Why timing is the whole thing
Several of these plants are worth gathering for a fortnight and worthless outside it. Young birch leaves are sticky and pale in early spring and leathery by midsummer. Nettle tops are worth picking before the plant flowers and coarse after. Linden flowers over roughly two weeks in June and goes hay-like if picked late.
Roughly when
Spring: nettle tops, birch leaf, young raspberry and blackberry leaf. Early summer: elderflower, linden, blackcurrant leaf. Midsummer: yarrow, chamomile, hop cones later. Late summer: heather, sideritis, elderberry into autumn. Autumn: roots — dandelion, chicory, marshmallow, angelica — lifted once the top growth dies back.
Roots are the autumn job
A root is lifted when the plant has finished putting energy into leaves, which is why almost every root in this catalogue is dug in autumn of the second or later year. Lifting in spring gets a thinner root with less of everything.
What this means when buying
Bulk material sold year-round from an unnamed harvest is often out of window. This is the practical reason a spring-picked nettle is a brighter green than the coarse late material, and why buying from someone who names a picking date is worth doing where you can.