Help with garlic plant

Umm... Could it be that your garlic is a particularly early maturing variety? Did the garden centre give any information/labeling with it?
Garlic bulbs are like onions in that when they reach maturity, the leaves start to die off. Maybe it's just ready for harvesting. I must admit I haven't heard of growing garlic in a pot before. What about watering? To much or too little can be equally devastating to any plant.
I'd have a little furtle in the soil at the base of the stem. If the bulb seems quite well developed then it maybe you need to buy a French stick and a pack of butter. If not..Anyone else?

I've never even heard of buying garlic in as a ready-grown plant!
18" also seems quite big to me? Is it standard garlic or something special?

Is the soil in the pot well watered (so the pot feels heavy but not so it is waterlogged as this will drown the plan)?

Also, did you repot the plant before it started dying or was it still in its original pot when it started dying? It sounds like the roots may have been damaged, perhaps by a pest or perhaps when potting on, as this would cause the plant to keel over. Did you notice when repotting whether the plant's roots were filling the pot or not? What colour were the roots?

If none of this helps, perhaps a picture would be useful, can you upload one?

Welcome to the Vine Desertgecko.
I'm surprised you bought a garlic plant in a garden centre. Never seen such a thing.
Garlic is a hardy plant, mine were planted outside in October and I expect them to be ready for harvest around July. I will know they are ready because the leaves will start dying off.
Maybe your garlic is some special variety and it's just reaching maturity. Can't see where you are going wrong.
If the plant fails take it back to the garden centre. Most give a guarantee.
As a side issue, one garlic plant would give you one bulb of garlic. What did the garden centre charge you for it ?

And here's a link to common garlic diseases - just in case.
Growing Garlic At Home - Plant Diseases

I'm intrigued .. a garlic plant, in a pot? How much did they charge you?

Thanks for the replies everyone. I take it it's uncommon to buy garlic as a plant! I got it from a large garden centre in Newhaven, East Sussex. I can't remember the exact price but it would've been around £3 - £4, I wouldn't have paid more then that. It did have a label but whether I put it in the new pot I'm not sure. I'll have a hunt for it as I wouldn't have thrown it out.

I noticed the plant starting to wilt, so that made me remember to repot it. So the leaves were already starting to flop before I repotted. It's just got worse now. I do recall there being loads of roots, but I didn't check them, I just quickly transferred it with as little fuss as possible. I have been making sure I'm watering as it's been warm and windy down here over the last couple of days. Maybe I let it dry out too mcuh and then over watered?

I agree it sounds like it could be mature already, as I know the leaves do die off when their ready, but I can't believe it would be ready yet. I'll have a feel in the soil, but if not I'll risk checking the roots.

I did attempt garlic last autumn with a supermarket bought bulb. I planted the cloves separate and they all grew quite well, but in the end the plant died and no bulb had formed. I put this down to the source of the bulb more then me.

I'm not at home atm, and wont be until tonight, but I'll definitely get some pics to show you what I mean.

I'm intrigued with this. Please post a pic!