Western Washington gets two turnip sowings: a spring one, and a late-summer one for fall harvest. East of the Cascades, the Seattle numbers below do not carry over, and this page has no verified frost dates for that side of the state. What follows is the western calendar, what its dates actually measure, and where it stops being reliable.
The short answer for Washington
Sow spring turnips in western Washington after the last-frost window of February 27 to March 18, and only when the soil has dried out enough to work. Sow the fall crop in late summer, counting back from a first-frost window of November 13 to November 30.
Both windows come from NOAA’s 1991-2020 Climate Normals for the Seattle station (USW00024233). They are not dates on which frost stops or starts. Each is the span where the probability of a 32°F freeze sits between 30 and 70 percent. A freeze on March 25 would not break any rule, and neither would a mild stretch that runs past November 30. The numbers describe odds.
Why the frost date is not the sowing date
Here, frost is a weak gate for turnips. The last-freeze window arrives early, but cold, saturated soil and low light hold spring crops back well past it. A seed sown on March 1 into soil that stays wet and cold can sit for weeks, or rot, while one sown later catches up and passes it. Let the soil, not the calendar, set the day. The method for judging that is in the last section.
The coldest month at the reference station has a mean daily minimum of 37.1°F, according to the same normals. That figure puts typical winter nights above freezing, so cold survival is rarely the limit on the west side. Timing is: roots need enough cool, lit weeks to size up before light fades and soil chills. The fall window is why late-summer sowing works. A turnip sown then grows into the cool part of the year and can take a light freeze on the way.
The gap between the late end of the spring window and the early end of the fall window is nearly eight months. That is long enough to fit more than one sowing, so a spring batch and a fall batch are both realistic. For which varieties suit each, see The Best Turnips and Rutabaga for Washington. For the wider picture, Growing Turnips and Rutabaga in Washington covers the full crop.
What will not work here
Copying the Seattle window across the state is the biggest failure. The frost facts for this page are western, mild and maritime. Eastern Washington is high desert with a far shorter season, and a date borrowed from the coast can land a sowing in the wrong half of the year. This page does not have eastern frost dates, so it will not guess them.
Sowing on the frost date
West of the Cascades, a gardener who reads “last frost February 27 to March 18” and sows that week is acting on the wrong signal. The frost risk is low, but the soil is the problem. Cold, wet ground slows germination, and low light thins the seedlings that do come up. The result is a ragged row, patchy stands and roots that bolt or turn woody before they size up. Waiting is cheaper than resowing.
Putting the slow crop in late
Rutabaga needs more growing time than turnips, which is why it is the first crop a short season defeats. On the mild west side that is manageable with a timely sowing. On the east side, or at altitude, a late sowing leaves it racing the first fall freeze with no margin. If you garden there, favor turnips over rutabaga unless a county source says otherwise.
Close spacing and weekly watering
The maritime west is damp for much of the growing season. Crowded rows hold moisture against the leaves and keep the root zone wet, so thin early and give roots room. Over in the high desert, the opposite problem shows up. A shallow weekly soak will not keep a fast, shallow-rooted crop evenly moist, and uneven moisture cracks and toughens roots. Dry-side gardeners need frequent light watering, or mulch, to hold the moisture steady.
None of this is exotic. It is just what the numbers say: a very early freeze date that misleads, a wet spring that punishes early sowing, and a state split by a mountain range.
What changes across Washington
The USDA Plant Hardiness Zone Map (2023, built on 1991-2020 normals) places Washington across zones 4a to 9a. That spread is five full zones. There is no single answer for the state, and a zone range this wide means the Seattle calendar describes one corner of Washington, not the whole thing.
The zone system tracks one variable: the average annual extreme minimum temperature. It tells you about winter cold. It does not tell you how many frost-free weeks you have, how wet your spring soil stays, or how early the first fall freeze arrives. For a quick-growing cool-season crop like turnips, those are the things that decide the sowing date. Treat the zone as one clue, not the whole recommendation.
If you garden in the north or at altitude, shift the calendar the same way each time. Spring sowing moves later, because the ground thaws and dries later. Fall sowing moves earlier, because the first freeze arrives sooner and the root needs its weeks before then. How much to shift depends on your location, and this page has no figure for it. The Seattle window is a reference point, not a starting offset to copy.
I will not tell you which zone you are in. Look it up on the USDA Plant Hardiness Zone Map by ZIP code, then read the zone as the winter-cold half of the story. The frost dates and soil behavior in your own yard are the other half.
Where to check this for your own county
WSU Extension is the place to confirm any of this locally. Washington State University’s county offices are staffed by people who know the soils and microclimates in their area, and they work from local experience, not a statewide average.
Look for two things. First, a local planting calendar for your county, or the nearest one, with spring and fall sowing windows for cool-season vegetables. Second, the varieties that have been trialed or recommended where you live. A turnip or rutabaga that performs in a coastal county may sulk in a dry eastern one, and a local trial list says so more reliably than a catalog does.
A county recommendation beats a state one, plainly. This page is the state one: it uses one reference station on the west side and a zone range that covers everything from coastal lowlands to high country. Where the two disagree, go with the county, then with your own garden. If your county office has no turnip-specific calendar, ask what they publish for other cool-season root crops, such as carrots or beets, and use the same timing as a starting point. Treat that as a starting point, not a rule.
Checking this against your own garden
Your own yard outranks every date above. Three observations are free, and each one tells you something about your microclimate.
- Soil by feel. Grab a handful from a few inches down. If it feels cold against your palm and clumps into a slick ball that will not crumble, it is too early. Soil that crumbles loosely and feels cool but not icy is ready. A spot that gets there earlier than the rest of the yard is your warm, well-drained corner.
- First true leaves. After seedlings emerge, the first leaves that look like turnip leaves, not the round seed leaves, show how the plant is coping. Steady growth means the timing was right. A long stall points to cold, wet soil or too little light.
- Frost on tender plants. In fall, note when frost first blackens tomato or squash foliage in your yard. If that date is earlier than the Seattle window, you are in a colder pocket, and your fall sowing should move up accordingly.
Write the dates down for a year or two. Your own record becomes the calendar this page cannot give you.
The Moon calendar is a separate, older custom: see when to plant turnips by the moon, written as tradition rather than fact.
Common questions
Can I sow turnips in Washington in February?
West of the Cascades, February 27 is the start of the last-frost window, but soil is usually the limit. If the ground is saturated and cold, wait. East of the mountains, February is far too early.
Is the Seattle frost window valid statewide?
No. The February 27 to March 18 and November 13 to November 30 windows are from the Seattle station and describe western Washington. Eastern Washington has a far shorter season, and this page does not carry its frost dates.
Will a fall frost kill my turnips?
A light freeze is not the main threat to turnips. What matters is whether the roots had enough weeks to size up before cold and low light slowed growth. Sow in late summer and count back from the November 13 to 30 window.
What does the hardiness zone tell me about sowing?
Only about winter cold. The USDA map is built on average annual extreme minimum temperature, and Washington spans 4a to 9a. It does not measure season length or soil wetness, which drive turnip timing, so check your county’s calendar as well.