Growing Unusual Root Crops in Washington

In western Washington, the answer is yes, and the season is long enough to try all four of the odd ones: celeriac, kohlrabi, horseradish and Jerusalem artichokes. The catch is timing. The last freeze arrives early, but cold, wet soil keeps these crops waiting well after it. East of the Cascades, a much shorter season changes the plan entirely.

When the soil is warm enough in Washington

The date most gardeners ask about is the last spring freeze. For the Seattle reference station (NOAA station USW00024233, 1991-2020 Climate Normals), the 32°F freeze probabilities in the 30-70% band put it between February 27 and March 18. That is a probability, not a promise, and a late freeze can still land outside the band.

Why the frost date is only a proxy

Those dates describe western Washington, which is mild and maritime. They do not describe the state. Beyond the Cascades the land is high desert with a far shorter season, so nobody in Yakima, Spokane or the Tri-Cities should plant by a Seattle calendar.

Even on the west side, a February or March frost date is a poor planting signal. These root crops wait for warm soil, and soil warms after the frost risk ends, not with it. Cold, wet ground and low spring light hold crops back well past the last freeze. Seed sown into a saturated bed in early March often sits and rots, or sprouts and stalls. A cheap soil thermometer pushed a few inches down tells you more than any normal. For the soil temperatures each crop wants, ask Washington State University Extension.

The wait also helps. Waiting until the bed drains and warms means seedlings that grow instead of sulk. Celeriac and kohlrabi started indoors or bought as transplants can go out once the ground has caught up, with a row cover handy for a cold snap.

Site the permanent one first

Jerusalem artichokes need a warning before any planting advice. Once planted, they are effectively permanent. Every fragment of tuber left in the ground regrows, and digging only scatters more pieces. Of everything in a vegetable garden, this is the planting people regret most. Give it a corner you are happy to surrender: a fenced strip, a bed bordered by lawn you mow, or a large container you can watch.

A plain fact about the vegetable: it stores inulin instead of starch, and many people get digestive discomfort from it. Cooking and frost are not guaranteed to change that. Eat a small first portion and see how you do.

Horseradish behaves similarly in the ground, with root pieces that resprout, so it deserves the same boundary thinking. Celeriac, kohlrabi and the turnip-type roots are annuals or biennials grown as annuals. They leave when you pull them.

How long the season lasts here

The same station gives a first fall freeze between November 13 and November 30, again from the 30-70% probability band. Set the latest spring date against the earliest fall date, March 18 to November 13, and you get about 240 days. Use the earliest spring date and the latest fall date, February 27 to November 30, and it stretches to about 276 days. That is the frost-free span in western Washington, and it is generous by national standards.

Frost-free is not the same as growing. Cold soil in March and short, dim days in November mean the productive stretch is shorter than the arithmetic says. Think of the span as an outer boundary. What it allows is the useful part.

A window this long allows a second sowing. A quick root like kohlrabi or a turnip can go in during spring, and a later sowing in summer can mature into the cool, moist days of autumn, which many of these crops prefer anyway. The Celeriac, Kohlrabi, Horseradish and Jerusalem Artichokes overview covers how each one grows. The succession sowing guide on this site shows how to space those later sowings.

The long window also makes transplanting celeriac worthwhile. Celeriac is the slowest of the group, and a transplant gets a head start indoors while the garden is still too cold. Celeriac also dislikes root disturbance, so handle transplants gently and keep the roots intact when you set them out.

East of the Cascades, none of this applies. This page doesn’t carry a frost window for the eastern side, so I won’t invent one. What a far shorter season does is simple. The first sowing becomes the only sowing, and a transplant is worth its fuss only if it lets a slow crop finish before fall. For a dated plan in Wenatchee or Walla Walla, the local extension office has the data this page does not.

Heat and water in Washington

I am not quoting a count of days above 90°F or a summer rainfall total here. The verified climate figures behind this page do not include either, and a number made up to fill the gap is worse than none. In western Washington, summers are generally mild and dry between wet springs and autumns. That shapes the crops more than heat does.

