Mint...well, I have a lot to say about it (and this is just my experience in my zone)....

First off I would say a well-drained soil is going to be best for ANY herb. They all love that. Imagine how you'd feel standing with wet feet all the time.

I don't know, for me, mint has been one of those no-brainers. I plant it, and it just grows. And like growers on this forum have stated, it can take over, if not in a container. I don't think mint really needs any fertilizer. I've never fertilized mine and all my mint is growing like crazy. And it's in soil that isn't considered a "rich" medium. Mint, in my experience, will grow just about anywhere.

Sooner or later, grown in a pot, mint will strangulate itself with its own roots. It just grows that way. Clipping it back is essential to encourage new, bushy growth and delaying the flowering heads (which can be quite pretty). I've been kinda lax in that department, as the mint growing in my beds have all flowered. But I will wack them off about a foot or so and in a few weeks, new growth will appear and it'll be dense as ever.

Some consider it the scourge of the garden, but I planted it on purpose where it is because I love the smell and the lush green growth it provides (such a beautiful backdrop). I just love mint and if you have the space, why not? I love adding small touches of it to salads, teas, lemonades, certain alcoholic beverages and some cooking dishes. Try adding some mint to cut up fresh strawberries with a sugar and balsamic vinegar reduction and you'll be wow'ed by the way it tastes! Incredible! But I have way more than I'll ever use. But I just love the way it looks. Balsamic vinegar by itself, when reduced, takes on a very sweet flavor that compliments strawberries quite nicely.

It's been my experience that mint doesn't require full sunlight. I have mine growing in two 8'x8' planting boxes with crape myrtles in the middle and the crapes shade them for a good part of the day, although they do get some direct sunlight.

If your mint is turning brown, my first reaction would be that you're over-watering it.

As I said, mint for me has always been a no-brainer. I've never grown in from seed. I buy seedlings of different species at the nursery, plant them, and sooner or later it's coming up everywhere, nice and dense and lush. It's really not one of those herbs you have to fuss over. And will thrive even despite a certain amount of neglect.

If you're going to grow mint in a container, I would chose a nice BIG and deep one and mix 'em up (spearmint, peppermint, orange mint, apple mint, chocolate mint, lime mint, the varieties are endless!). It's nice to have different types. Water them once in a while, clip them back and they'll reward you always with lush, dense and green growth. Don't be afraid to trim them regularly. They won't suffer because of it, they'll benefit from it. And when the season is over, do what I did. Cut them all back to sticks, and when the next season rolls around you'll be surprised how quickly new growth appears and sooner you'll have more mint than you know what to do with. Just take the soil from the container (by now root bound in a solid ball the shape of the container), and slice off a 3-4" disc of it and replant it, and it will grow like crazy. That's the way I did mine, and I didn't have to buy a single new mint plant this year. And I'm surrounded by more mint than you can shake a Mojito or Mint Julep at. :-)

I have so much much, I cut it into bouquets and bring in inside in vases sometimes mixing it in with fresh cut flowers, and the smell is wonderful. Beats silk plants inside (which, I'm sorry, is just tacky) any day of the year!

Herboholic gave you some excellent information, but s/he didn't comment on the window issue. You're trying to grow it inside, and it doesn't like it. Fresh air and some sunlight is what it needs more than anything. Light through glass is filtered, so you're plants don't get certain sun-rays that they need.

Those brown spots are rust, and that's a fungal disease common to mint - and it usually happens when a plant is in an unhealthy environment. You'll need to repot, using fresh potting mix. Remove as much soil as you can from the roots, then wash the whole plant in water to which you've added a dash of bleach (or use water from a chlorinated swimming pool) and a dash of dishwashing liquid. Wash the pot thoroughly the same way.

Why would you grow mint and not want to harvest it? Aside from the myriad ways of using mint in your cooking, harvesting is an excellent way of pruning - which encourages lush growth.

Mint has a very aggressive root system by which it spreads at a rapid rate. It also spreads by runners, above and below the ground, and by seed, and by layering and just about every other method known to botanists. So it needs a LARGE pot - if it fits on your windowsill, it's a tiny fraction of the size it needs. The pot doesn't need to be deep, but keep it off the garden by placing it on 'feet' - those roots can and will grow down and escape!

Back to the indoor thing. I'll bet your pot is sitting on a saucer which is almost always full of water. That's a big no-no. Tip out that water whenever you see it - look for it frequently! You'll find that the soil at the bottom of the pot is waterlogged - which leads to sick (drowning) roots which leads to one very dead plant. I honestly suspect you've been overwatering, despite your claims to the contrary - it's a common fault that beginners kill their plants with kindness. Apologies if I'm wrong!But that fungal disease tells its story.

Mint will grow in most soils - you've got yours in a pot, so you should use a potting mix - any will do. In a cooler climate it will take full sun. In a hot climate it does best in partial shade. Mints prefer moist soil with excellent drainage and the soil should never be allowed to dry out completely. Mint is susceptible to rustÂ. Keep manure away and remove any yellow-streaked sprigs to prevent the spread of the rust virus. If badly infected, spray with a copper solution, or cover the entire plant with straw and set it alight. New growth will soon appear. If preferred, dig up the entire plant and discard it. Do not plant mint in the same position for several years. Ideal soil pH is 6.5.

Mint will grow in any sort of pot as long as it is a decent size (think BIG) and has plenty of drainage holes in the bottom. You will need to repot every year at least, dividing the plant as you do it to prevent it becoming rootbound.

You shouldn't need to worry much about fertilising, since you'll be repotting frequently and using fresh potting mix each time. Stick a rusty iron nail into the soil - that'll give the plant the iron it likes.