Plan on late April to early May as the earliest frost-free window on the Maine coast, then wait longer for the soil. Celeriac, kohlrabi, horseradish and Jerusalem artichokes can all be grown here. Portland, the reference station, gets about 23 to 26 weeks between freezes, and it is the mild end of the state. Inland and northern gardeners should push every date later.
When the soil is warm enough in Maine
At Portland, the NOAA NCEI 1991-2020 Climate Normals put the last spring 32F freeze between April 22 and May 1 (the 30-70% probability band). A frost date is a probability, not a promise, so a cold night after May 1 is always possible. That is also the coastal number. Inland and northern Maine run substantially later in spring, sometimes by weeks.
Frost is only a proxy. Celeriac, kohlrabi, horseradish and Jerusalem artichokes wait for warm soil, and soil warms after the frost risk ends, not along with it. Cold, wet ground makes seed sit and rot, and chilled young celeriac or kohlrabi plants can run to seed instead of swelling. A cheap soil thermometer settles the question better than the calendar. So does a squeeze test: a handful that crumbles is ready, and one that smears into a ribbon is not. Your University of Maine Cooperative Extension office can give you the soil temperatures that suit each crop locally, and this page deliberately does not guess at them.
Jerusalem artichokes and horseradish: site them first
Settle the location before you buy anything. Jerusalem artichoke is effectively permanent once planted. Every fragment of tuber left in the ground regrows, and digging a patch out rarely finishes the job. It is the most regretted planting in a vegetable garden. Give it a corner you are willing to surrender for good, away from any bed you rotate, and keep the edge mown or bordered so it cannot wander into the lawn.
With that settled, planting is simple. Set tubers in the ground once the soil has warmed, and harvest after the tops die back in fall. Plan on a small first portion at the table. Jerusalem artichokes store inulin rather than starch, and many people get digestive discomfort from it, so a little is the sensible way to find out how you react.
Horseradish behaves in a similar way. Root pieces left behind will resprout, so give it a fixed spot too. Preparation needs care. Grating a fresh root releases compounds that burn the eyes and airways. Open a window or run a fan, and keep your face well away from the bowl, especially when you lift the lid on freshly grated root.
Celeriac and kohlrabi: timing the start
Celeriac is the long-season crop in this group, and it is usually started indoors rather than sown in the ground. Kohlrabi is quicker and can go in as soon as the soil is workable and no longer cold. Its young plants tolerate cool conditions better than celeriac’s do. Harden off any transplants before they go out, and cover them if a late cold night threatens.
How long the season lasts here
Both frost dates come from the same Portland normals. The first fall 32F freeze falls between October 9 and October 21. Pair the earliest last-spring date with the earliest first-fall date (April 22 to October 9) and you get 170 days. Pair the latest ones (May 1 to October 21) and you get 173. The extremes are 161 days (May 1 to October 9) and 182 days (April 22 to October 21). So the frost-free gap on the coast runs from about 23 to 26 weeks.
That is a gap between freezes, not a promise that every crop finishes in it. It also describes the mildest corner of the state. Move inland or north and the number shrinks from both ends. For a sense of how a shorter window changes the planning, Growing Unusual Root Crops in South Dakota covers another cold-winter state with its own constraints.
What the window allows is the useful part. At the long end, on the coast, there is room for a second sowing. Kohlrabi is the obvious candidate: sow it early, then again in summer for a fall harvest, because its cool-weather habits suit a September and October finish. The site’s succession sowing guide lays out how to stagger those sowings so they don’t all ripen the same week.
At the short end, the first sowing is the only one, and every week counts. That changes the transplant decision. Celeriac started indoors buys weeks it cannot otherwise find, so the risk to its roots at transplanting is worth taking. Handle the plants by the leaves and keep the root ball intact. For kohlrabi, the weeks gained rarely justify the disturbance, so sowing direct is usually the simpler call when the window is tight. Neither crop forgives a rushed planting into cold soil, which brings the calendar back to the thermometer.
Fall matters as much as spring. A hard freeze ends the harvest for anything that must still be growing. Plan the end of the season from your own nearest frost data, not from Portland’s numbers, if you garden anywhere other than the coast.
