When to Plant Unusual Roots in Vermont

In Vermont, sow hardy unusual roots such as salsify, scorzonera and burdock as soon as the soil can be worked, and hold tender ones until the freeze risk has passed. At Burlington, that means waiting until after the April 30 to May 10 band. Those dates describe the mildest corner of the state, so most gardens should run later. Below is how the calendar works, where it breaks, and how to correct it for your own ground.

The short answer for Vermont

At Burlington, the last spring 32F freeze falls between April 30 and May 10, and the first fall one between October 3 and October 13. Sow hardy roots at the front of the spring band, set tender ones out after the end of it, and plan every harvest around the fall band.

What the dates are a probability of

Both ranges come from the NOAA NCEI 1991-2020 Climate Normals for the Burlington station (USW00014742). They are the 30-70% band of 32F freeze probabilities. They are not guarantees, and they are not a date when frost “ends.” Inside the band, the chance of a 32F freeze is neither remote nor certain. Outside it, the odds lean clearly one way.

A freeze after May 10 is therefore possible, just less likely than not. The same logic runs in reverse in autumn, where a freeze before October 3 can still happen. Gardeners who treat the band as a hard fence tend to get burned by exactly those off-years.

Between the two bands, Burlington has somewhere around five months. That is the best-case budget in the state, and it is the number every slow root crop has to fit inside.

Sowing windows by type

Timing depends less on the individual plant than on how it handles cold. Three groups cover most of the unusual roots a Vermont gardener will meet.

Which of these deserve bed space is a separate question, covered in The Best Unusual Roots to Grow in Vermont. For the wider picture of how these crops fit a northern garden, start with Growing Unusual Root Crops in Vermont.

What will not work here

The crops that fail in Vermont fail on arithmetic. Anything that needs more than the season on offer is a gamble, and the gamble gets worse the further you get from Lake Champlain.

Anything that needs a long, frost-free season

Yacon and oca are the clearest cases. Plant them out after May 10 and the first fall freeze band opens on October 3, which leaves under five months even at Burlington. Oca tends to bulk up its tubers as days shorten in fall, and in Vermont that is just when frost arrives to cut the plant down. Expect small harvests, and expect to throw row cover over them in early October to buy a few extra weeks. In the hills, where the season is substantially shorter than Burlington’s, these crops are closer to a novelty than a harvest.

Anything that must overwinter in the ground

The coldest month at Burlington has a mean daily minimum of 12.9F. That is an average of the nightly lows, not the record, so individual nights run well below it. Tender tubers and storage roots left in the bed do not survive that. They freeze, then rot. Dig them before the fall band and store them indoors.

The hardiness zone adds context. The USDA system rates Vermont at 3b to 5b, and it is built on one variable only: the average annual extreme minimum temperature. It tells you whether a perennial root such as horseradish or Jerusalem artichoke will live through winter. It tells you nothing about when to sow.

Sowing into cold, saturated ground

Vermont’s April soil is often wet from snowmelt. Pushing slow-germinating seed into cold, waterlogged clay on the first warm weekend is a reliable way to lose the row. The seed sits, rots or crusts over, and the gardener resows in late May anyway. If a handful of soil squeezed in your fist stays in a sodden lump, wait.

Borrowing Burlington’s dates

Burlington is the reference city, and it is also the easy case. The Green Mountains and the Northeast Kingdom run substantially shorter. A gardener in either place who plants on Burlington’s schedule will be sowing into a season that has not caught up, and harvesting into one that ends early.

One thing this page does not have is summer rainfall or humidity figures for the state. It makes no claim about watering schedules or spacing for wet summers, and you should treat any advice that does with caution.

What changes across Vermont

There is no single answer for the state. The USDA Plant Hardiness Zone Map (2023, built on 1991-2020 normals) places Vermont across zones 3b to 5b. That is a spread of two full zones inside one set of borders, and it is the reason a Burlington calendar does not transfer to a Northeast Kingdom hillside.