Where heat stress is modest, the troubles are different. A cool, damp spring or fall brings slow ripening, mildew and rot. Dense foliage that never dries out invites disease. Space plants so air moves through them, and water at the base in the morning rather than overhead in the evening. A dry summer on the west side still means irrigation, since rain is scarce just when roots are swelling. East of the mountains, expect real heat and dryness, and that means shade, mulch and steady water for the roots.

Two facts govern this group. The first is season length, which tells you whether celeriac, the long one, can finish. On the west side the roughly 240 to 276 frost-free days say yes, as long as you start it early enough and let the soil catch up. On the east side, it is a gamble that needs a head start indoors and early transplanting.

The second is the winter low, which decides whether the hardy roots stay in the ground under mulch or come out before the soil freezes. At the Seattle reference station, the mean daily minimum of the coldest month is 37.1°F (NOAA 1991-2020 Climate Normals). That is above freezing, and it means west-side lowland winters are mild enough that horseradish, Jerusalem artichokes and hardy roots can usually stay put under a layer of mulch. It does not mean a freeze can never come. It is an average of minimums, not a record low.

The USDA Plant Hardiness Zone Map 2023 gives Washington a range of 4a to 9a. That range spans the whole state, from cold interior valleys to the mild coast, and it does not describe your yard. East of the Cascades, with winters far colder than Seattle’s, lifting and storing the less hardy roots before the ground freezes is the careful choice.

Stored roots call for inspection, not trust. Check each one for a soft patch, a weeping spot or a moldy crown. Any one of those ends the question, and the root should be discarded. A root that merely looks sound is not proven safe. I won’t promise a keeping time, because it depends on the root, the storage conditions and the condition it started in.

Which these root crops suit Washington

The choice comes down to type, and here it follows from this territory’s facts. A short season favors the earliest types, which matters east of the Cascades. A hot, dry one favors types given airflow and shade at the root. A humid, cool one, which describes much of the west side in spring and fall, makes disease resistance the first criterion.

Turnip, rutabaga and kohlrabi are brassicas. They share their pests and diseases with cabbage, kale and mustard, so a gardener who rotates them into a cabbage bed has rotated nothing. Keep them in a separate rotation from the rest of the brassica family, and see the crop rotation page in this series for how.

When it comes to preparing horseradish, grating a fresh root releases compounds that genuinely burn the eyes and airways. Grate it with good ventilation, such as an open window or outdoors, and keep your face and eyes well away from the bowl. This is not a joke or a rite of passage.

This page names no varieties, and that is deliberate. The only recommendation that means anything is one trialed where you garden, and I don’t have those trial results. A variety that thrives in a cool, damp valley near the Sound may sulk in a dry basin east of the Cascades. Washington State University Extension and its county offices can say which types have been tested for your conditions. For comparison, Growing Unusual Root Crops in Newfoundland and Labrador shows what a cold, short season does to the same list, and Growing Unusual Root Crops in Mississippi shows the opposite problem of long, hot, humid summers.

Common questions

Can I plant on the last frost date?

Not for these crops. The last freeze at the Seattle reference station falls between February 27 and March 18, but cold, wet soil and low light hold root crops back well past it. Wait for the soil to drain and warm, and let a thermometer tell you.

Do the Seattle dates apply across Washington?

No. They describe western Washington, which is mild and maritime. Eastern Washington is high desert with a far shorter season, and it needs its own dates from your local extension office.

Can I leave roots in the ground over winter?

On the west side, hardy roots and perennials like horseradish and Jerusalem artichokes can usually stay under mulch, since the coldest month’s mean minimum at Seattle is 37.1°F. In colder parts of the state, lift tender roots before the soil freezes. Inspect anything you store, and discard a root with a soft patch, a weeping spot or a moldy crown.

Are Jerusalem artichokes safe to eat?

They are a standard vegetable, but they store inulin instead of starch, and many people get digestive discomfort from it. Start with a small portion to find out how you react. Choose the planting spot carefully, too, because any tuber left behind will regrow.