When to Sow Tomato Seeds: A Zone-by-Zone Timetable

Key facts at a glance

Contents

• Key facts at a glance
• When should you sow tomato seeds?
• Counting back from your last frost date
• Sowing chart for USDA zones 3 to 6
• Why your hardiness zone is not your frost date
• Greenhouse, tunnel or open ground — how much earlier can you sow?
• Does the variety change your sowing date?
• Light, not the calendar, decides how good your seedlings are
• How to sow tomato seeds step by step
• After germination: potting on, temperature and hardening off
• Is it too late to sow tomatoes in May?
• Common mistakes
• Frequently asked questions

When should you sow tomato seeds?

Six weeks before you plant out. That is the working answer for almost every cold-climate gardener, and it comes straight from how a tomato seedling develops rather than from tradition. Iowa State University Extension puts the figure at five to six weeks before the intended outdoor planting date. Renee's Garden and most seed houses say six to eight. The overlap, six to seven weeks, is where sturdy plants come from.

At six weeks a tomato seedling has four to six true leaves, a stem about as thick as a pencil, and roots that just fill a 9 cm (3.5 in) pot. That plant transplants without checking. At ten weeks in the same pot it has set flower buds, dropped its lowest leaves and stalled. It will sit in the ground for two weeks doing nothing while a younger plant next to it overtakes it.

Sow by the weather, not only the calendar

The Royal Horticultural Society has been arguing for several years that gardeners should shift from sowing by the calendar to sowing by the weather, and that waiting three or four weeks longer than the traditional date often produces better crops. Their trials found that later sowings caught up with and overtook the early ones. In zones 3 to 6, where March can swing 20 degrees in a week, that advice is worth more than a fixed date.

In practice: check the extended forecast before sowing, and be willing to move your date by a week either way.

You probably need two sowing dates

If you grow both under cover and outdoors, split the sowing. Plants for a polytunnel or greenhouse can go out two to three weeks earlier than plants for open ground, so they should be sown two to three weeks earlier too. Sowing everything on one day guarantees that half the batch spends too long indoors.

Container growers get a third date. A large pot against a south wall warms faster than a bed, which brings the planting date forward by roughly a week. There is more on that in the guide to growing tomatoes in pots.

Counting back from your last frost date

The zone chart below is a good starting point, but zones are broad and microclimates are not. Work out your own date in three steps.

Step 1 — find your average last spring frost

This is the date after which frost becomes unlikely where you live, based on decades of local records. Enter your postcode or ZIP into a frost-date calculator rather than guessing from the zone map. It is an average, not a promise: in roughly one year in three a frost arrives after the stated date.

Step 2 — add a safety margin for planting out

Tomatoes do not merely need to avoid frost. Growth stops when night temperatures fall below 10 °C (50 °F), and cold soil damages roots long before the thermometer reaches zero.

• Open ground: last frost plus 10–14 days, with night temperatures reliably above 10 °C (50 °F).
• Unheated greenhouse or polytunnel: last frost minus 7–14 days, provided you can throw fleece over the plants on cold nights. Air and soil should both be above 10–12 °C (50–54 °F).
• Heated greenhouse: set by your thermostat rather than the weather. Mid to late April suits most hobby setups in zones 5 and 6.

Step 3 — subtract the growing-on time

Five to six weeks for dwarf and bush varieties. Six to seven for cherry and standard slicing types. Seven to eight for beefsteaks, which are slower to build a frame. Add up to two extra weeks only if you have proper grow lights.

A worked example

You garden in zone 4 with an average last frost of 12 May. Outdoor planting is therefore around 25 May. Six weeks back gives a sowing date of 13 April for the outdoor plants. Your tunnel plants go out around 5 May, so they get sown around 24 March. Two dates, both fitted to your garden rather than to a national average.

Sowing chart for USDA zones 3 to 6

The chart combines typical last-frost windows for each zone with the six-week rule. Harvest windows assume a variety needing 65–75 days from transplant to first ripe fruit — most cherry, plum and mid-sized slicing tomatoes. Beefsteaks run two to three weeks later throughout.