Burlington sits at the warm end. Lake Champlain moderates both ends of the season, which is why its frost bands are the friendliest in the state. Move east into the Green Mountains or north into the Northeast Kingdom and the spring band moves later, the fall band moves earlier, and the winter minimum drops.

For a gardener in the north or at altitude, the practical shifts are straightforward:

• Sow hardy roots later than the Burlington window, and wait until the soil has actually drained.
• Start tender, long-season roots indoors, or skip them in favor of hardier types.
• Plan harvests ahead of the earlier fall frost, and keep row cover on hand for the first cold night.
• Favor perennials rated for the cold end of the range.

Elevation is only part of it. Cold air pools in valley bottoms and low corners of a yard, so two gardens a few miles apart, or even two beds on one lot, can frost on different nights. A hardiness zone does not capture that, because it measures winter extremes and not the dates of the last spring freeze.

This page cannot tell you which zone you are in. Look up your ZIP code on the USDA Plant Hardiness Zone Map and read the result for your own address.

Where to check this for your own county

Your state extension service is the next stop. In Vermont that is University of Vermont Extension, part of the land-grant system. Extension staff work with local growing conditions all year, and they are the people who know what a county’s frost pockets and clay soils do to a planting date.

Two things are worth looking for there:

• The local planting calendar for your county or region, with frost dates that match your area and not Burlington’s.
• Varieties trialled nearby, meaning cultivars that have been grown and evaluated under similar conditions to yours.

Be prepared to look up calendars crop by crop. Extension calendars are usually written for a single crop or a single species, not for a whole category such as “unusual roots.” This page does not have a Vermont calendar for these specific crops, so check whether the crop you want is covered, and if it is not, ask.

Many extension offices also run a gardening helpline or a Master Gardener program. Ask whether yours does, and bring your approximate elevation and soil type to the conversation.

A county recommendation beats a state one. This page is the state one. It is built on a single reference station at the mild end of a state that spans two hardiness zones, so use it as a starting frame and let local guidance override it.

Checking this against your own garden

Every date on this page is an average from somewhere else. What your own ground is doing this week outranks all of them, and three observations will tell you more than any chart.

Soil warmth by feel. Push a bare hand a few inches into the bed in the morning. If the soil feels distinctly cold, slow-germinating seed will sit and sulk. If it feels merely cool, hardy roots can go in. Compare a sunny, sheltered bed with a shaded or low one, and you will learn how much your own yard lags the regional picture.

The first flush of male flowers. If you grow squash or cucumbers, the male blooms open before the female ones. The date they appear, counted from when you planted out, tells you how warm your site runs in early summer and whether tender transplants like yacon or oca are likely to keep pace.

The first true leaves. After a seedling’s initial seed leaves comes the first set of true leaves. The number of days between sowing and that point is a direct readout of soil warmth in your spot. A slow arrival means a cold pocket, and next year’s sowing date should move later.

Write all three down, with dates, for a single season. By the second year you will have a calendar that belongs to your yard, which no state-level page can supply.

Common questions

Can I sow unusual roots in April in Vermont?

Hardy ones, yes, if the soil is workable and not waterlogged. At Burlington the last spring 32F freeze falls between April 30 and May 10, so late April sowing of salsify or burdock is reasonable. Tender roots should wait until after May 10, and in colder parts of the state, later still.

Is the Burlington date the right one for my garden?

Probably not exactly. Burlington is the mildest corner of Vermont, and the Green Mountains and Northeast Kingdom run substantially shorter. Treat the Burlington window as the earliest sensible date and shift later if you garden at elevation or in the north.

When should I harvest ahead of fall frost?

The fall band at Burlington runs from October 3 to October 13 for a 32F freeze. Tender roots such as oca and yacon should be dug before then, or protected with row cover. Hardy roots can generally stay in the ground longer, though in colder parts of the state the band arrives sooner.

Will roots left in the ground survive a Vermont winter?

Hardy perennials generally do. The coldest month at Burlington has a mean daily minimum of 12.9F, and the state’s USDA zones run from 3b to 5b. Tender tubers do not make it through, and the colder end of that range is harder still. Check the hardiness rating of any perennial root against your own zone before leaving it in the bed.