Answer: Unfortunately, these basins are difficult to plant. The surface must allow for the smooth flow of water in and out and the planting must hold the soil against erosion. The plant must be able to survive the occasional inundation -- as well as fluctuating soil moisture levels. These paramenters account for the seemingly uncreative use of lawn grasses in these areas. Grass is probably the best alternative for an evergreen, even though it is difficult to mow on the slope. Of the two plants you mentioned, vinca is more likely to survive these conditions, although given the moisture levels I would not expect a uniform look from it. In my experience phlox would not survive the periodic moisture. Another possibility is to line it with attractive stones and omit the planting. You might check with the local authorities as to the legality of using different plants and materials as well. In some cases this is very strictly regulated.
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