Don’t Spray That Ditch!
N.C. Cooperative Extension
Louisburg, NC 27549
It is hot. It is humid. It has been raining for most of the summer and the grass just grows and grows. The ditches are the worst places to mow. It is hard to mow them with a mower since they are not flat and may be deep. It takes a long time to weed-eat them as they line the entire road front. What to do? Spray the ditch with weed killer? DON’T DO IT! Take a look at the photo below. The photo was taken on a road just outside of Louisburg in the middle of July. You can see where the landscaper has sprayed the ditch and killed the vegetation and now a head cut has started. The fellow looking at the head cut is a newly graduated civil engineer who works with my program and he, like me, is dismayed to see the dead ditch and the eroding ditch bottom. Sediment, or muddy water, is the number one water pollution problem in North Carolina and the United States. Where does sediment come from? Poorly maintained ditches like this!
The eroded place in the ditch is called a head cut. A head cut is a stream geology term meaning an abrupt drop in a stream bed. The study of streams (and ditches) is called fluvial geomorphology, and streams and ditches have characteristic behavior and are greatly affected by human activity. As water falls over the drop, the force of the water eats away at the soil and the head cut moves up stream and becomes deeper and deeper. The longer the head cut is allowed to cut upstream, or up the ditch, the deeper and wider it will become. The head cut in the photo has just begun, but if you look up above it you will see an older head cut that has become much deeper and wider. What was a difficult ditch to maintain is rapidly becoming a gully that will be impossible to maintain.
Ditches are designed to convey water. In the stormwater world they are designed to carry a lot of water and we call them vegetated swales. A properly designed swale will have a wide, relatively flat bottom that allows water to spread out, slow down, and infiltrate. Spreading water out and slowing it down are the keys to avoiding erosion and head cuts because water wants to concentrate as it flows downhill. A well designed swale will be easy to mow with a lawnmower, and the grass in the swale or ditch should be allowed to grow as tall and as dense as possible so that it can slow and spread the water that flows through the ditch. The State of North Carolina’s Division of Water Quality has recognized the value of swales in flood control and pollution prevention and has assigned them nitrogen, phosphorus, and sediment removal credits in the state’s Stormwater Best Management Practice Manual (see: http://portal.ncdenr.org/web/wq/ws/su/bmp-ch14).
Some people seek to solve their ditch problems by adding rock or concrete to reinforce the ditch. Hardened repairs with rock or concrete are prone to failure because heavy flows of water can go under or around rock and concrete and cause more erosion. The best way to protect ditches, swales, and stream banks is to allow natural vegetation to grow and to manage it as little as possible. If a ditch is prone to erosion then it can be lined with plastic grid or net-like turf matting also known as turf reinforcement mat or TRM. Grass can be planted in and through the matting. The grass and matting will grow together and make a strong, erosion resistant barrier.
So what to do with your ditch? Let it grow. The manicured look is not always best. A ditch with tall, dense vegetation is actually good for the environment. You may want to give it a ‘hair cut’ every few weeks but remember, vegetation is the best way to hold soil in place and to prevent flooding and preserve water quality.