12-05-2010, 01:25 PM
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These are stem plants, and all stem plants behave much the same. With the cut ends rooted in the substrate, they will continue to grow; once they reach the surface, they will usually continue to grow along the surface, in which case the lower leaves frequently yellow and die. Regular pruning/trimming, which means pulling u[p the stem, cutting off the lower portion, and planted the top portion back in the substrate is how you keep them "uniform". Some stem plants need this every week due to very fast growth.
Sometimes a stem plant may grow above the surface rather than along it, depending upon the species. Pennywort and Wisteria will not normally do this, unless they are physically forced into it.
To have just floating plants, lay the stem on the surface and let it grow. Prune/cut it back same as previously mentioned to keep it in check, depending upon how much cover you want. I find Pennywort makes a superb floating plant. Wisteria is less effective.