When to Plant Potatoes: Soil Temperature and Zone Chart

Quick facts: when to plant potatoes

Contents

• Quick facts: when to plant potatoes
• What actually decides when you can plant potatoes?
• How to measure soil temperature at planting depth
• Three planting windows: early, main and late
• Zone chart: when to plant potatoes in USDA zones 3–6
• How close to the last frost can you plant potatoes?
• Shoots are up and frost is forecast tonight – what now?
• What signs in the garden tell you it is time?
• Step by step: how to pick your planting day
• Does the variety change the timing?
• How late can you plant potatoes?
• When to plant potatoes in bags, tubs and raised beds
• Common mistakes
• Frequently asked questions

What actually decides when you can plant potatoes?

A seed potato is a tuber, not a seed. It already carries everything it needs as starch and water. What it waits for is a signal from the soil that starting is worth the effort, and that signal is temperature. Below roughly 7 °C (45 °F) almost nothing happens: sprouts do not extend and no roots go out. The tuber just sits there, fully available to everything that lives in cold, wet ground.

The threshold is 7 °C (45 °F) at 10 cm (4 in)

Extension services on both sides of the Atlantic land on the same number. University of Maryland Extension states that seed pieces need a soil temperature of at least 45 °F before planting, and North Dakota State University Extension gives the same figure for a northern-plains climate. Nordic guidance says 7–8 °C – the same threshold in another scale.

The important word is sustained. One sunny March afternoon can lift the top few centimetres over the threshold without saying anything about the soil as a whole. So read the thermometer in the morning, several days in a row.

What happens if you plant into cold soil

The tuber does not freeze at 5 °C (41 °F). Something more insidious happens: nothing. The seed piece sits for two, three, four weeks while cut surfaces stay soft and soil-borne fungi get unlimited time to work. What you get is gaps in the row, weak plants and a harvest well below what it should have been.

From the other side: a potato planted into 10 °C (50 °F) soil emerges in two to three weeks and goes straight into growth. One planted into 5 °C soil may need five weeks, exposed to rot for all of them. You gain no time by planting early into cold ground. You lose it.

Moisture is the other half of the answer

Temperature is what you measure, but soil moisture matters just as much. Wet clay in April is cold and short of oxygen, precisely the environment rot fungi want. The old test still works: soil is ready when it crumbles off the spade instead of sticking together in a slab. In practice a light sandy soil can be planted a week before heavy clay across the road. If you garden on clay, patience is not a virtue but a method.

Three planting windows: early, main and late

There is no single correct date. There are three ways to plant potatoes, each with its own purpose. Choose deliberately and you harvest in three waves instead of all at once in August.

Window 1: early planting under cover

The window for the first new potatoes of the year. You plant two to three weeks ahead of normal, in the warmest and best-drained bed you have, using chitted seed of a very early variety. Then you cover: two layers of horticultural fleece laid loose over the bed, or black plastic put down a fortnight before planting to pre-warm the soil.

Three things to know. Cover buys roughly two weeks, not four. The fleece must lie loose with plenty of slack so shoots can lift it. And vent it as soon as it runs over 25 °C (77 °F) underneath, or the foliage comes up long, pale and floppy.

Window 2: main outdoor planting

The big one, giving both summer eating and storage potatoes. You wait for soil temperature with no cover and plant once the threshold has been passed three mornings running. In USDA zone 6 that is usually late March to mid-April, zone 5 mid to late April, zone 4 early to mid-May, zone 3 the second half of May.

This window is wider than people think. Anywhere in the three weeks after the soil reaches temperature, the exact day barely changes the final yield. Potatoes planted on 5 May and on 15 May differ by about five days at harvest, not ten – the later one comes up into warmer soil and catches up.

In the UK and Ireland the same window is expressed by variety group rather than zone. The Royal Horticultural Society puts first earlies in around late March, second earlies early to mid-April, and maincrops mid-April to early May, later in colder districts.

Window 3: late planting for a fall crop

The least known window, and possibly the most rewarding. Plant an early variety in late June or early July and you get fresh potatoes in September with paper-thin skins and a flavour stored potatoes never have. It works comfortably in zones 5 and 6, with a narrow margin in zone 4, and not at all in zone 3. The arithmetic: a very early variety needs 70–80 days, an early one 80–90. Add ten days because growth slows as day length falls, count back from your average first fall frost, and plant no later than that.

