ShelfYears

Does granular fertilizer expire?

No — Milorganite states flatly that its fertilizer has no expiration date and works indefinitely kept cool and dry, and the same holds for any straight dry lawn food. The bag is what fails, not the food.

Updated

Granular fertilizer
Sealed
Doesn't meaningfully expire
Opened
Indefinite, kept dry
MOISTURE CAKING
Harmless

Sealed: Doesn't meaningfully expire Tier 1

Milorganite's own answer to "Does Milorganite expire?" is one line: it has no expiration date, and it will work indefinitely if stored properly in a cool and dry area. The company adds that a dry shed or garage is fine — seasonal temperature swings do not affect quality or effectiveness. Scotts says the same about its dry plant foods: they should not expire if stored properly and kept where the particles will not get wet.

After opening: Indefinite, kept dry Tier 1

Opening the bag does not start a chemical clock — it starts a moisture one. Scotts' description of the failure is exact: exposed to air and moisture, the granules absorb that moisture and harden. The nutrients are still in there; what you lose is a product that pours and spreads evenly.

How to tell it's gone off

  • Granules fused into lumps or one solid slab — the bag has taken on moisture.
  • A bag that feels damp, or whose inner liner is stained, stiff or split.
  • Granules that pour and rattle freely with no clumps: Milorganite's position is that a dry bag works indefinitely, whatever its age.
  • Colour tells you nothing here. Dry fertilizer does not fade, darken or change smell as it ages.

If you use it anyway

Harmless

Dry granular fertilizer is the rare garage product that genuinely does not go off. Milorganite says it has no expiration date and works indefinitely stored cool and dry, and Scotts says its dry plant foods should not expire if kept from getting wet. The only thing that changes with time is physical: a bag that has taken on moisture sets into lumps, which is a handling nuisance and nothing more.

Why it goes off — MOISTURE CAKING

The nitrogen, phosphorus and potassium in a bag of lawn food are mineral salts, and mineral salts do not age. They are, however, thirsty: left open to humid air the granules pull in water, swell, dry again and fuse. That is the whole failure mode — a physical one, in the bag, not a chemical one in the granule.

Every page here commits to one of four states — Harmless · Degraded performance · Equipment damage · Safety issue. How the verdicts work.

How to store it

Scotts' storage answer for leftover lawn food: keep it in its original container and store it in a cool, dry place out of reach of children and pets. Milorganite asks for the same cool, dry area and notes that in a dry shed or garage the seasonal temperature swings do not matter — only damp does.

Sources