Stop planting by the calendar. Start planting by the soil.
Tomatoes go out when the soil holds about 16°C, whatever the date says. The kitchen thermometer test.
Warm-season crops are planted by soil temperature, not by the date. Push a kitchen thermometer about 10 cm into the bed at around 10 am on three mornings and average the readings. Plant tomato seedlings out once the soil holds 16°C, and 18 to 20°C is better, and only after the last frost has passed. Sow beans, basil and the pumpkin family direct into soil of much the same warmth, or the seed can rot. In most of our cities that point arrives between early September and late October. Brisbane is usually there first; Canberra and Hobart come last.
Melbourne Cup Day. Hobart Show Day. The first week of October. The long weekend. Every city has a tomato date, and the plants have never read a calendar. What they read is the soil.
A warm week in September lifts the air quickly and the ground slowly. The seedlings planted on the strength of that warm week go into cold soil and sit there, and the ones planted a fortnight later into warm soil often overtake them. This lesson is about knowing which fortnight you are in.
Why soil temperature beats the calendar
For warm-season plants the soil is the thermostat. Many seeds need a warm, consistent soil temperature to germinate well, and a tomato seedling in cold soil cannot take up water and nutrients properly however sunny the afternoon is. That is often the stalled, purple-tinged tomato seedling people photograph in October: in many cases not sick and not starved, just too cold to take up what is there. Cold roots are the common cause, though nutrient problems can look similar, so if a plant does not come good as the soil warms, look further.
Soil also behaves differently from air. It warms slowly and it holds what it gains, so three hot days do not make warm soil and one cold night does not undo a month of spring. A reading tells you where the season actually is. The calendar only tells you where it usually is.
The numbers that matter
Three figures cover the spring patch.
- Tomatoes: Margot White, a horticulturist at the Royal Tasmanian Botanical Gardens, puts it plainly on Gardening Australia: do not plant out until the soil reaches a minimum of 16°C, and 18 to 20°C is even better. Plant after the last chance of frost as well.
- Beans, basil, zucchini, pumpkin and cucumber, sown direct: much the same warmth, from about 16°C up, and corn wants soil that is properly warm, around 16°C and higher. Bean seed sown into cold, wet soil tends to rot before it can sprout, and the same cold, wet soil slows the others and makes for patchy emergence.
- Capsicum, chilli and eggplant: warmer again, above 18°C, which is why they are almost always raised in a punnet somewhere warm and planted out last.
How to take the reading
You do not need a soil thermometer. A kitchen or meat thermometer with a probe does the job.
- Push it about 10 cm into the bed where you intend to plant, the depth Agriculture Victoria uses for sowing decisions. For fine seed, a reading at sowing depth is closer to what the seed feels.
- Take the reading at around 10 am, before the sun has done much to the surface.
- Do it three mornings running and average the three. One warm morning can be a fluke; three readings give you a steadier picture.
Read the actual spot. A raised bed against a north-facing wall can run well ahead of a shaded bed in the same yard, and pots run warmer than the ground again. Bare soil warms faster than mulched soil, so pull the mulch back for a couple of weeks before planting and put it back once the seedlings are in.
A free signal, if you have no thermometer
Watch for volunteers. When last summer's nasturtiums start coming up on their own, the soil is warming towards what beans, basil and the pumpkin family want. It is a useful local clue rather than a precise gauge: nasturtiums come up from around 13°C, a little below what the warmth-lovers need, so treat it as a nudge to start checking, not a green light for everything.
What to do while you wait
Waiting is not idle. This is the fortnight that sets up the season.
- Sow tomatoes, capsicum, chilli and eggplant in punnets somewhere warm from late winter into early spring. In roughly six to eight weeks they are around 10 cm tall with two sets of true leaves, which is planting-out size. Capsicum and chilli are slower off the mark, so start them at the earlier end, about eight weeks ahead.
- Direct-sow the crops that do not mind cold soil: beetroot, carrot, lettuce, radish, spring onion and silverbeet can go in through early spring in most gardens.
- Dig compost through the beds and put the tomato stakes in before planting, so the roots are never disturbed later.
