The planting field guide

Turn a date into a growing plan.

A calendar suggests when to start. A practical plan also accounts for your variety, the space you have and the conditions in your bed. Work through these checks, then use your own dates in the calculators.

By Safe to Plant · Published 19 September 2026 · Guidance and worked examples, not a record of field trials.

01 / Choose your starting point

A last-frost date is a risk estimate.

The median last spring frost marks the middle of a historical set of last-frost dates. Roughly half those years had their final frost later. It does not mean a 50% chance of frost on that particular night, or promise that this year's last frost is over. The weekly frost chart answers a different question: how often the record contained frost in that part of the year.

Match the risk setting to the consequence of a mistake. Losing spare seedlings is different from losing the only expensive plant you have. A cautious setting uses a later spring threshold and an earlier autumn threshold, leaving a shorter season. It still retains historical risk; check the forecast and your garden before planting.

Make the decision in this order

Open your town's calendar and choose a risk setting. Note whether its record contains regular frost at all.
Read the crop's planting method. Indoor sowing, direct sowing and planting out are different operations.
Check the forecast before the outdoor operation. Delay a tender crop if frost is forecast, even after the historical date.
Check the bed and plant. Cold, saturated soil or an unacclimatised seedling can make a frost-free day unsuitable.

Our estimates use a gridded NASA temperature record. A valley bottom, balcony or sheltered wall can behave differently from the grid. Local observations help explain that mismatch; they do not turn a historical statistic into a forecast. See the source resolution and calculation limits.

02 / Work backwards

Start seeds for the space you actually have.

Choose a realistic outdoor planting date, then count back the indoor growing time stated for your variety. Starting earlier only helps if you can provide the light, warmth, container space and care the plants need while they wait. A large seedling is not automatically a better transplant.

Check what the packet's timing includes. Some suppliers count from sowing; others describe transplant age. Germination can be slow, and a cold or dim growing space changes development. This calculator performs date arithmetic with your inputs. It does not decide whether a seedling is ready.

Keep two operations separate

Indoor growing produces a seedling. Hardening off gradually accustoms it to outdoor light, wind and temperatures. Allow time for that transition rather than moving a sheltered plant straight into full exposure. Bring tender plants under cover when conditions demand it. Hardening off does not make a frost-sensitive crop frost-proof.

For direct-sown crops such as carrots, skip the indoor calculation and prepare the final bed. For transplants such as tomatoes, check root development, moisture and the outdoor conditions before committing.

Worked example: a 20 May planting-out date and a packet instruction of 6–8 weeks from sowing gives 25 March–8 April. If that indoor period includes hardening off, do not add it again. The calculator shows a separate reminder so you can interpret the packet correctly.

03 / Leave time to finish

Will this variety fit the remaining season?

Days to maturity need a starting event. Sixty days from transplanting is not sixty days from sowing. Use the packet's definition, then add an allowance for uncertainty. A crop sown as days shorten may take longer than one grown in a supplier's trial. The allowance here is your planning choice, not a crop-specific prediction.

Choose an end date that fits the crop and garden. For a tender outdoor crop, that may be a cautious first-frost planning date. A hardy leaf crop can sometimes continue later, but slow growth, poor light or waterlogged ground can be the real limit. First harvest also differs from a useful harvest period: leaving one day before frost is fragile when you want several weeks of picking.

Worked example: transplant on 1 August, allow 60 days to first harvest and 14 extra days, and the target becomes 14 October. With a 15 October end date, just one day remains. Compare an earlier variety, an earlier start or another crop. The arithmetic does not say the plant will definitely fail or succeed.

Use a small second sowing as a test

For crops suited to repeated sowing, keep the first batch modest and note emergence and growth. Do not repeat a fixed interval after the useful season closes. Actual harvest records are more informative than assuming every batch will match the packet. Perennials, bulbs and biennial flowering need their own seasonal cycles; this single-season calculator is not designed for them.

04 / Check what the calendar cannot see

Four reasons to change a planting date.

Your soil is colder or wetter than the air suggests

The site's soil-warming check uses air temperature as a proxy. A sunny afternoon cannot tell you the temperature at seed depth. Measure where you intend to sow and look for a sustained pattern. Soil that smears and compacts when worked is a reason to wait. In a pot, drainage holes, volume and exposure can matter as much as the town's average.

The warm season is also the wet season

A frost-free climate can still have a poor establishment season. Monthly rain totals conceal whether rain fell steadily or in a few storms. Before a monsoon planting, check where water collects, how quickly the bed drains and how you will water during later dry spells. TNAU identifies very high rainfall as harmful to tomatoes. Do not copy a regional sowing month to every Indian city; use the tomato guide alongside your town's rainfall panel.

The plant needs a winter cycle your site may not provide

A hardiness zone describes winter cold. It does not measure winter chill or show that a fruit tree will crop. A species-level chill figure cannot choose a cultivar. Ask a nursery for its exact cultivar, the chill model behind its rating and evidence from comparable local conditions. Our warm-winter screen is a reason to investigate; monthly means cannot calculate chill hours.

This matters for flowers too. A temperature window for planting a peony or snowdrop is not evidence of repeat flowering in a tropical lowland garden. Where winter suitability is unresolved, our calendars withhold flowering or harvest predictions and mark planting windows as conditional. In frost-free climates we also omit fixed catalog harvest-month predictions that have not been localized.

Your planting material differs from the catalog

A dormant bare-root tree, container plant, seed and division do not share one establishment procedure. Read nursery or packet instructions before using catalog depth or spacing. Use mature spread to plan permanent plants and the variety's spacing for annual beds. The site's figures are general references, not instructions to bury every crown or graft union at a universal depth.

05 / Make next year's plan better

Keep a record that explains a result.

Record the cultivar, sowing date, planting-out date, first harvest or flower, and bed condition. Add weather, protection and setbacks. “Tomatoes failed” is hard to learn from; “transplanted 12 May, unprotected, cold night two days later, leaves damaged” gives you a decision to change.

Compare actual results with the plan after the season. Early harvest may reflect a fast variety, purchased transplants or a sheltered site. Late harvest may reflect a cool season or an incorrectly interpreted maturity figure. Change one assumption at a time where practical. One successful year is useful evidence, but does not establish a new climate normal.

Use My Garden to save choices on this device and download a plan. Use the growing record worksheet for actual results. A predicted event should never quietly become an observation.