I have a hanging basket with Lobelia in it. I had one last year, too, and though it looked good at first, it started to dry out and die after a few weeks of having it. I figured I got a bad one, but the one I bought this year is doing the same.
It’s hanging in a spot that’s shaded most of the day.
I water it every day, until the water runs out from the bottom of the basket.
I fertilize every few weeks.
I bought it from a reputable local dealer. Every other plant I have from them is beautiful.
I’m in Texas, where the summers are brutal, and around here, I thought lobelia is considered a cool-weather plant. It seems to show up for sale more in the fall. Maybe you’ll have more luck if you try it after the worst of summer is over?
The problem could actually be too much water; the only way to know is to stick your finger in the pot (if there are insufficient drainage holes, the soil could be soggy at the bottom of the container, something that’d be most obvious if you turned the pot over and gently removed the entire soil ball to check).
Overly soggy soil causes plants to wilt due to inability of the roots to take up oxygen, which will result in an appearance similar to that of overly dry soil.
If for some reason you’re using a soil mix that dries out with great rapidity even in the shade, you can try again with a more moisture retentive mix, or add those water absorbent crystals that release moisture as needed (this works nicely in dry spots in the garden).
Also, annual lobelia should get at least half day sun.
I’ve found that the plants for sale have almost always well overgrown their pots by the time they appear in the stores. Are you sure your plant isn’t root-bound and in need of a larger hanging basket?
Like I said, the place I get them from is pretty reliable, and every other plant I get from them is healthy and not root bound. So though I’m not sure how to tell, I sorta doubt it.
How to tell on average-size pots: take pot in one hand, place other one around stems on soil with fingers spread wide, turn pot over semi-quick (flip it over basically) and lift pot off of soil-ball. If you see more root than soil or a ‘wall’ of encircling roots - its root-bound. You can do this with pot on its side or however works best for the soil within the pot.
It helps to do this when the soil is barely moist and semi-firm, or when very dry if it is clay-like soil in pot. It varies, but usually you can tell what is going to work so it does not fall apart in your hand(s).
If the soil, especially the lower part of the root-ball is kinda slimy and smells bad, too much water (!) and it is anaerobic soil. That is bad, of course, and is not unusual with cheapo potting soils or old plants that are root-bound and not draining well (plugged drains basically).
If all looks good in the root-ball, then its likely either too much sun or bugs in soil eating upon roots (which is obvious when doing the ‘inspection’, fwiw).