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Sadly, the brand names for bagged mulch found in the "Big Home Improvement" stores are pretty low quality nowadays. I put down some recently in my garden bed and almost immediately after a big rain noticed what I assume is some sort of white powder mold or mildew:

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  1. Should I be concerned about it?
  2. If so, just apply some typical fungicide spray or...?
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    The best mulch for your garden is from an arborist or tree cutter. In my area of the US, this is free from my municipality. It does not mildew or otherwise cause problems. If this isn't available where you live, then you can get good results simply by contacting one or more tree cutters and asking them to dump their chips on your driveway of front lawn when they're in your area. They should be happy to oblige, since you'll be saving them money.
    – Jurp
    Commented Jun 1, 2023 at 11:29
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    The best mulch depends on what you are using it for. Different mulch for different plants and situations.
    – Boba Fit
    Commented Jun 1, 2023 at 12:55

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There are many reasons why you add bark or wood chip mulch to your beds but one is that it provides a slow release of organic nutrients as it breaks down.

it doesn't break down of its own free will it requires fungi to break down the very complex molecules of lignin in the wood before other micro-organisms can also begin on the plant tissue.

Whilst you might cover the entire bed with mulch in the autumn, come spring, you would ideally need to don a pair of gloves and slightly pull it away from trees, shrubs and new growth of other plants so they can grow on, free of an contact with the mulch but it is no big deal if you don't.

The fungal mycelium is only interested in eating dead or dying matter and therefore your living plants are 99% safe from their clutches, so I would say to you to not worry one bit about it and worry more about reducing the amount of water on those beds since such blooms tend to rise when the soil is overly wet as seems the case in your picture.

As soon as the soil warms up and dries off, the fungal blooms will die back and the mulch will perform its second and third function - reducing the chance of weeds to take hold and helping the soil to resist evaporation.

The quality of mulch does vary but from my experience here in the UK, you need fungi on your mulch for it to perform the first function - slow release of nutrients.

In the words on the front cover of The Hitchhikers' Guide to Galaxy - Don't Panic !

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  • PS: Looking back at your other posts, you were considering using pecan shells as a mulch for some reason, probably cost, which whilst organic would take a lot longer to break down. Is there any reason why you decided against those shells please ?
    – Nikki
    Commented Jun 1, 2023 at 21:43
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    If I remember right, I think I had an opportunity to get a modest amount of them and saw where they might make better mulch up for against tree bases inside a retaining ring for aesthetic purposes. I eventually went with bark chips/chinks, I think...no significant reasoning behind, it just worked out easier that way.
    – AA040371
    Commented Jun 1, 2023 at 23:25

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