Seedsweeper

How to survey

• Dig a square with a click (a tap on a touch screen). A dug square shows how many seeds lie in the eight squares around it.
• Mark a square you believe holds a seed with a right-click, a long press, or M. On a phone, the button beside the board switches taps between digging and marking.
• Click a number whose seeds are all marked, and its other neighbours are dug for you.
• The survey is complete when every square without a seed has been dug.

Field notes

• Under a stand of giant hogweed, some 3,760 living seeds lie in every square metre in autumn, 95 per cent of them in the top five centimetres of soil (see Seed bank).
• A seed dug up in autumn does not germinate at once: it needs a winter first. The exercise is less patient.
• A plot is clear only when it has stayed clear: follow-up surveys continue for at least five years after the last plant.