Have you had any recent very dry/hot periods followed closely by very wet periods? I've had them split if they get lots of water like that. The stems can't quite cope with the surges of water and swell to bursting.

pretty common. next time plant them deeper. it helps alittle.

Thanks for the help... do you think they'll recover... or should I plant some more to cope with it? Should I water more regularly... to even out the dry spells that we've had?

they should recover and yes to even watering.