In western Washington, sow hardy unusual roots such as salsify, scorzonera and parsnip in mid-spring, once the soil has drained and warmed, not on the last frost date. Frost in the Seattle area ends somewhere between February 27 and March 18, but cold, wet ground holds seeds back for weeks after that. Eastern Washington runs on a different and shorter calendar.
The short answer for Washington
The last spring frost at the Seattle reference station falls between February 27 and March 18. The first fall frost arrives between November 13 and November 30. Both ranges come from the NOAA NCEI 1991-2020 Climate Normals for station USW00024233, and both are a probability, not a promise. They mark the dates where the chance of a 32°F freeze sits in the 30-70% band. On the early date, a freeze after it is still a coin flip. On the late date, one before it is just as likely.
That is a frost calendar, not a sowing calendar, and the two diverge sharply on this side of the Cascades. Spring here is cool, wet and dim. Soil can stay cold and waterlogged well past the last freeze, and low light slows everything that germinates. A parsnip or salsify seed dropped into sodden ground in early March does not get a head start. It gets a long wait, and sometimes rot.
So treat the end of the frost window as the earliest the question even opens. The real trigger is a bed that crumbles in your hand instead of smearing, and a few mild days in a row. For most gardeners that lands in mid-spring, though it shifts year to year and from one bed to the next.
Here is how the main types of unusual roots sit against those dates.
| Type | Examples | Timing against the frost window |
|---|---|---|
| Hardy, slow to germinate | Salsify, scorzonera, parsnip | Direct sow in mid-spring once soil drains; frost is not the limit |
| Hardy, long-season | Celeriac | Start indoors ahead of time, transplant after the frost window closes |
| Tender, warmth-hungry | Sweet potato, yacon | Plant only after the window has clearly passed and soil is warm |
The back end works in your favor. With a first fall frost no earlier than November 13, the west side gives slow roots a long tail to finish. The mean daily minimum of the coldest month at the Seattle reference station is 37.1°F, above freezing. That figure is an average of nightly lows, not the record cold, but it explains why hardy roots here can often wait in the ground well into winter. For a deeper look at which roots earn their bed space, see The Best Unusual Roots to Grow in Washington.
What will not work here
Sowing on the frost date is the first casualty. The last-freeze range of February 27 to March 18 looks like an invitation, and it is a trap. Slow germinators like salsify and scorzonera, already stubborn, will sit in cold muck and either rot or emerge ragged and uneven. Waiting three or four weeks usually costs nothing, because seed sown into warmer soil catches up fast.
Tender, heat-loving roots are the second casualty on the maritime side. Sweet potato and yacon want warm soil and warm nights for months. Western Washington can be mild in the sense that winters rarely bite, but mild is not the same as hot. A late frost date and a 37.1°F coldest-month minimum say the winters are gentle. They say nothing about summer heat, and these crops live or die on it. Without a warm microclimate or some cover early on, expect small, disappointing harvests.
Wet autumn ground defeats close spacing and lazy harvesting. Because the first fall frost does not arrive until November 13 at the earliest, there is a long stretch of damp, mild weather when roots left in crowded rows can rot or draw slugs. Space them generously, grow in raised or well-drained beds if your soil is heavy clay, and lift anything you do not trust the bed to hold.
Eastern Washington breaks different rules. Beyond the Cascades the climate is high desert, and the season is far shorter than the Seattle figures above. This page does not have frost dates for any station east of the mountains, so it will not borrow the Seattle window for them. What can be said is that the slowest crops, the ones that need a long stretch from sowing to a fat root, are the first to fail where the season is short. The dry summer there also punishes anyone who waters once a week. Roots that go from bone dry to soaked tend to split, fork or turn woody, so steady, regular moisture matters more than a big weekly soak.
Altitude is the same story in miniature. A bed in the foothills or in the mountains has a shorter run between frosts than the Seattle range, and dates measured at sea level do not travel up the slope with you.
