Cover bare earth with grass or other low, dense plants as soon as the digging ends, because roots bind the topsoil and the leaves break the force of raindrops before they strike the ground. Seed berms, cut banks, fill slopes and ditch sides immediately, hold the seed under a thin mulch, and keep traffic and grazing off until the cover has closed. Grass slows water and resists scour, but with a typical grass roughness a grassed ditch carries about 37 percent less than the same ditch bare.
Part of Flood Control and Drainage
Vegetation cover is the living protection on soil surfaces: grasses, ground covers and deep roots that hold earth in place against rain and running water.
Plants Do Two Jobs: Roots Hold, Leaves Shield
Roots bind soil particles together and anchor the surface against flowing water. The leaves and stems above ground absorb the impact of raindrops and slow the water running across the surface. Bare soil has neither, and the first storm tests it hard. The parent article puts it simply: vegetation is your best erosion control, and bare earth on berms and slopes erodes the structure within months.
What You Need
- Seed or cuttings of a hardy local grass — choose species that already grow well nearby, spread by roots or runners, and tolerate wet and dry. Take advice from whoever farms the area.
- Mulch — straw or similar to keep seed from washing away and to hold moisture. See Mulching.
- Fencing or stakes — to keep animals and people off the new cover.
- A prepared surface — loosened topsoil, free of stones and roots.
Method: Establish Cover
Step 1: Prepare the surface. Rake or loosen the top few centimetres so seed contacts soil. Leave the surface rough across the slope rather than polished smooth down it.
Step 2: Sow straight after earthwork. Seed any bare slope, especially cut banks and fill slopes, the same day if you can. Bare soil that waits for the next storm loses seed and soil together.
Step 3: Mulch and firm. Spread a thin layer of straw and press or tread it in lightly. This holds the seed against rain and wind.
Step 4: Protect while it grows. Keep animals and foot traffic off until the cover has closed. The new roots are shallow and weak.
Step 5: Water or time sowing. Sow before a rainy period if you can. In a dry spell, water lightly rather than flooding, which washes seed down the slope.
Step 6: Maintain. Cut grass in ditches short so it does not slow the flow too much, and reseed bare patches at once.
Worked Example: What Grass Costs A Ditch
Take the 0.5 m by 0.4 m trapezoidal ditch from Ditch Construction at 0.5 percent slope. With a bare earth roughness n of 0.022, it carries 0.42 m3/s brim-full. With an assumed grassed roughness of 0.035:
- Q = 0.42 x (0.022 / 0.035) = 0.42 x 0.629 = 0.27 m3/s.
- Loss: 1 - 0.629 = 0.371, about 37 percent.
So a grassed ditch resists scour better but passes about three-eighths less water. Either enlarge the ditch where it will be grassed, or keep the grass short. The roughness values are typical textbook figures, not measurements of your ditch.
Common Mistakes
- Delaying seeding — erosion starts at the first storm.
- No mulch — seed washes off a slope before it roots.
- Sowing a non-native or fragile species — it dies in the first dry spell. Use local, hardy plants.
- Letting animals graze it too early — young plants are pulled out by the roots.
- Forgetting capacity — grass in a ditch reduces its flow. Enlarge the section.
What’s Next
- Learn next: Riprap — where grass cannot hold the water
- Combine with: Cover Crops — the farming use of the same principle
- Combine with: Earth Berms — seed the berm at once