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I found this yellow looking patch underneath the branches of my rosemary plant. I did not see it near either of the other 2 plants in the container. I dug the yellow stuff up, it seemed surface level, but the soil underneath where it was seemed very light in color.

I am a total newb gardener here. What is this thing and how do I make sure it doesn't come back?

This is to give you an idea of where it is located, that's a rosemary plant and I had to really pull back the plant to see this yellow stuff

This is a close up image of the yellow stuff

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It is a fungus digesting the organic matter in the soil underneath the plant, and it is, if anything, beneficial. See the bark chips in the soil? They are what the fungus is breaking down. The white stuff you saw under the surface is mycelia. The part on the top is the reproductive material.

It really only eats the undecomposed material, rendering it into something the plant can access. If the look bothers you, it would be fine to remove the patch, but it won't harm the plant. As stormy notes, this type of fungus usually only appears when the soil surface doesn't get much time to dry out. This constant damp is hard on a potted plant. Make sure the top 1-2" of soil dry out in between waterings.

But from what I can see, the rosemary looks fairly healthy. I also see a little sprout (weed?) coming out, usually a sign that the surface soil isn't drying out.

  • The fact this fungus is growing shows that you might be watering too much. Let your plant's soil dry out a bit more before you water. When you water, SOAK the soil making sure it drains well. Lift the pot. Remember the weight...allow the soil to dry. Pick up the pot and if it is significantly lighter it is probably time to water. If moist, don't water! Rosemary likes some dryness. – stormy Aug 26 '14 at 1:14
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Looks like Fuligo septica, one of the slime moulds, also known charmingly as dog vomit slime mould. Harmless, but not attractive, and very common where there are bark chips within the compost mix or on top of the soil. Allowing it to dry out between waterings might help a bit, but you might find it returns, so if it does and you really hate it, you may need to change the potting compost, this time using something without woody residue.

  • By the way, slime moulds are really interesting creatures, some of them showing some strange behaviour, exhibiting some learning capabilities. – Ariser Sep 1 '15 at 20:29

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