Any plant material in the ditch will trap debris floating downstream during heavy rains. Over time, this build up can add several inches to the bottom. I have a ditch/gutter on one side of my yard. I have to clean it out every year from the soil build-up.
The process is similar to the formation of a river delta. Rain water carries soil and debris along the ditch. Somewhere, the stuff gets snagged. This acts as a natural dam and begins to collect more stuff on the upstream side, until you have a ridge or something build up and obstruct the free flow.
If you do plant anything, keep it to the sides. Ditch lilies, with their myriad of leaves, will trap debris even quicker. The debris build-up will even include the leave themselves.
Let's say you planted a forsythia hedge on our side of the ditch.It would look beautiful in the spring and give you a wall of green all summer. But, by the end of the 2nd season, you will have branches touching ground and taking root. Before you know it, the ditch will have a natural damn of forsythia coming up all across the ditch.
