I grow the tender annual lobelia (Lobelia erinus) in Zone 7/7b. The blue is to die for!
Lobelia loves cool weather and plenty of water. I like Peter's 20-20-20 fertilizer but Miracle-Gro does just as well.
It does NOT like hot, dry summers.
Last year I planted it in a raised, well drained bed in full sun. Big mistake.
And in my area, we have to grow it in partial shade/full shade. It just can not take the hot summer sun.
Good luck.
Z7/7b