Romanian deadlift will work the lower back quite bit. You could also grasp the db across your chest with both hands do a goodmorning. Though not specifically targeted at the lower back, a one armed overhead squat will certainly activate those muscles. How doing "superman" with no weight at all?

Bent over rows. Deadlifts can also target your back, however seeing as you only have one dumbell the row will probably be more effective

Do you mean this kind of dumbell row -> click here

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Yeah that is the single dumbell row, and that should do good for your back. What i like to do is use a barbell or two dumbells to do a standard bent over row Bent Over Barbell Row Exercise Guide and Video . However both workouts should be good exercises for your back. Other examples of back exercises (these are not neccecerally dumbell exercises however) are inverted rows, Chinup/pullups, Deadlifts.

Bent over rows are not the best choice for lower back. They will hit the lower back to some degree but there is no movement of the lower back itself as opposed to something like a good morning for instance. Rows hit the lower back more if they are done unsupported.

I would recommend the Superman, Horse Pose, Bar Hangs, etc..Work on posture, and make sure all of your exercises are being completed with proper mechanics. If you are having back pain see a doctor or a professional in your area, as I doubt anyone here is qualified to treat someone with back pain, let alone over the internet.