If the seed is already down, wait until the new grass has been mowed a couple of times before a wash crew walks on it. Here’s why the order matters, and how to plan the calendar.

Why the two jobs clash

A homeowner in Massachusetts asked a lawn-care forum this exact question. They planned to overseed around the start of September and also wanted their vinyl siding cleaned, and they didn’t know which job should come first. It’s a common squeeze, because the fall timing for both jobs overlaps.

There are two separate risks to new grass:

  • The wash mix. Some house washes are bleach-based. A Maryland Extension horticulturist says chlorine bleach can be harmful to landscape plants. University of Maryland Extension also notes that grass seedlings are sensitive to chemical injury.
  • Feet and gear. A wash crew drags hoses, sets ladders and walks the edges of the house, often right where the lawn meets the beds. Maryland Extension says young seedlings are easily injured, and it advises keeping foot traffic off new seeding for at least a month after the seed germinates.

The second risk is easy to forget.

When fall seeding happens

Ohio State University Extension in Fairfield County says the whole month of September is acceptable for seeding in central Ohio. It says to postpone until spring if you can’t seed before October 20, because late seedlings may not survive the winter. Another Ohio State extension educator, writing from the Sandusky area, calls late August to early September a great time to overseed.

Germination is quick under good conditions. A University of Kentucky turf specialist says tall fescue typically takes seven to 10 days to germinate and Kentucky bluegrass 15 to 21 days.

The new grass isn’t ready for a crew the day it sprouts. Maryland Extension says to start mowing when the grass is a third taller than your normal height. That is typically four to six weeks after seeding in good conditions.

Plan the calendar

Your situation What to do Why
You haven’t seeded yet Wash now, then seed The crew finishes before any seedlings are there to step on
Seed went down in the last few weeks Wait until the lawn has been mowed at least a couple of times Maryland Extension’s foot-traffic rule for new seeding
You seeded in late September Consider booking the wash for spring First mowings may land in late October or November
The wash can’t wait Hand-wash the worst spots, or have the crew work from the driveway and beds only Keeps feet, hoses and ladders off the seedbed

Ohio State’s Fairfield County office says to sprinkle a new seeding lightly at least twice a day, which leaves the ground wet. Seeding second means you don’t have to pause that watering for wash day.

Protecting the lawn on wash day

Whether you hire it out or do it yourself, follow the cleaner’s label. Two house-wash makers’ data sheets say the same thing: wet the surrounding plants with water before you apply the product and rinse them again afterward. One of them, a bleach-based wash, says this even though it also claims it won’t harm plants and shrubs.

A few more steps from extension experts:

  • Don’t let the mix dry on plants. Maryland Extension says to wet foliage first and wash any solution off right away.
  • Covers need care. The same answer suggests plastic sheeting over plants, but warns it can burn leaves if the plants sit in the sun.
  • Soak the soil if a spill gets in. A Michigan extension answer suggests drenching soil that got bleach, to dilute it and push it past the roots.

What to tell the wash company

When you call for a quote, say the lawn was just seeded (or is about to be). Then ask:

  1. Where will you park the rig and run the hoses?
  2. Do you use a bleach-based mix, and how do you pre-wet and rinse the lawn edges?
  3. Can you set ladders from the beds or the driveway instead of the grass?

Our questions to ask a house washing company cover the rest of that call. To check whether a fall quote is fair, see what a house wash costs.

Keep the machine off the grass, too. Another homeowner on the same forum reported that grass laid about six weeks earlier looked burned after a worker briefly ran a gas pressure washer on it during a pool acid wash (the cause isn’t certain). Ask the crew to keep the engine on pavement.

If something does go brown after a wash, take dated photos before you call. Our guide on wash damage versus damage that was already there explains how to raise it. For more plant mistakes to avoid, see pressure washing mistakes homeowners make. Fertilizer stains are a separate problem; see rust spots on the driveway after a lawn treatment.

The dates above are general extension guidance, so check with your county extension office for your own area. For how we source and check every claim, see our methodology.

Sources