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Recently I was watching my plant which has stalled growth. Lately, it feels like there is fungus outside the pot. Is that fungus? Is it coming from inside out?

The brown dead plant is coriander.

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That isn't fungus, it is mineral salts (aka nutrients) from within the substrate that are being taken into solution when you water and moved into the terracotta pot and the water they were in evaporates, leaving the mineral salts precipitating on the outside.

It is nothing to worry about of course, unless you find it unsightly wherein you could always try to scrub them away with a little water but they will continue to precipitate out every time you water. It is because the terracotta is breathable.

It can also occur in plastic pots but it tends to be on the top of the substrate or even on the sides of the pot, again capillary action means water will go up the side of the pot, above the level of the substrate. The water evaporates, leaving the precipitant salts on the side - or even on the substrate.

One of the greatest sources of these nutrients which houseplants certainly need is tap water that contains limestone deposits. They are good for the plants since commercial fertilisers do not contain Calcium or Magnesium salts for the very reason they are already in tap water.

If you were to water with distilled water, other salts (from the fertiliser and decomposing organic matter) would still precipitate out of course but your plants would then be deprived of calcium, magnesium and other micro-nutrients that tap (and mineral) water contains of course.

You could try moving to occasional Drench Watering, where you stand your plant pots in a bucket, kitchen sink or even your bath and stand the pots in water for 30 minutes, so the entire substrate is drenched and then stand the pots outside or on a draining board for at least an hour and any excess salts will dissolve in the sink and once on the draining board the force of gravity will draw the solutions downwards and out of the drainage holes in the pot.

Then, you leave the plant un-watered for much longer than you might from Trickle Watering but the next watering should contain fertiliser.

On a separate note, Fungi are our friends.

Plants work in harmony with microbes in the substrate. Just like we humans cannot digest the food we eat without our own billions upon billions of bacteria and fungi in our guts, plants rely upon the microbes in the substrate to do the digestion for them. Roots are just big vacuum cleaners sucking up small molecules of nutrients that have been digested by the microbes.

In return, the roots seep out simple sugars from leaf photosynthesis into the substrate to feed the same microbes and help them grow.

It is all to do with eco-balance in the Rhizosphere and occasionally the fungi get a new lease of life and go up for some air as it were. If you are particularly lucky, you might even get some fruiting bodies growing aka mushrooms as in your case.

Don't worry, they will die back and in turn get digested by the bacteria and the nutrients from their tiny little bodies will go back to feeding the plant roots.

The Circle of Life.

PS: Do not put anything other than water or water+fertiliser in container grown plants. Anything else will disrupt the balanced diversity in the rhizosphere. That means no vinegar, table salt, cinnamon, neem oil, hydrogen peroxide or other internet gardening myths and useless ' home remedies’. All these things do is temporarily or permanently pollute the substrate, destroy the balance of beneficial bacteria, fungi, nematodes and other microbes and might even kill roots as a consequence.

You only need to worry is if the substrate begins to stink and then it is other types of pathogenic fungi who only wake up and attack once a substrate is anaerobic (no oxygen) and waterlogged. That isn't down to the beneficial fungi but down to plant owners overwatering and not providing a well-draining substrate and pot.

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  • > You only need to worry is if the substrate begins to stink Do you have any good resources on what to do in this case? It happened to one of my three (plastic) potted hortensias. I accidentally overwatered one of them and it was sitting in 5 inches of water for 3-4 days. After 4 days the substrate still seems wet and smells bad. Jul 27, 2023 at 23:11
  • It is Root Rot and frankly in most cases it is terminal. The process is too detailed to explain here. Search for Root Rot in this group or better still, look on YT since Show Not Tell is more effective than the written word in that regard.
    – Nikki
    Jul 28, 2023 at 22:02
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    Thanks for the pointers. There's some green buds, hopefully it comes through. Aug 8, 2023 at 20:22
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It the white stuff kind of crusty, like dried salt?

If so, it's just efflorescing. Clay pots are slightly permeable to water. As you water the plant, some of that water dissolves salts in the soil, seeps through the pot to the outside, and then dries leaving the salt behind in a fine crust. After that happens enough times, the salt builds up visibly.

If it is efflorescence, you should be able to wipe it away with a damp rag (might need a little elbow grease) or ignore it. It will not harm your plant.

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