What changes across Washington
Washington does not have one garden climate. The USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 normals) places the state anywhere from 4a to 9a. That is a spread of five full zones, so there is no single answer to when or what to plant. Anyone who tells you otherwise is describing their own corner.
Keep in mind what that map measures: one variable, the average annual extreme minimum temperature. It tells you whether a perennial root like horseradish or Jerusalem artichoke survives the winter. It does not tell you how long the growing season is, how warm the summer runs, or how fast spring soil thaws. Two places in the same zone can have very different planting calendars.
The dates on this page are for the Seattle reference station, which is the dated anchor. If you garden in the north, in the mountains or at elevation, shift your thinking in a consistent direction. Sow later in spring, expect frost to come sooner in fall, and favor roots with a shorter run from seed to harvest over the slowest ones. Consider starting celeriac-type crops indoors to buy back weeks. In colder spots, hardy roots may need mulch to be held in the ground, while on the mild west side they often manage without.
To find where you sit, use the USDA map and look up your own location. Do not guess from a neighbor’s advice or from a zone quoted in a plant catalog. If you are new to these crops, the overview in Growing Unusual Root Crops in Washington is a good place to match crops to conditions before you worry about exact dates.
Where to check this for your own county
The best next stop is Washington State University Extension, the state’s land-grant extension service. It has county offices across Washington, and that local knowledge is the part this page cannot supply.
When you get there, look for two things. First, a planting calendar written for your county or region, ideally one that lists sowing windows for root crops, not just the usual carrots and beets. Second, any variety trials or recommended-variety lists from your area. A root that thrives on the wet west side may stall in a dry eastern valley, and trial results from nearby ground tell you more than any catalog description.
This page does not have a WSU calendar specific to unusual roots, so it cannot say whether one exists for your county. If you cannot find one online, call or write the county office and ask what they recommend for the crop you have in mind. Master Gardener programs attached to extension offices often field exactly these questions.
A county recommendation beats a state one, every time. This page is the state one: a single reference station on the mild western side, a hardiness range that spans five zones, and general crop types in place of local trial data. Use it to get oriented, then let the county-level answer overrule anything here that does not match what your neighbors are actually harvesting.
Checking this against your own garden
Your own observations outrank every date above. Three are worth watching, and none needs equipment.
- Soil warmth by feel. Push your bare hand into the bed. If it feels cold and the soil clumps into a sticky ball, wait. A bed that warms and drains first, usually one that is sunny or sloped or raised, tells you where your earliest sowings belong. A bed that stays cold is your late one.
- The first flush of male flowers. If you grow squash anywhere on the property, the first male blossoms signal real summer warmth. Where they show up early, tender roots like sweet potato have a better chance. Where they lag, plan for cover or smaller expectations.
- The first true leaves. Seedlings first show seed leaves, then the leaves that look like the adult plant. Time how long that takes after sowing. Slow seedlings point to a cool or shaded spot, and fast ones point to a warm pocket.
Write down what you see for a season or two. A notebook of dates beats any generalized calendar, this one included.
Common questions
Can I sow unusual roots at the last frost date in western Washington?
You can, but you probably should not. The last frost falls between February 27 and March 18 at the Seattle reference station, yet spring soil is often cold and wet well after that. Wait for the bed to drain and warm.
Do these dates apply to eastern Washington?
No. They describe western Washington, which is mild and maritime. Eastern Washington is high desert with a far shorter season, and this page has no frost dates for it. Check local sources for that side of the state.
Can I leave roots in the ground over winter?
On the west side, hardy roots often hold well, since the coldest-month mean minimum at the Seattle reference station is 37.1°F. That is an average, not an extreme, and colder or higher areas may need mulch or earlier harvest.
Are the frost dates a guarantee?
No. They mark the band where the chance of a 32°F freeze runs from 30% to 70%. A freeze after the early date or before the late date remains possible.