I'm not sure why, initially it was doing great, but then started to wilt and go dry. We may have forgotten to water it, but there was rain and all. It's meant to do well in full sun too, which is what it was getting.
My guess is the soil was not drained enough, it seems like the stems just rotted away at soil level.
The soil itself was nice black soil from our compost heap, so it should have had enough nutrients, etc. A friend told me that they do better in the soil than in pots, and maybe that's because they need good drainage.
Ah well, we'll just have to try again :)