The four things that degrade a botanical
Oxygen oxidises the aromatic compounds; light drives the same reactions faster and destroys pigment outright; heat accelerates everything; and time does the rest. None of these makes a dried botanical unsafe — it makes it inert, which is a different and more common problem.
What fades fastest
Ground seed and spice, by a wide margin — mahleb is noticeably stale within weeks. After that, aromatic leaves whose character sits in surface glands: lemon verbena, curry leaf, tarragon. Then flowers, which lose scent before they lose colour, so a violet that still looks perfect may have nothing left.
What keeps well
Whole hard seed, whole nutmeg, roots, barks and roasted grains. Whole nutmeg keeps for years and takes ten seconds to grate. Dried root is often still usable after a couple of years. Aged mandarin peel is the extreme case: it is deliberately stored for decades and improves.
Practical storage
Opaque or dark containers, airtight, away from the cooker, and not in the cupboard above it. Buy quantities you will finish rather than the economical large bag, because the saving disappears the moment the material goes flat. Valerian gets its own rule: keep it away from anything else aromatic, because the smell transfers to a whole cupboard.
How to tell it has gone
Smell it. If it does not smell of itself, it will not taste of itself, and nothing in the brewing recovers that. Colour is a poor guide in both directions: faded calendula is genuinely degraded, while a perfectly purple violet may have no aroma at all.