Swale Desk Field notes on swales and yard drainage. One desk, three numbers.
Season This page is seasonal: the planting window closes 6–8 weeks before first frost. Past it, wait for spring — a winter swale dig is a spring erosion project.

Plant a swale that does not wash out this winter

Timebox: half a day, inside a real window — 6 to 8 weeks before the first hard frost, or wait for spring. Output: a channel whose plants hold the soil you shaped. A swale dug in September and left bare is a gully by March; the plants are not decoration, they are the retaining system.

Checked 2026-09-10 · seasonal page — revisit before each planting window

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.

First-winter survival list

CheckPass mark
Germinated before frost?Yes — roots, not just shoots
Soil covered?Mulch or blanket until canopy closes
Inlets armored?Riprap pad at every entry point — riprap
Spill path exists?Overflow goes somewhere legal — see swale

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.

Where to go next

Sources

  1. USDA NRCS — Critical area planting (code 342), conservation practice standard — supports critical area planting specification for establishing vegetation.
  2. USDA NRCS — Grassed waterway (code 412), conservation practice standard — supports vegetated waterway establishment and maintenance.
  3. Minnesota Stormwater Manual — Design criteria for wet swale (wetland channel) — supports the swale vegetation requirements this job satisfies.