A close relative is the UK trick of planting cold-stored seed potatoes in July or early August into a large container kept bright and frost-free, for new potatoes at Christmas.

Zone chart: when to plant potatoes in USDA zones 3–6

The chart gives averages for a normal year in a cold-climate garden: the northern US, most of Canada, Scotland and the Nordic countries. Frost dates vary enormously inside one zone, so treat the first column as a placeholder for your own local average and let the thermometer make the final call.

Two things stand out. Between zone 6b and zone 3a there are only about nine weeks of difference in planting date, but the usable growing window shrinks from over five months to barely three. That is why variety choice matters more than planting date the further north you garden.

And in zone 3 there is no late window at all. Everything there is about starting as early as the soil allows, with chitted seed and very early varieties, and protecting foliage from June frosts.

How close to the last frost can you plant potatoes?

Two different things get muddled here, which is why the question causes so much confusion. The tuber underground and the foliage above it do not share a frost tolerance.

The tuber is protected, the foliage is not

A seed potato at 10 cm (4 in) is unaffected by a −3 °C (27 °F) night in the air above it. Soil is an excellent insulator and 10 cm is plenty for a cold spring night. That is why NDSU Extension advises planting up to two weeks earlier than the average last frost date as standard practice.

Foliage is another matter. Potato tops are fully frost-tender: at 0 to −1 °C (30–32 °F) at ground level the soft shoots go watery and blacken within a day.

The arithmetic for a planting date

1. Find your local average last frost date. It varies wildly inside one zone – a frost hollow can run three weeks behind a slope five kilometres (three miles) away.
2. Subtract two weeks. That is the earliest you should plant, provided the soil allows it.
3. Check the soil temperature. If the soil has not reached 7 °C (45 °F), the frost date is irrelevant – the soil is in charge and you wait.
4. Allow 2–4 weeks to emergence. Two weeks in warm soil with chitted seed, up to four in cool soil with unchitted seed.
5. Check where emergence lands. If the shoots will be up before your last frost date, plan on covering. That is manageable, not a reason to wait.

A worked example for zone 5a: average last frost around 25 April, so two weeks before is 11 April. The soil reaches 7 °C around 15 April, so the soil, not the frost date, sets the day. Plant on 16 April with chitted seed and shoots appear around 30 April, just after the frost date, with fleece on standby.

Why chitting is a time machine

Chitted seed emerges roughly two weeks earlier than unchitted and matures two to three weeks sooner. That is the whole reason chitting is standard in short-season gardens, and it matters most in zones 3 and 4. The full method – timing, light, temperature and what a good sprout looks like – is in the guide to chitting potatoes.

Shoots are up and frost is forecast tonight – what now?

The classic spring panic, and it has a simple answer. You have three methods and they stack.

Hill soil over the shoots

The most effective move is also the cheapest. Pull soil up over the young shoots so they are buried or nearly so. If the foliage is 10 cm (4 in) high, cover all of it; at 25 cm (10 in), cover the bottom 15 cm (6 in). Shoots push through again within days, and hilling protects more stem so the plant has living tissue left even if the tip is lost.

Cover with fleece or straw

If the foliage is too tall to bury, lay doubled horticultural fleece over the row in the evening and take it off once the sun has warmed things up. Fleece buys 2–4 °C (4–7 °F). Straw, grass clippings, cardboard or an old sheet all work, and the RHS suggests a cloche, hessian or straw overnight for exactly this situation. Avoid plastic directly on the leaves – where it touches, it insulates nothing.

Water in the afternoon

Moist soil stores more heat than dry and releases it overnight. A thorough watering the afternoon before a clear, still frost night lifts ground-level temperature by roughly a degree. Not much – but one degree is often exactly the margin.

If the foliage blackens anyway

Do not dig anything up. A potato that loses its tops almost always pushes new shoots within one to two weeks, because the tuber is an intact energy reserve. The harvest comes later and a little smaller, but it comes. Cut away blackened growth only if it starts to mould, and let the plant restart at its own pace.

What signs in the garden tell you it is time?

Before thermometers, gardeners read plants instead. Those markers still work, because they measure the thing that matters: accumulated warmth in the ground.

The dandelion

The best-documented marker for potatoes is the dandelion. Wisconsin Horticulture at the University of Wisconsin-Madison Extension sums it up in one line: plant potatoes when the first dandelion blooms. It works because dandelions open at about the point where the ground has steadily passed 7–8 °C – exactly the potato's threshold.

