deplete nitrogen during decomposition as the microorganisms doing the decomposition use up additional nitrogen in the process. However, its a slow process, only affects the top few inches of the soil, won’t affect deeper plant roots (but is known to inhibit seed germination, hence being good as a weed suppressor) and it apparently all rebalances out in the end. I would think the benefits of weed suppression and adding organic matter to the soil over a long period generally outweigh the small, temporary loss of nitrogen.

    A thick blanket of fine sawdust may ‘cap’ the soil, depleting soil oxygen levels so that might be more of an issue for plant health. Mixing in some rougher chippings, leaves, bark, etc., with the sawdust would prevent that, so maybe thats what they meant by ‘not enough bark’ for a good mulch? 
    Mountainous Northern Catalunya, Spain. Hot summers, cold winters.
  • Hostafan1Hostafan1 Posts: 34,889
    "depletes nitrogen" might mean it "depletes" 1% or 99%. As I've said before, I seen no evidence it actually has any detrimental effects on plants.
    Devon.
  • GrannybeeGrannybee Posts: 332
    The sawdust is from a mixture of apple, sweet chestnut, and ash. And a mixture of fine and coarse. None of the trees have been treated with chemicals. It is likely that some oil from the chainsaw is on the sawdust but not enough to bother me. Or the heathers!
  • Hostafan1Hostafan1 Posts: 34,889
    I'd happily use it as a mulch , or store it for 6 months before digging it in. 
    I'm not telling you, or anyone else, what to do, but that's what I'd do.
    I have woodchip delivered by local tree surgeons and  I've used it to mulch on day 1 and never regretted it.
    Devon.
  • Paul B3Paul B3 Posts: 3,154
    I used to do an appreciable amount of chainsawing for our open-fire at home ; finished with a lot of 'chippings' which I purposely put around my perennials .
    I never noticed anything untoward .

  • Hostafan1Hostafan1 Posts: 34,889
    can we start a revolution to debunk the myth about nitrogen depletion? Our collective experience suggest it's a load of nonsense.
    Devon.
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