Erodium Bishop's form

Flowers all summer

I have some clumps of this doing well, and one or two struggling.
What conditions do you think it prefers ?

I put in them a very griity well draining soil then a thick collar of pea shingle seem to like light shade and water, I do not feed them. I have six those I water and in a bit of shade seem to be the best this is a new bed to cover a drain so only planted them june/july. I think the others sit on clay even though in lots of gritty soil, it seems clay does not do for this type. But that's just my observation possibly do not drain as well sit in water?

thanks...
The clump of mine which is struggling is in full sun... and I wondered if the roots were being eaten by ants or similar... there is another clump about a metre away in almost as much sunshine which is thriving... sounds like very dry is not what they like ?

Mine seem to love a good drink because of the drainage I suppose. Ihave ants under a couple of plants in the rockery but nothing ever seems to get damaged I only see then if I pull out a weed.