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Photo: Coffee grounds in the pot — Corn cheese · CC BY-SA 4.0. Photo cropped from the original.
Feeding

Coffee grounds in the pot

MYTH

Coffee grounds make a great fertilizer for houseplants.

FACT

Grounds hold a little nitrogen, but not enough food to replace fertilizer. And while they break down, soil microbes can lock up nitrogen for a while.

What actually matters

  1. Not straight into the potUsed grounds still contain caffeine, which can hold back root and shoot growth. How much depends on the amount and on the plant.
  2. They pack into a crustGrounds are fine and pack down: a thick layer lets less water and air through to the roots.
  3. They won't acidify soilBrewed grounds are close to neutral and won't noticeably acidify the soil. They won't help acid lovers either, such as azaleas and gardenias.
  4. Compost them insteadIf you have a compost heap, add your grounds there – no more than a fifth of the pile.
  5. Feed with real fertilizerLook for one labeled for houseplants. An all-purpose one works too, mixed at half or a quarter of the label strength.
Sources: Oregon State Extension, Arizona Extension (after WSU), RHS, Illinois Extension, NC State Extension, Iowa State Extension.