Old potting compost can be reused in some situations, but it should not be reused directly without amendment or assessment.
Fresh potting compost provides three main benefits: nutrients, aeration, and appropriate moisture retention. After one to two years of use, the nutrients are largely depleted, the organic matter has broken down (making the compost denser and less aerated), and the pH may have shifted from the accumulated effects of watering and root activity. Bark-containing composts break down particularly significantly: bark particles that provided aeration in fresh compost become compressed, fibrous fragments that hold moisture and pack tightly. Old compost from a plant that had root rot or a significant pest infestation also carries a biological legacy that is not desirable to pass on to the next plant.
When Reusing Is Appropriate
Old compost from a healthy plant (no rot, no significant pest history) can be reused for lower-priority plants, for garden use (beds, borders, garden containers), or as a component blended with fresh compost and amendments. For a high-value or sensitive houseplant going into old compost, amend it thoroughly by adding a significant proportion of fresh perlite to restore aeration, and a portion of fresh houseplant potting mix to restore organic structure and some nutrient content.
A mix of roughly 50% old compost plus 30% fresh compost and 20% perlite is better than old compost alone.
When Not to Reuse
Do not reuse compost from a plant that had root rot, from a plant with a persistent or unresolved pest infestation (particularly mealybug, which can survive in the root zone, or fungus gnat larvae, which live in the compost), or from a plant that died for unknown reasons. Composting garden plants is reasonable recycling; using the compost immediately in new pots for houseplants risks carrying over the problems that affected the previous plant.
Bag and bin compost from plants that died from disease or severe pest infestation.
Refreshing Old Compost for Garden Use
Old houseplant compost is suitable for use in outdoor containers, garden beds, or as a mulch. The depleted nutrients are less of a concern outdoors, where the plant has access to a larger soil volume and the organic matter contributes to soil structure beneficially. Mix old houseplant compost into garden borders rather than using it as the primary growing medium in outdoor pots, where plants need a nutrient-rich starting point. It composts down fully in an active compost heap over a season.