Reading the chart in your own garden

Every zone spans a wide band of latitude and altitude. Zone 4 covers Minneapolis, where the last frost is usually in the first week of May, and parts of interior New England and the Canadian prairies where it is closer to the end of the month. If your local frost date sits at the early end of the range, sow at the early end of the sowing window.

Coastal and lakeside gardens run late. Water holds winter cold into spring and summer warmth into autumn, so the last frost arrives later and the first frost does too. Sow a week later than the chart, then pick a week or two longer.

Zone 3: what it takes to make it work

Zone 3 gardeners have roughly 90 frost-free days. That is enough for tomatoes only with three conditions in place. Use a tunnel, cold frame or greenhouse to buy three extra weeks in spring and two in autumn. Choose varieties rated at 55–65 days from transplant. And accept that the last flush will be picked green and ripened indoors on a windowsill. Variety choice does most of the work here — see the roundup of best tomato varieties for short-season options.

Why your hardiness zone is not your frost date

This is the single most common source of bad sowing dates, and almost no seed packet mentions it. USDA hardiness zones are calculated from average annual minimum winter temperature — how cold it gets in January. They say nothing directly about when spring frost ends.

The two correlate loosely at best. A maritime zone 6 in coastal Britain has mild winters and a cool, slow spring. A continental zone 6 in the Midwest has harsh winters and a fast one. Their last frost dates can differ by three weeks.

What to use instead

Use three numbers together. First, your average last spring frost date from local records. Second, your average first autumn frost date, which tells you how long the season is and therefore which varieties can finish. Third, soil temperature measured at 10 cm (4 in) depth in your own beds, which is what actually determines when a tomato can be planted.

Hardiness zone remains useful for perennials, shrubs and overwintering. For an annual crop sown in spring and killed by the first frost, it is the wrong tool.

Greenhouse, tunnel or open ground — how much earlier can you sow?

An unheated greenhouse or polytunnel buys roughly three weeks in spring and two in autumn — five extra weeks of season, which in zone 3 or 4 is the difference between a partial crop and a full one. That head start moves straight into the sowing date: plants destined for cover get sown two to three weeks before the outdoor batch.

It is not free, though. Glass and plastic cool fast once the sun drops, so keep fleece within reach for the first fortnight after planting.

Soil temperature beats air temperature

A warm afternoon says very little. Below 10 °C (50 °F) soil, tomato roots take up phosphorus poorly, leaves turn purple and the plant sits still for weeks. Push a soil thermometer 10 cm (4 in) down mid-morning for three days running. Once you get 15 °C (60 °F) consistently, planting is safe.

You can speed this up. Covering the bed with black plastic or landscape fabric two weeks before planting typically raises soil temperature by 2–4 °C (4–7 °F) and can bring the planting date forward by close to a week.

Does the variety change your sowing date?

It changes both ends of the calculation — how many weeks the seedling needs indoors, and how many days it needs in the ground.

Determinate or indeterminate?

Determinate bush types grow to a set height, set their trusses and ripen over a few weeks. They suit anyone who wants a concentrated harvest for sauce or preserving, and they need almost no training. Indeterminate types keep growing and setting new trusses until frost stops them. They yield more in total but need support and regular removal of side shoots to stay manageable.

In a short season, early always beats large. A beefsteak sown on schedule in zone 3 will set fruit and then run out of warmth before it can ripen. An early cherry in the same tunnel will be picking from the first week of August.

Check the days-to-maturity figure carefully

On almost every seed packet, days to maturity counts from transplant, not from sowing. Add your six weeks indoors to get the real total. A variety listed at 75 days is roughly 117 days from sowing — nearly four months.

Light, not the calendar, decides how good your seedlings are

At 50° north — Winnipeg, southern Britain, the Netherlands — daylight reaches ten hours around 20 February. At 55° north it is early March. At 60° north, mid March. Below roughly ten hours of daylight, a tomato seedling cannot build a thick stem and short internodes. It stretches instead.