Borrowed markers that do not apply

Be careful about lifting markers from one crop to another. The rule about planting corn when oak leaves are the size of a squirrel's ear is for corn, which wants far warmer soil. Lilac in full bloom is the signal for beans, cucumbers and squash. Potatoes sit early in that sequence, at roughly the same point as peas and carrots. Plant them the same week you would grow carrots and you are close.

And the markers that fail

St Patrick's Day and Good Friday are both traditional potato-planting days, and the Old Farmer's Almanac lists them among the folk dates gardeners still use. St Patrick's Day at least is fixed, on 17 March, which suits zones 6 and 7 and is far too early for zone 4. Good Friday wanders between 20 March and 23 April – a four-week swing in soil temperature. Charming traditions, poor agronomy.

Step by step: how to pick your planting day

1. Decide what you want. Early new potatoes, a big main harvest or storage potatoes? That decides which window you aim at and which variety you buy.
2. Look up your local average last frost date. Add your average first fall frost date too if you plan a late planting.
3. Count back to chitting. If you are chitting, start four to six weeks before the planned planting date, in good light at 10–15 °C (50–59 °F).
4. Start measuring soil temperature two weeks before the chart date. Ten centimetres deep, 8 am, same spot every time.
5. Check the moisture. If the soil crumbles in your hand it is ready. If it sticks together in a slab, wait, however good the temperature looks.
6. Plant once you have three mornings above 7 °C (45 °F). Go shallower early in spring, around 8 cm (3 in), because the surface soil is warmest. Go deeper, 13 cm (5 in), for later plantings.
7. Write down the date and the variety. Next year that note is worth more than all the general advice combined.
8. Keep fleece within reach until your last frost date has passed. Emergence happens when it happens, and the weather does not ask first.

If you still need to settle varieties, soil preparation and hilling, the whole season is covered in the guide on how to grow potatoes.

Does the variety change the timing?

Yes, but not the way most people assume. The variety does not change when the soil is warm enough. It changes how many days you need afterwards – and therefore how late you may plant and how early you can dig.

Two conclusions. In zone 3 and cold parts of zone 4, late storage varieties are rarely worth the space: they do not finish, and the tubers have poorly set skins and store badly anyway. The return is in very early and early varieties, planted as soon as the ground can be worked.

In zones 5 and 6 the opposite holds: split the crop. One bed of a very early variety under fleece, one of mid-season or maincrop for storage, and a small late planting in late June for fresh potatoes in September. Three beds, three harvest periods, the same work.

How late can you plant potatoes?

Later than almost anyone expects, if you garden far enough south and choose the right variety.

The rule

Take the variety's days to harvest, add ten days because shortening late-summer days slow growth, and count back from your average first fall frost. For a very early variety at 70–80 days that means roughly 90 days before frost. If your first frost usually lands around 10 October you can plant until about 10 July; if it lands around 20 September, your cut-off is late June. Growers in colder parts of the Midwest plant into June for a fall harvest, using at least 110 days before the average first frost as the rule for a full maincrop.

What a late planting does

They come up fast in warm soil, often in ten days rather than three weeks. The plant then runs into falling day length, which in itself encourages tuber set, but total growth slows and tubers stay smaller. Expect about two thirds of a normal yield. Two details make a large difference: mulch with straw or grass clippings right after planting, because soil above 25 °C (77 °F) in July strongly suppresses tuber formation, and water properly, since a late planting has no spring moisture to live on.

When it really is too late

In zone 3 there is no late planting; the question there is how early you can start. In zone 4 the limit is around mid-June for a very early variety, and that means small potatoes in September rather than a real crop. In zones 5 and 6, July is genuinely workable. One exception worth knowing: seed potatoes bought in spring and kept cool ever since usually plant perfectly well in July, as long as the sprouts are short and firm. Gone soft, shrivelled or covered in long white threads, they are spent.

When to plant potatoes in bags, tubs and raised beds

Growing in a bag, tub or raised bed moves the whole schedule forward – the easiest gain available to anyone who wants early potatoes.

Raised beds

A raised bed of loose, humus-rich soil warms faster than surrounding ground and drains freely, which removes half the risk of planting early. In practice you can plant about a week ahead of open ground in the same zone. Measure anyway, at 10 cm (4 in), inside the bed.

