Plant them after the soil warms, not when the calendar says spring. For a Boise-type valley that points to early or mid-May. Higher Idaho ground runs later, sometimes by weeks. Celeriac, kohlrabi, turnips, rutabagas, horseradish and Jerusalem artichokes can all work here, but the state’s range of elevations decides which ones, and when.
When the soil is warm enough in Idaho
The date most gardeners ask about is the last spring frost. For the Boise station (USW00024131), the NOAA NCEI 1991-2020 Climate Normals put it between April 23 and May 7. That is the band where the chance of a 32 F freeze sits between 30 and 70 percent. A frost date is a probability, not a promise, so a late freeze after May 7 is still possible.
Treat that date as a proxy. Celeriac, kohlrabi, turnips and the rest do not respond to air frost alone. They respond to soil, and soil warms after the frost risk ends, not alongside it. Cold ground that is also wet holds seed back and stalls transplants. A transplant that sits chilled for two weeks can bolt or stunt before it ever gets going.
A cheap soil thermometer settles the argument better than any chart. Take the reading in the morning, a few inches down, on several days in a row, and plant when the trend is steady rather than on the first warm afternoon. Handfuls of soil should crumble, not smear. Your University of Idaho Extension office can tell you which soil-temperature thresholds suit each crop in your county.
One warning about the numbers above. Boise sits in a low valley, and most of Idaho is higher and colder, with far shorter seasons. If your garden is in the mountains, on the Palouse, or up on the eastern plateaus, the Boise dates are a best case, not a starting point. Ask a neighbor with a long-running garden when the ground actually becomes workable.
Siting Jerusalem artichokes before you plant them
Choose the spot first, because this is the most regretted planting in a vegetable garden. Once established, Jerusalem artichoke is effectively permanent. Every fragment of tuber left in the soil regrows, and digging a patch out never gets all of it. Give it a corner you can wall off with a mowed strip or a hard edge, away from the raised beds, the asparagus and anything you plan to rotate. Never plant it where you may want a clean bed in five years.
Planting comes after that decision. Set the tubers in spring once the ground is workable and no longer waterlogged, and give them room. They are among the hardiest things in the garden, so soil warmth matters less for them than for celeriac. They also store inulin instead of starch, and many people get digestive discomfort from it. A small first portion is the sensible way to find out how you react.
How long the season lasts here
The same Boise normals give the first fall frost as October 7 to October 21. Count from the end of the spring band to the start of the fall band, and the shortest window is May 7 to October 7, which is 153 days. Count from the early end of one to the late end of the other, April 23 to October 21, and the longest is 181 days. So the frost-free stretch at the Boise station runs from about 153 to 181 days, depending on which years you pick.
That spread of nearly a month matters more than the average. In a good year, it allows a spring sowing of kohlrabi and turnips, a harvest, and a second sowing in late summer for a fall crop. In a short year, the second sowing might get caught by the early end of the fall band before roots size up. Plan the second sowing as a bonus, not as the base of your plan. The succession sowing guide on this site lays out how to stagger sowings so a bad week does not wipe out the whole crop.
Season length also settles the transplant question. Celeriac is the slow one in this group and has the least room for error. Start it indoors, and it gets a head start, but every transplant risks root damage. Before you plant, check the root ball. Look for roots circling the pot, a crushed base, or a pot-bound mat that will not unroll. In a long season, a setback costs a little. In a short season, it can cost the whole crop.
Turnips, rutabagas and radishes sit at the opposite end. Their taproots resent disturbance, so direct sowing usually beats transplanting, and the choice of sowing date does more for you than the choice of seedling. If your elevation means one sowing and one season, put your effort into that sowing date and the first weeks of care.
Gardeners who live far above Boise should expect the true window to be shorter than 153 days, and this page cannot give that figure. Frost-date data for a nearby station, or a conversation with your county extension office, will give you a local number to work from.
Heat and water in Idaho
The brief for this page calls for a count of days above 90 F and the summer rainfall total. This page does not carry verified figures for either, so it will not quote them. What it can say is that Boise is a low valley, and the lower valleys of southern Idaho are known for dry summers where water comes from irrigation rather than rain. Higher ground is cooler, often with a shorter growing window and chilly nights.
