There is nothing that you can do to get rid of mushrooms, that are growing in the ground. However as in your case they are in a planter, you may be able to do something about it. Look for a piece of decaying wood under the soil. More then likely the fungi spores are coming out of it. If you have a lot of organic mulch incorporated into your potting soil, that may also be the source of your problem. If you have good clean soil, and a wooden planter, the planter may actually be the source of the problem. If so, take everything out, and stick it in your oven, on its lowest setting for a few hours. Be careful, and check it frequently to make sure it does not get too hot, and start burning. If need be, leave the door open a crack. This will sterilize the wood killing any spores, that may still be present.
If your planter is painted or stained, please do not try this though. As it will more then likely reach the flash point of the paint and ignite, creating a much larger problem then the one you all ready have.
I would not bother using a fungicide, as they do nothing to stop mushrooms.

Good luck

Mike

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