What is the best time of year to clean out ditches?

Answer: 

In ditch cleaning, you should be concerned about creating an erosion problem. Soil that is eroded in one place will settle as sediment someplace downstream, so ditch erosion causes two problems - one where the soil is removed and another where it is deposited.

Vegetation in the ditch helps to protect it from erosion. The roots bind the soil, holding it in place at the point of contact with the water. Also, the vegetation slows down the water velocity. Fast moving water can pick up larger particles of sand and gravel and cause erosion. While slowing the water, ditch vegetation also promotes sedimentation, and thus the ditches fill up over time and must be cleaned out.

Ditches that are cleaned out in the fall may have to wait until the following spring before new vegetation will be able to grow. Such ditches may lie unprotected during the heavy spring runoff period. This gives erosion a great opportunity! Once erosion gets started, it is hard for new plant roots to grow. You may need to place a lining of large stones in a ditch that is eroding badly. Seeding, even in the fall, as soon as possible after the ditch is cleaned will allow the moisture in the soil to promote the growth of the new seedlings. Waiting until the soil is dry will reduce the percentage of seed growth.

Ditch cleaning in the spring, after the peak runoff period in early spring, is the better time. Warm weather follows and thereby promotes revegetation. This is not to say that spring cleaning is risk-free with regard to erosion. It still takes a few weeks to get the grass growing again. But in comparison to fall cleaning, which is followed by a long period where the ditch may lie bare, spring cleaning is preferable.