Not sure what's wrong but I have to ask, why are you growing it indoors? It's a hardy plant that would much rather be outside.

Rodney

Funny, I have some that is doing that too. I would just make some mojitos with the rest, cut it back and plant it in a pot outside, if that is an option.

Unfortunately I live in an apartment and don't really have the option of growing plants outside. Thus, indoor windowsill plants.

Julia, can I infer from your comment that cutting away the leaves that are like this is a good thing to do, rather than leave them on?

The leaves are still green and functional, except for the brown parts. If the browning is too much for your eyes to bear, then lop them off; otherwise leave them.

Mint is the herpes of the plant kingdom and it will not be harmed if you remove the browning leaves and harvest the rest. It will come back again and again.

If you cut off a sprig of mint, you can very easily make more plants from it. Just take the last couple inches of the sprig, remove all of the big leaves, and push the cut end deep into some moist dirt or shaded water. Mint is arguably the easiest plant to replicate, and the cutting will likely start growing in a few days without any skill or extra effort/products.

The brown on your leaves looks like two things to me. One looks like a bacterial spot that was likely spread by insects. The other may be from water issues like the leaves being wet for too long, and/or overwatering.

The reduced light of being indoors will decrease the plant's ability to overcome all forces by growing though, and you may be better off with a plant that naturally grows in deeply shaded environments. If you don't have plant-eating cats, Philodendron Scandens is both attractive and effortless to maintain.

Plants like mint grow so fast that they don't seem to care about leaf damage this minor though, and like Julia has stated, you can cut the brown part off and make Mojitos out of the rest of the sprig. The plant will likely appreciate the extra sunlight from not having its branches in its way.

Thank you Buck, that's very helpful.

I'm afraid it might be a bacterial thing, as I'm beginning to see similar browning with tiny black specks on the basil and cilantro I have in pots next to the mint.

Is there any way to stop the spread of this stuff, or are my plants doomed?