What can you do on top of a drainfield?
The drainfield dos and don'ts come down to air, weight and water. Thurston County says grass or shallow-rooted plants are the right cover, trees and large shrubs stay at least 30 feet away, and nothing drives, parks, grazes or gets built on the field. Sprinkler lines keep 10 feet from every septic part. The reserve area, your spare drainfield site, gets the same care.
Street to Drainfield · updated · sources listed at the end
What can you plant on a drainfield?
Grass is the ideal cover, says the county's septic questions page. It can be mowed lawn, an unmowed meadow or ornamental grasses, and groundcovers and ferns also work. Pick low-maintenance, low-water, shallow-rooted plants; the county's "Landscape Your Drainfield" guide, linked from its care page, names specific ones.
Skip the vegetable bed. The county gives three reasons: vegetables need watering that reduces the soil's ability to treat wastewater, deep roots and rototilling can damage pipes, and food crops risk contamination. Landscape plastic or fabric is out too, and so is mulch or bark over the field, because they cut the air the soil needs and hold water.
How far should trees stay from a drainfield?
At least 30 feet. The county says trees and large shrubs have root systems that seek out wet places like drainfields and can clog and damage the lines. Some soils need even more distance; if you want trees anywhere near the field, the county suggests asking an expert who can judge species and spacing for your soil.
Roots also find the pipe from the house to the tank. The Department of Health's signs-of-failure page lists root blockage of that line, especially on older systems, as a common cause of slow drains.
Can you drive, park or build on a drainfield?
No to all three. The county's care page says cars and heavy equipment can crack pipes, and suggests a barrier wherever cars, livestock or machines could get onto the field. Its questions page rules out a carport, camper pad, tennis court or hot tub, because vehicles compact the soil and concrete or asphalt cut off oxygen and evaporation.
Animals count as weight. The county says livestock trample and muddy the soil in winter and compact it in summer, and that even one horse is not recommended.
Where can sprinklers and irrigation go?
Not in the field and not draining toward it. The county says not to install an irrigation system in the drainfield, and that water lines should sit at least 10 feet from all septic components, with approved backflow prevention on every sprinkler line. Any irrigation set up on city water also falls under the city's backflow rules, described in city water in Olympia, Lacey and Tumwater.
Drains and walls need a call first. Planning a French, curtain or interceptor drain, or a new wall, inside the 30-foot zone around any septic part? The county wants Environmental Health consulted beforehand, and it never allows cutting through a drainfield for drains, walls or lines.
Why does the reserve area matter so much?
It is your next drainfield. The county requires every new home on septic to have a designated replacement area, and says one should have been set aside on the permit for systems installed after 1980. It must be treated exactly like the working field: no building, paving, parking or deep digging. When a drainfield fails, the county says pumping will not fix it and the new field has to go somewhere else, and the reserve may be the only spot on the lot with suitable soil. Learn the warning signs in signs of a failing drainfield.
How do you find the drainfield before a yard project?
Start with paper. The county keeps a record drawing, once called an as-built, for most permitted systems; ask for it before you plan anything, as described in Thurston County septic records. Drawings from before 1980 can be rough sketches, newer ones show the tank, field and reserve.
Then use the yard. The county says that in late summer, if you stop watering, green stripes may show over the lines, and in winter frost may melt first above the pipes. White-capped tubes at or near the surface are monitoring ports at the ends of the lines. If you probe, avoid heavy steel bars that can damage the tank top or other parts, and stay well back from tank lids.
| Thing | On the drainfield? | County reason |
|---|---|---|
| Grass or groundcover | Yes | Shallow roots hold soil and leave air for treatment |
| Trees and large shrubs | Keep 30 feet away | Roots seek out wet trenches and clog lines |
| Vegetable garden | No | Watering, digging and contamination risk |
| Carport, patio or hot tub | No | Compaction and lost oxygen |
| Sprinkler lines | Keep 10 feet from all parts | Too much water near the system |
| Horses or other livestock | No | Trampling and compaction |
Yours to do
- Mow and keep grass or shallow-rooted plants on the field.
- Mark the edges so vehicles stay off.
- Turn downspouts and runoff away from the field.
- Keep the reserve area open and unpaved.
- Ask for the record drawing before any yard project.
Leave to a licensed or certified pro
- Drains or retaining walls within 30 feet are checked with Thurston County Environmental Health first.
- Advice on planting trees near the field from someone who can judge your soil.
- Locating buried components with equipment, by a septic professional.
- Irrigation backflow assemblies installed and tested by certified people where the city requires them.
On the record
- 30 feetMinimum distance for trees and large shrubsThurston County septic FAQ
- 10 feetMinimum distance from water lines to any septic partThurston County septic FAQ
- 1980Year after which permits should show a reserve areaThurston County septic care page
Common questions
Can I put a children's play area on the drainfield?
A grassy play space is fine, but skip anything heavy, paved or dug in. The county's rules against building, paving and digging apply to play structures too.
Is bark mulch acceptable on a drainfield?
The county does not recommend it, because mulch and bark reduce air exchange and hold water in the soil.
Can I store a boat or trailer on the reserve area?
No. The county says the reserve area should be left undeveloped and protected from compaction.
Will mowing equipment damage the drainfield?
The county's warnings are about cars, heavy equipment and livestock. An ordinary lawn mower is part of keeping grass cover, but keep larger tractors and loaded trailers off.
Sources
- Septic Frequently Asked Questions, Thurston County — planting, trees, mulch, structures, livestock, sprinkler setbacks, drains and walls; read 6 November 2026
- Care & Maintenance of Your Septic System, Thurston County — dos and don'ts list, reserve area, locating the drainfield; read 6 November 2026
- Signs of Septic System Failure, Washington State Department of Health — roots in the house-to-tank line; read 6 November 2026
- Frequent Questions on Septic Systems, US EPA — no driving or building on drainfields; read 6 November 2026
How this page was put together. This page follows Thurston County Environmental Health's drainfield guidance, with support from the Washington State Department of Health and EPA, read on 6 November 2026. It does not list plant species, because the county's own landscaping guide covers them, and it leaves tank access and component locating to professionals.
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