You can see it happen within three to five days of germination: the seed leaves stay perched on a thin, pale stem that leans hard toward the window. That plant is more prone to damping off, snaps easily during potting on, and takes longer to recover after transplanting than a compact plant sown a month later.

What grow lights actually change

An LED fixture delivering roughly 150–200 µmol/m²/s at leaf level for 14–16 hours a day lets you sow about a month earlier than the window alone allows. Hang it 15–25 cm (6–10 in) above the plants and raise it as they grow. A desk lamp will not do it; light falls off with the square of distance, so a bulb 60 cm (24 in) away delivers a fraction of what the plant needs.

Getting more from a windowsill

Use the brightest window you have, ideally south-facing. Turn the trays half a turn daily so plants grow straight. Stand a sheet of white card behind them as a reflector, which noticeably reduces leaning. And run the room cool: at 16–18 °C (61–64 °F) growth slows but stays compact, which offsets low light rather neatly.

Move the air

Seedlings that get moved grow sturdier. A small fan on its lowest setting for a few hours a day, or brushing your hand across the tops twice daily, produces measurably thicker stems. It costs nothing and reduces damping off by keeping the surface dry.

How to sow tomato seeds step by step

1. Fill trays with seed compost. Seed compost is deliberately low in nutrients and fine in texture; rich compost at this stage gives lazy roots. Moisten it before sowing so it is evenly damp, never soggy.
2. Space the seeds 2–3 cm (1 in) apart. In cell trays or small pots, one or two seeds per cell is plenty. Crowding is the most common cause of stretched seedlings.
3. Cover with 0.5–1 cm (1/4 in) of compost. Sieve it over and firm gently. Tomato seeds germinate in darkness, unlike lettuce, so they must be covered.
4. Water from below or mist. Stand the tray in shallow water until the surface darkens, or use a spray bottle. A watering can from above washes the seeds out of place.
5. Hold 21–27 °C (70–80 °F). Cover with a propagator lid or clear bag to keep humidity up. A heat mat underneath makes a real difference: at 25 °C (77 °F) the first seedlings appear in 6–8 days, while at 15 °C (59 °F) it takes about 14 days.
6. Remove the cover the moment seedlings appear. As soon as the first loops break the surface, the lid comes off and the tray moves to the brightest spot you have. Leaving it on for one extra day causes mould and stretch.
7. Drop the temperature to 18–21 °C (65–70 °F). Cooler but bright gives compact plants. Do not let it fall below 15 °C (59 °F) or growth stalls.
8. Label everything. Variety name and sowing date on every row. Tomato seedlings look identical for a month, and by the time they do not, it is too late to remember.

Is it too late to sow tomatoes in May?

In zones 5 and 6, no. A cherry tomato sown on 10 May goes out in late June and starts ripening in early September, which still gives four to six weeks of picking before frost. The crop is smaller, but it is a crop, and the plants are usually healthier than February-sown ones.

In zones 3 and 4 a May sowing is a stretch. The plants will flower and set, but there is not enough warmth left to ripen much. Buying started plants is the better move there — it puts you six weeks further along on the same day.

The quiet advantages of sowing late

Late-sown seedlings grow into rising light and warmth, so they stay compact without lights. They spend less time indoors, which means less trouble with fungus gnats and damping off, and they go into soil that is already warm.

The comparison that surprises people: a tomato sown on 1 April and one sown on 25 April are often the same height by mid June, and the difference in first harvest is about a week. If you want a genuine head start, invest in light and warmth, not in earlier calendar dates.

What to sow alongside

Tomato sowing overlaps with cucumber sowing in the calendar, but cucumbers race from seed to transplant-ready in three to four weeks rather than six. Sow them on the same day and you will have leggy, oversized cucumber plants waiting for a bed that is not ready. The timing is covered in the guide to how to grow cucumbers.

