How does a seed know when to grow?

It waits for the right combination, usually enough water, the right warmth, and sometimes light or the cold of a winter that has passed. When those arrive together, water swells the seed and wakes the chemistry inside it.

Until then it can wait a very long time. Some seeds have sprouted after centuries. And a seed carries its own packed lunch, enough food to push out a first root and a first leaf before it has to feed itself.