Plant a swale that does not wash out this winter

What you need

• Seed — native sod-formers for the slopes, mowable grass for the bottom (see the note on grass vs rock)
• Straw with tackifier, or an erosion blanket and stakes on steeper grades
• Water, on a schedule you will actually keep

Steps

1. Time it. First frost date minus 6–8 weeks, minimum. Past that, don't plant; wait. The frost does not grade on effort.
2. Seed by zone. Side slopes: dense, low sod-formers — the species your local extension lists for wet feet; the desk does not pick cultivars for a continent. Bottom: something that tolerates mowing and occasional standing water. No fertilizer along the wet line: it feeds the runoff, not the swale.
3. Hold the soil until the roots arrive. Straw with tackifier on gentle runs; erosion blanket staked into the channel on 3%+ grades or any slope that will take a storm before germination.
4. Water on schedule. Frequent and light until germination, then deeper and rarer. Never so hard the channel runs — the point of the exercise is to stop teaching the swale to be a ditch.
5. Mow the bottom, not the slopes. A mowed bottom keeps the channel open; mowed side slopes lose their footing. Clippings out of the wet line — they dam the flow and rot into muck.

Where people stop

• Planting late anyway. The first storm does the re-grading you declined to do.
• One grass everywhere. A single mow-height species on 3:1 slopes and a wet bottom cannot do both jobs; one of them loses, usually the bottom.
• Watering like a lawn. Lawn watering on a swale moves soil downhill. Light and frequent, then deep and rare.