02-28-2010, 01:31 AM
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found this in a google search. it's relevant
Most rosette plants cannot be “pruned” like a stem plant. With Vallisneria and Sagittaria, some people trim the ends of the leaves if they lay across the water surface and block light. The disadvantage of this method of trimming is that the damaged end of the leaf often begins to rot. Eventually, the whole leaf will die back. The best answer is to use those species that will not outgrow the height of the tank they are kept in. Alternatively, it is preferable to remove the longer outside leaves while allowing the shorter, young inner leaves to grow rather than to damage the leaves by shortening them.
Vallisneria, Sagittaria, Cryptocoryne and some Echinodorus species spread by runner, just below the surface of the substrate. A healthy bed of Vallisneria will need regular thinning or it will begin to take over the tank. Large, older plants can be thinned from the middle of the bed by feeling around them to find the attached runners and severing them as you slowly work the plant out of the group. Removing young plants from edges of the stand is easier. These can just be pulled up — roots, runners and all — and pinched from the parent stand at any convenient point.