Reference point
A handful of fully dried straw or grass, crushed and then smelled.
Do this before you read on. An entry only works when you have just smelled the thing.
Sweet, warm, with a faint vanilla note and a layer of dust. Nothing of fresh grass is left — no green, no cold, no wet.
The change has a clear chemical cause. As grass dries, a compound in the plant is converted into coumarin, a heavy molecule that smells sweet like vanilla and baked goods. Fresh grass gives light compounds that evaporate within minutes; hay gives a heavy molecule that stays for months.
The three base-stage entries of the green family — hay, dry rice straw and dry tea — are all dried leaves. No fresh leaf in this family reaches the base stage, and that is no coincidence. There is no way for a fresh leaf to give a base note.
How to smell it
Crush a handful of dry straw or grass that has dried for a few days, and smell at once. Compare it with freshly cut grass if you can — the two come from the same plant, a few sunny days apart, and have nothing left in common. This is the strongest contrasting pair in the whole dictionary.
Why this one is hard
Because people do not think of hay as a smell worth noticing. It is light, familiar, and easily overlooked as the backdrop of the countryside.
Don’t confuse with
This entry
Hay
Green family
The same plant, a few sunny days apart. Fresh grass is sharp, green and cold; hay is sweet, warm, vanilla-like and stays for a very long time.
Scientific note
Coumarin is not present as such in fresh grass. It forms as the plant dries, when enzymes release it from a glycoside precursor. It is the cleanest example in the dictionary of a smell created by the death of plant tissue rather than existing in the living tissue.
In Vietnamese life
In European perfumery the smell of hay is a category of its own, called foin. In Vietnam the smell of dry straw is tied to drying yards, straw beds and the harvest — the same molecule, two entirely different sets of associations.