Bags and tubs

Here the gain is larger, because the container can be moved. Plant a 40–60 litre (10–16 US gal) bag three to four weeks before normal outdoor planting, stand it in a greenhouse or against a sunny wall, and carry it under cover when frost threatens. New potatoes in early June are realistic in zones 5 and 6. Fill the bag only a third full at planting and top it up as the foliage grows; the smaller volume also warms faster at the start.

Unheated greenhouses and polytunnels

An unheated greenhouse moves you roughly two zones south. A bag planted under glass in late March in zone 5 gives fresh potatoes in early June. You count backwards exactly as when deciding when to sow tomato seeds: control the climate and you control the calendar.

Common mistakes

• Planting on a date instead of a temperature. St Patrick's Day, Good Friday and the Easter weekend are holidays, not growing signals. Measure the soil.
• Taking the soil reading in the afternoon. A sunny April afternoon shows 10 °C (50 °F) at the surface while planting depth sits at 5 °C (41 °F). Read at 8 am, at 10 cm (4 in).
• Planting into wet, sticky soil. The temperature can be right and the crop still rot, because waterlogged ground has no oxygen. Wait until the soil crumbles.
• Waiting for all frost risk to pass. A tuber at 10 cm (4 in) is not bothered by a night frost. You can plant up to two weeks before your average last frost date.
• Forgetting the fleece once shoots appear. Foliage blackens at 0 to −1 °C (30–32 °F). Keep cover within reach until the last frost date has gone.
• Digging everything up after a frost night. The plant almost always regrows within one to two weeks. The harvest is later, not lost.
• Planting long-season maincrops in zone 3. They do not finish and they store badly. Stay with very early and early varieties in short-season gardens.
• Planting the whole crop on one day. Two or three plantings a fortnight apart give you fresh potatoes all summer instead of one week of glut. It is the same logic as timing how to grow garlic at the other end of the year: the date is half the harvest.

Frequently asked questions

When should you plant potatoes?
Plant when soil at 10 cm (4 in) has held 7 °C (45 °F) for three mornings running, which is typically up to two weeks before your average last frost. In USDA zone 6 that means late March to mid-April, zone 5 mid to late April, zone 4 early to mid-May and zone 3 the second half of May. The thermometer always beats the calendar.

What soil temperature do potatoes need for planting?
At least 7 °C (45 °F) at planting depth, ideally 8 °C (46 °F) and rising. Below that the tuber does not grow at all; it simply sits in cold, wet soil and risks rotting. Measure at 10 cm (4 in) at 8 am rather than in the afternoon or near the surface, and take three readings in a row before you commit.

Can you plant potatoes before the last frost?
Yes. A tuber at 10 cm (4 in) is insulated by the soil and is unaffected by air frost above it. Extension guidance puts normal practice at up to two weeks before the average last frost date, provided soil temperature allows. It is the emerging foliage that is vulnerable, so keep fleece or loose soil for hilling within reach.

Will frost kill potato plants?
It kills the foliage but not the plant. Potato tops blacken at 0 to −1 °C (30–32 °F) at ground level. Hill soil over young shoots before a frost night, or lay doubled fleece over the row in the evening. Even if the tops go black, the plant almost always pushes fresh shoots from the tuber within one to two weeks.

How late can you plant potatoes?
Take the variety's days to harvest, add ten days for shortening days, and count back from your average first fall frost. A very early variety needs about 90 days in total. In zones 5 and 6 that often allows planting into early July, in zone 4 until about mid-June, and in zone 3 there is no late window at all.

Do you need to chit potatoes before planting?
No, but it buys roughly two weeks and gives a more even crop. In zones 3 and 4 chitting is close to essential for the season to be long enough. Start four to six weeks before your planned planting date, in good light at 10–15 °C (50–59 °F), until the sprouts are short, stubby and about 1–1.5 cm (1/2 in) long.

How long do potatoes take to come up after planting?
Two to four weeks. In soil at 10–12 °C (50–54 °F) with chitted seed the shoots often show in a fortnight. In soil just over 7 °C (45 °F) with unchitted seed it can take four weeks. A late planting into genuinely warm June or July soil sometimes emerges in ten days.

Should you plant potatoes on St Patrick's Day or Good Friday?
Only if the soil agrees. St Patrick's Day, 17 March, suits USDA zone 6 and warmer but is weeks too early for zones 3 and 4. Good Friday is worse, because it moves between 20 March and 23 April depending on the year. Use the traditions as a reminder to go and check the thermometer, not as the decision itself.

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