When should I sow?

What this planner calculates

The date you sow is a step backwards from the date a young plant can sensibly go outside. Our calculation first adds or subtracts a crop allowance from your last-frost date. It then subtracts a range of indoor growing weeks. The result is a window, because a seed packet, a room temperature and the weather cannot be reduced to one universally correct day.

For tomatoes we use six to eight weeks indoors and a provisional planting date one week after the frost date. Peppers use eight to ten weeks and a two-week delay. Cucumbers use three to four weeks and the same two-week delay. Lettuce uses four to five weeks and cabbage four to six, with an indicative planting date two weeks before the frost date for hardened plants in suitable conditions. Basil uses four to six weeks and a two-week delay. These are planning assumptions for a temperate outdoor garden, not promises about frost tolerance.

Choose the frost date carefully

A last-frost average is a probability estimate, not the last possible cold night. Use a local weather service or extension office rather than a broad hardiness-zone label. Hardiness zones describe winter cold; they do not give the spring planting date for your particular garden. A low hollow, exposed balcony or sheltered urban courtyard can differ from the nearest weather station.

Check whether the date you found represents an average or a more conservative probability. Write that distinction in your garden notebook. If the source gives a range, running both ends through this tool is more useful than picking a middle date and forgetting the uncertainty. The tool does not fetch a forecast or detect your location.

A worked example

Suppose your last-frost planning date is 15 May and you want six tomato plants. The provisional transplant date becomes 22 May. Counting back eight weeks gives 27 March; counting back six gives 10 April. That is your first indoor-sowing window. If your room is cool and growth is slow, the earlier end may suit. If light and space are limited, the later end may produce smaller, more manageable plants.

At an assumed 80 percent germination, six divided by 0.8 equals 7.5, rounded up to eight seeds. That number only allows for germination. It does not include seedlings lost to handling, disease, pests or weather. If you add spare plants, decide where they will go before sowing an entire packet. A windowsill filled with crowded, stretched plants is not a useful insurance policy.

Use a real germination estimate

The default 80 percent is an example, not a claim about your packet. Replace it with a recent packet figure or your own small germination test. Ten seeds provide only a rough indication; one seed changes the result by ten percentage points. Twenty or more give a less coarse estimate, but the test still represents the conditions you provided rather than every possible sowing environment.

Count normal seedlings, not merely seeds that have cracked. Record when you began, how you kept the test moist and how long you waited. If a crop has a specific dormancy treatment, a simple damp-paper test may be inappropriate. Follow species-specific instructions and avoid using this vegetable planner for unfamiliar wildflowers.

Read the plants before planting out

Harden seedlings off by gradually introducing outdoor light, wind and temperature. A tray that has lived in a warm room should not spend its first afternoon in strong sun. Bring plants under protection when conditions deteriorate. Watch the compost as well as the leaves: small cells can dry surprisingly quickly outdoors.

Delay tender crops when the soil remains cold or the forecast is unsettled. Earlier sowing cannot make a cold garden warm. If plants need to stay inside longer, give roots suitable space and maintain good light. For cucumbers in particular, avoid planning such a long indoor stay that roots become crowded before the bed is ready.

Keep next season's useful evidence

Record the actual sowing date, first emergence, potting-on date and planting-out date. Note what happened after planting, including a cold spell or a week when watering was difficult. A short record of the conditions is more useful than a verdict such as “bad seed”. Next season, compare the same crop and variety before changing every part of the method.

The date arithmetic uses UTC at midday so daylight-saving changes do not move the displayed day. Your entries stay in this browser and are not saved. Print the result if you want a paper record. For the practical work around those dates, read our seed-starting guide and visit the kitchen-garden references.

Sources and limits

The planning ranges are informed by University of Minnesota Extension's seed-starting guidance and Iowa State Extension's indoor propagation guide. Local guidance and the seed supplier's variety instructions take precedence. The calculator is not designed for tropical growing calendars, heated greenhouse production or commercial transplant scheduling.