I have L. cardinalis and L. syphilitica, all in full sun. Not especially damp soil either. They're been there for years and do well.

I have both red and purple lobelia in full sun and red lobelia in partial shade. They seem to hop back & forth between 2 gardens.

Lobelia cardinalis grows wild here in damp areas along streams and rivers. I've seen it growing happily in both full shade and full sun, so it should be fine in just about any aspect given adequate moisture.

L. siphilitica escaped from my garden (from whence it has long been removed) and has continued to grow sneakily from old seed in the lawn (full sun), the field (also full sun) and in the shrub borders under the shrubs (full shade.) So aspect seems pretty variable for this Lobelia as well, though it tends to grow in drier areas than L. cardinalis, in soil of average moisture.

I have several patches of Lobelia cardinalis. It is one of the hummingbirds' top 3 favorite plants in my gardens (the other 2 being Monarda didyma and Salvia guaranitica 'Black & Blue').

It does fine in part sun, but pouts in my back garden when it is droughty. I will give it some extra watering, along with the veggies, and butterfly plants, because the hummingbirds love it so much. Also this encourages it to reseed along the edges of the garden. I agree with NHbabs, that it would prefer to have adequate moisture, over full sun.

I've posted this pic before, but here is a pic of my Cardinal flower from 2011 when we had abundant rainfall that summer -
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Thanks everyone!
The two lobelias I have are the cardinalis and syphilitica and everyone has provided lots of good info for both.
I wasn't sure if the syphilitica, like the cardinalis, reseeded but I think nhbabs set me straight on that.
Hi terrene. I guess you know where these lobelias are going. Yes, my butterfly/hummer garden.. And wouldn't you know it: beebalm and salvia feature prominently in it!
gary