Common mistakes

• Sowing in January or February without grow lights. The classic error. Seedlings stretch in low light and end up weaker than plants sown six weeks later.
• Sowing too deep. Seeds buried 2–3 cm (1 in) germinate slowly or not at all. Half a centimetre to one centimetre is right.
• Leaving the propagator lid on too long. Once the first seedlings show, it comes off. A single extra day in a humid, dim box produces mould and stretched stems.
• Keeping them too warm after germination. 25 °C (77 °F) is right for germinating and wrong for growing. Drop to 18–21 °C (65–70 °F) once seedlings are up.
• Never potting on, or potting on late. Root-bound plants flower early on the windowsill and never make up the lost ground.
• Planting out by the calendar instead of the soil thermometer. Below 10 °C (50 °F) soil, the plant stands still for weeks and the leaves go purple.
• Skipping hardening off. Going straight from a windowsill to full sun scorches leaves and costs up to two weeks.
• Sowing far too many. Two or three plants per person is generous. Twenty seedlings on one windowsill become twenty poor seedlings.

Frequently asked questions

How deep should tomato seeds be sown?
About 0.5–1 cm (1/4 in). Place the seed on moist seed compost, sieve a thin layer over the top and firm gently. Tomato seeds need darkness to germinate, so they must be covered, but anything deeper than a centimetre means many seeds exhaust themselves before reaching the surface. A useful rule is to cover a seed to about three times its own thickness.

How long do tomato seeds take to germinate?
At 25 °C (77 °F) soil temperature the first seedlings appear in 6–8 days and nearly all within ten. At 20 °C (68 °F) allow about eight days, at 15 °C (59 °F) roughly fourteen, and at 10 °C (50 °F) germination can take six weeks or fail altogether. A heat mat under the tray is the cheapest way to get fast, even emergence. Fresh seed germinates faster than seed several years old.

Can I start tomato seeds in January?
Only with grow lights. In January, daylight in zones 3 to 6 runs from about eight to nine hours, and a tomato seedling in that light stretches badly. With an LED fixture giving 14–16 hours of strong light, January sowing works, particularly for dwarf varieties intended to fruit indoors. Without lights you gain nothing — March-sown plants catch up and pass them by May.

Do tomato seeds need light to germinate?
No, the opposite. Tomato seeds germinate best in darkness and should be covered with a thin layer of compost. Light becomes critical the instant the seed leaves break the surface. At that moment the tray must move to the brightest position available, because seedlings begin stretching within 24 hours in dim conditions. Many failed sowings trace back to exactly that one day.

Do I need grow lights to start tomatoes?
Not if you sow from mid March onward and have a south-facing window. Grow lights become necessary for earlier sowings, or if your only windows face north or east. A cheaper alternative is to sow later and compensate with warm soil at planting time by pre-warming the bed under black plastic; the harvest date usually ends up much the same.

When should I sow tomatoes for a greenhouse or polytunnel?
Two to three weeks earlier than for open ground, because the plants go out that much sooner. In zone 6 that means late February to mid March, in zone 5 early March, in zone 4 mid to late March, and in zone 3 late March to early April. The condition is that air and soil in the structure sit reliably above 10–12 °C (50–54 °F) at planting, with fleece ready for cold nights.

How many weeks does it take from sowing to harvest?
Allow 16–18 weeks for an early cherry tomato and 21–26 weeks for a beefsteak. The breakdown is 5–8 weeks from sowing to transplant plus 55–100 days from transplant to first ripe fruit, depending on variety. A tomato sown on 20 March in zone 5 typically gives its first ripe fruit in mid to late July, with the main flush through August.

Can tomato seeds be sown directly outdoors?
Not usefully in zones 3 to 6. Soil in these zones rarely reaches 21 °C (70 °F) before June, and once you subtract the germination time there are too few warm weeks left to ripen fruit. Direct sowing can work in zone 6 in a hot year with a 55-day bush variety, but the six-week head start from indoor sowing is almost always worth keeping.

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