Hi,

Yup, the plant is Lobelia Cardinalis indeed. As quoted from the Plant Finder at APC : It tends to produce many white roots at the nodes, which will have to be concealed with careful pruning. Pruning should be done by topping and replanting the tops. At one time I let it grew tall, with a lot of white root at the nodes (same as taipan's, see mine here and here), but then I keep trimming it, now the plant producing a lot of sideshoots and more compact...it will grow tall if you let it..

Since this plant have quite intensive root system, some root tab will surely help...and intense ligthing makes this plant grow more compact between nodes, IME...

Maybe it is still in emersed form (well maybe semi..hehe)..that is why some lower leaves are darker...how long since you planted it?..

Regards