GardenCalc
The date on the seed packet is a hint, not an instruction. It is printed for the whole of the UK, and the whole of the UK does not warm up together. What actually governs whether a seed comes up is the temperature of the soil it is sitting in.
Soil temperature beats the calendar
The RHS puts a number on it: “Most seeds won’t germinate below a soil temperature of about 7°C (45°F)”, and therefore “you must delay early sowings until the soil has at least reached this temperature”.1
That is why sowing on the packet date in a cold spring gets you nothing, and sowing a fortnight later catches up and overtakes it. Seed sitting in cold wet ground does not wait politely — it rots.
A soil thermometer costs a few pounds and settles the argument in ten seconds. Push it in to seed depth, mid-morning, not straight after rain.
Indoors and outdoors are two different questions
Most sowing confusion comes from collapsing them into one. Sowing indoors is about having plants ready at the right moment; sowing outdoors is about the ground being warm enough to receive them. A tomato started indoors in March and a carrot sown outdoors in March are answering different constraints, and only one of them cares what your soil is doing.
Spacing is the third question, and the one most often skipped until the plants are already in. It is easier to decide before you sow than to thin a crowded row afterwards.
The three tools here, and when each one helps
- Planting calendar — what can go in this month where you are, as the overview.
- Sow window planner — your indoor and outdoor windows for a specific crop, worked from your own frost dates rather than a national average.
- Plant spacing calculator — how many plants a bed actually takes, before you sow too many.
Sources and checking notes
Every figure here was read off the source named, on 19 September 2026.
- Royal Horticultural Society — Getting started with vegetables: seed sowing techniques. rhs.org.uk. Checked 19 September 2026: “Most seeds won’t germinate below a soil temperature of about 7°C (45°F)” and “so you must delay early sowings until the soil has at least reached this temperature.”