In a hot, dry valley garden, the root crops that struggle are the cool-season ones. Kohlrabi can turn woody, and turnips and radishes can grow pungent or tough when heat arrives early. The fixes are practical: shade at the root with mulch, steady moisture rather than a cycle of drought and flood (which splits roots and invites rot), and sowing dates that bring maturity into cooler weeks. Where summers run cool and damp instead, the picture reverses. Expect slow ripening, mildew and root disease, and look for types that tolerate wet soil.
Two facts matter most for this group. The first is season length, which says whether celeriac, the long one, can finish at all. The second is the winter low. The coldest-month mean daily minimum at the Boise reference station is 25.4 F. That is an average of nightly lows, not a record, so real cold snaps go deeper. The USDA Plant Hardiness Zone Map (2023, 1991-2020 normals) spans zones 3b to 7b across the state, which is a huge spread. Your own zone may sit far from Boise’s.
What that decides is who stays in the ground and who comes out. Horseradish and Jerusalem artichokes are perennial and cold-hardy, and they can stay under mulch through winter in most of the state. Hardy roots can be left under a deep mulch in milder valley gardens, where soil freezing is shallow. In colder parts of the range, lift them before the soil freezes hard and store them. Celeriac does not take a hard freeze well and is best lifted before one.
Storage touches food safety, so inspect every root before you keep it. Look for soft patches, weeping spots or mold at the crown. Any of those ends the question for that root, and a sound look on the outside is not proof that the inside is fine. This page will not give a keeping time, because conditions vary too much. Check stored roots often and discard anything that has turned.
Horseradish needs a second warning when you prepare it. Grating a fresh root releases compounds that burn the eyes and airways. Work with the window open or a fan running, keep your face well away from the bowl, and lift the lid of a food processor slowly. This is a real irritant, not a quirk.
Which these root crops suit Idaho
The choice comes down to type, because this page cannot source locally trialled varieties and will not invent them. Gardeners spot a made-up variety name instantly, and a wrong recommendation costs a season. The right source is the University of Idaho Extension, whose county offices know which varieties have been tried in your elevation and soil.
Reasoning from Idaho’s facts, a short season favors the earliest-maturing types and quick crops like kohlrabi and turnips. A hot, dry valley favors types that tolerate heat and gain from airflow, mulch and shade at the root. Cool and damp conditions make disease resistance the first criterion on a seed packet or in a catalog. The table sets out how that reasoning plays out by crop type.
| Crop type | Pressure in Idaho | What to look for |
|---|---|---|
| Celeriac (long season) | Short seasons at elevation; early fall frost | Earliest-maturing types; transplants with a healthy root ball |
| Kohlrabi (quick) | Heat turning it woody | Types that hold quality; timed to cooler weeks |
| Turnip and rutabaga (brassicas) | Heat in valleys; disease in cool, damp spots | Early types for short seasons; disease resistance where damp |
| Horseradish (perennial) | Winter cold at high elevation | Hardy stock; a contained, permanent site |
| Jerusalem artichoke (perennial) | Spread, not cold | A walled-off site you will never need for anything else |
Turnips and rutabagas are brassicas. They share their pests and diseases with cabbage, kale and mustard. A gardener who rotates a turnip bed into a cabbage bed has rotated nothing, because clubroot, flea beetles and the rest are waiting there. Use the crop rotation guide on this site to keep brassicas off ground that has just grown any of their relatives, and group kohlrabi with them too.
Gardeners comparing northern conditions can look at how the same crops fare in Growing Unusual Root Crops in South Dakota and Growing Unusual Root Crops in Ontario, which deal with colder winters and different rainfall. For the crops themselves, Celeriac, Kohlrabi, Horseradish and Jerusalem Artichokes covers growing habits in more depth.
Common questions
Is the Boise frost date right for my town?
Only if you garden in a low valley near Boise. The April 23 to May 7 band comes from the Boise station, and most of Idaho is higher and colder. Check a nearby station or ask your county extension office.
Can I get a second sowing in?
At the Boise station, the window runs from about 153 to 181 days between the frost bands, which can leave room for a late-summer sowing of quick crops. At higher elevations, plan on one sowing and treat a second as a bonus.
Can I remove Jerusalem artichokes if I change my mind?
It is very difficult, because every tuber fragment left behind regrows. Decide on the site before planting, and choose a spot you can wall off permanently.
Where do I find varieties that suit my area?
Your local University of Idaho Extension office is the place to start. Its county staff know what has been trialled at your elevation, and that local knowledge counts for more than any general list.