Long before calendars, people noticed that certain plants
reliably signal when others are ready — one flowering as another
fruits, one dying back as another comes into its own.
This is sometimes called phenology, though you don't need
the word to use the practice.

As covered in section 9.3, a fixed date doesn't account for
how much weather varies year to year. But two plants in the
same location are responding to the same local conditions
at the same time — so when one reaches a particular stage,
it's a more accurate, locally adjusted signal than any calendar
could be.

Many foragers in Suomi use bird cherry (tuomi) blossom as a
rough marker — when it flowers, several other spring growth
events tend to be happening around the same time, including
good early nettle picking. This isn't a precise rule,
but a useful local rhythm worth noticing.

As covered in section 6.1, many mushrooms have specific tree
relationships. Some foragers watch for early autumn leaf colour
change in birch as a rough signal that certain birch-associated
mushrooms may be starting to appear nearby.

As covered in section 2.4, insect activity often correlates
closely with specific plant stages — certain pollinators
becoming active right as the flowers they depend on open.
Noticing insect activity can be as useful a timing cue
as watching the plants themselves.