Honestly, if you let that Water Wisteria of yours grow out some more you will have yourself a stunning background plant.

The Water Wisteria is on the left and right side of this wild tank of mine.
Here are a few tips to encourage the Water Wisteria you have to grow in a more bush like pattern (some may have already been mentioned).
1. Plant stems in close groups approximately 2 to 2.5 inches apart.
2. rip a few leaves off the bottom of the stem and place a good chunk of this stem in the ground. It should stay in place and begin growing thick roots provided the substrate is not disturbed.
3. Trim the plant once it begins growing side shoots off the mother stem. this will cause the mother plant to send out double the shoots and cause your Water Wisteria to gain a thick, bushy-like appearance, great background cover.
Keep in mind these tips are not only good for Water Wisteria, but many of the other stem plants the other members mentioned. Bacopas, Rotalas, Limnophilas, Hygrophilas and Pogostemen plants all follow this same approach, except #1 in which some stems can be planted much closer to one another depending on the species.