Short answer:
Bagged Multipurpose Compost doesn’t expire like food — but it does degrade over time. Most manufacturers recommend using unopened bags within 12–18 months of packing, and opened bags within 6 months.
What actually happens as compost ages
- Nutrients decline – soluble nitrogen and potassium can leach or volatilise.
- Microbial life slows or dies off – especially in sterile, packaged MPCs stored dry.
- Structure collapses – wood fibre and peat-free blends can slump, reducing aeration.
- Moisture balance changes – loss of moisture leads to hydrophobic material.
- Odours may change – stale or sulphurous smells can indicate anaerobic storage.
These changes won’t make compost unsafe, but they reduce germination, drainage, and nutrient release — all key to healthy plant growth.
Shelf life: unopened vs opened bags
| Condition | Expected Life | Notes |
|---|---|---|
| Factory sealed, cool dry store | 12–18 months | Most brands specify “use within 1 year” of packing date. Cooler, shaded storage extends life. |
| Opened bag, resealed & dry | 3–6 months | Re-seal after each use; store off the ground and out of direct sun. |
| Exposed to rain or frost | < 3 months | Structure collapse, nutrient loss, and anaerobic conditions likely. |
| Home-made or PAS100 compost | 6–12 months | Richer biology but less stable; best used fresh. |
Tip: Check the bag seam or printed date code — many brands include a pack date or batch number that helps estimate age.
Can you still use old compost?
- For seed sowing: avoid old composts — nutrients and structure are too unpredictable.
- For pots or borders: blend with fresh MPC (e.g. 50:50) to restore porosity and nutrients.
- For soil improvement: even aged compost adds organic matter; mix into borders or beds.
If compost feels slimy, smells bad, or won’t rewet — it’s likely anaerobic. Spread thinly on soil and allow to dry before reuse.
Extending compost life
- Store in cool, shaded, dry conditions (garage, shed, north wall).
- Keep sealed or cover open bags with breathable fabric.
- Add a handful of biochar granules or dry perlite to restore structure in tired mixes.
- Avoid freezing or waterlogging — both accelerate breakdown.
Summary
Compost doesn’t have a strict expiry date, but freshness matters. Within a year of packing, structure and nutrient profile are at their best. After that, performance depends on storage and intended use — fine for soil improvement, less ideal for precision planting or seed starting.
Disclaimer.
Brand names such as Westland, SylvaGrow, RocketGro, Dalefoot, Carbon Gold, and others mentioned on this site are registered trademarks of their respective owners. MultipurposeCompost.co.uk includes these examples for informational and comparative purposes only and does not claim endorsement, affiliation, or suitability for any specific use. Gardeners and buyers should always check current product specifications and manufacturer guidance before purchase or application.