- Pull the mulch back to let the sun in.
Planting day, when it comes
A week or so out, harden the seedlings off. Stand them outside in a sheltered spot for a couple of hours on the first day and lengthen it over seven to ten days, so the shift from a warm windowsill to the open bed is gradual rather than a shock.
Tomatoes are planted deep. Set the seedling so that only the top few leaves sit above the soil, having pinched off the leaves that would be buried. The buried stem grows roots along its length, which makes a stronger plant that handles heat and dry spells better and carries more fruit. Water in well and stake at the same time. If a cold night is forecast in the first week, an upturned pot over each plant at dusk is cheap insurance; lift it again in the morning.
City by city, in a usual spring
These are usual years, drawn from our own sowing calendar; your thermometer is this year.
- Brisbane, Gold Coast, Sunshine Coast: the soil is usually there by early spring. Tomatoes, beans, corn and zucchini can go in from then, with cherry and heat-tolerant tomato types the safest bet for the humid months ahead.
- Sydney, Newcastle, Wollongong: our calendar puts warm beds at planting warmth from mid to late September in most years, shaded beds a few weeks later. Beans direct once the soil reads warm and the frost risk has gone.
- Perth, southern WA and Adelaide: similar timing, and the dry heat arrives fast behind it. Plant as soon as the reading says yes and mulch straight after. Albany and the south coast run a little later than Perth.
- Melbourne and Geelong: the calendar groups them with the temperate metros, but their spring soil runs a few weeks behind Sydney's, which is why the Cup Day rule exists. Sow indoors in early spring and plant out when the reading says so, often from mid to late October.
- Canberra and the tablelands: a cool-zone spring. Under cover through early spring, out after the last frost, often late October or November.
- Hobart: under cover through early spring, out after the last frost, often November.
- Darwin, Cairns and the tropics: a different calendar altogether. Warm-season crops go in through the dry season, and the coming wet is the reason to hurry, not the cold.
- Alice Springs and the inland: frost is the gate, not soil warmth. Plant out once the last frost has passed, usually from October.
Our What to Sow Now page has the month-by-month chart for your postcode if you want the full list.
Grow it: seeds for this lesson
Tomato Heirloom Mixa spread of shapes and colours from one packet, started in punnets in late winter or early spring.See the seeds
Tomato Cherry Foxa quick, forgiving cherry for first plantings.See the seeds
Tomato Yellow Grosse Lissethe classic Australian slicer in its yellow form.See the seeds
Tomato Siberianbred for cold starts, the one to try in Canberra, Hobart and the hills.See the seeds
Tomato Tropicbred for heat and humidity, the one for Brisbane and the coast.See the seeds
Bean Bush Jadethe first bean to sow the week the soil reads warm.See the seeds
Plant by the soil and the plants stop sulking. It is a smaller correction than it sounds, one thermometer and three mornings, and it is the difference between seedlings that sit and seedlings that get on with it. For the rest of the sowing list for your area, our What to Sow Now page covers every zone.
Frequently asked questions
I have already planted tomatoes and they have gone purple and stopped. Have I killed them?
Usually not. A purple tinge and no growth in cold soil is the plant unable to feed itself yet, and it often comes right as the soil warms. Do not pile on fertiliser; keep them watered, cover them on cold nights, and be patient.
Can I warm the soil up faster?
A little. Pulling mulch back lets the sun into the soil, and Gardening Australia suggests a sheet of black plastic laid over the bed for about two weeks ahead of planting to hold the heat. Take it off at planting.
Do raised beds and pots warm faster?
Yes, often noticeably, which is why the first tomatoes of the street are usually in a raised bed against a warm wall. They also cool and dry faster, so watch the watering.
Do cold nights matter as well as cold soil?
They do. Tomatoes slow right down when nights stay cold, and early plantings gain little for it. Once nights are reliably mild and the soil reads warm, growth takes off.
Does this rule apply to seed potatoes?
No. Potatoes are planted into cooler soil and follow their own timing, and we cannot send them to Western Australia, Tasmania or Kangaroo Island due to interstate quarantine restrictions and our shipping policy. Our seed potato guides cover them separately.