Heat and water in Maine
This page does not carry a verified count of days above 90 F or a summer rainfall figure for Maine, so it will not quote one. What the sourced facts do show is a state with a mild coastal reference station and a cold winter, not a hot-summer problem. Gardeners worried about heat stress on roots are likely asking a question that matters more in warmer states. For that contrast, see Growing Unusual Root Crops in South Carolina. For your own county’s summer heat and rainfall, check the NOAA station nearest you.
What a cool, damp summer brings instead is the real list of worries: mildew, slow ripening, and leaf and root diseases that thrive on wet foliage. Space plants so air moves between them, water at the base in the morning, and mulch to keep soil from splashing onto leaves. Kohlrabi, turnip and rutabaga are brassicas. They share their troubles with cabbage, kale and mustard, so rotating them into a cabbage bed rotates nothing. Follow the rotation page on this site and keep that family moving as a block.
Two facts govern the rest. The first is season length, which says whether celeriac, the long one, can finish at all. On the coast the 23 to 26 weeks between freezes make it plausible. Inland, it depends on how much the window shrinks, and an early indoor start matters more.
The second is the winter low. The mean daily minimum of the coldest month at Portland is 15.6 F. That is the average minimum, not the record low and not the monthly average. The USDA Plant Hardiness Zone Map (2023) spans 3b to 6a across the state, so parts of Maine run much colder than Portland. Horseradish and Jerusalem artichokes are perennials that are commonly left in place under mulch. Celeriac and kohlrabi are not meant to winter in the bed, so lift them before the soil freezes.
Storage touches food safety, so inspect every root. Look and feel for a soft patch, a weeping spot or a moldy crown. Any one of them ends the question, and a root that merely looks sound is not proof that it is safe. Don’t trust a keeping time from anyone, including this page, because it depends on your conditions. For the crops side by side, read Celeriac, Kohlrabi, Horseradish and Jerusalem Artichokes.
Which these root crops suit Maine
Choose by type, and let Maine’s own facts do the reasoning. A shorter season favors the earliest-maturing types. A cool, damp summer makes disease resistance the first criterion on any seed packet or plant tag. A cold winter favors the perennials that can stay in the ground under mulch.
| Type | What Maine’s facts favor | Watch for |
|---|---|---|
| Celeriac | Earliest-maturing types, started indoors, especially inland | Long season, leaf diseases in damp weather |
| Kohlrabi | Early types sown direct, with a second fall sowing on the coast | Brassica pests and diseases, rotation |
| Horseradish | Hardy perennial, wintered in place under mulch | Fixed site, irritant vapors when grating |
| Jerusalem artichokes | Cold-tolerant, left in the ground | Permanent once planted, digestive discomfort |
| Turnip and rutabaga | Cool-season crops that suit a mild summer | Brassicas, so rotate with cabbage and kale |
Notice what is missing: variety names. This page refuses to give any, and the refusal is the honest answer. The only recommendation that means anything is a variety trialled where you garden, and Maine runs from a mild coast to a cold interior. Any name offered here would be a guess. Ask University of Maine Cooperative Extension which varieties have been tested in your county, and for disease problems it has seen locally. Then match what it names to the type that suits your site.
Common questions
When can I plant celeriac outdoors in Maine?
Wait until the frost risk has passed and the soil is warm. At Portland the last spring freeze falls between April 22 and May 1, and inland dates run later. That is a probability, and soil lags behind air temperature, so test the soil before planting out.
Can I leave roots in the ground over winter?
Horseradish and Jerusalem artichokes are perennials that can stay in the ground under mulch. Celeriac and kohlrabi should be lifted before the soil freezes. Portland’s coldest-month mean daily minimum is 15.6 F, and colder parts of the state fall well below it.
Do turnips and rutabagas need to rotate away from cabbage?
Yes. Turnips, rutabagas and kohlrabi are brassicas, and they share pests and diseases with cabbage, kale and mustard. Rotate the whole family together, and see the rotation page on this site.
Which varieties should I grow?
This page names none, because it cannot source varieties trialled in Maine. University of Maine Cooperative Extension can tell you which ones perform in your county, and which diseases are worth choosing resistance against.