With respect, we seem to have a little confusion here with 'Lobelia', there are two types which are very different ie lobelia erinus, the the half hardy annual commonly used as a bedding plant,which can usually be planted out in may in most UK regions, then we have lobelia cardinalis which is a border perenial and will thrive in sun or shade, it requirs constantly damp soil,its ideal position is as a pond marginal (watch out for slugs) It was the latter that I responded to. Hope that clears things up.