Are coffee grounds mixed with potting soil good for indoor gardening?
Coffee grounds are a source of nitrogen, which is necessary for all plants. Some references indicate that the grounds may make the soil slightly more acidic, but others state that the grounds are neutral with respect to pH, so should be fine for most plants. However, plants may differ in how they respond to coffee grounds. Some experts recommend starting with 15 mL (1 tablespoon) of coffee grounds, lightly working this in to the soil once a week or so. See how the plants tolerate the grounds and adjust the “dose” accordingly.
And if you use a fine coffee grind, beware that the grounds might pack together and form a crust, decreasing soil aeration and risking growth of fungus. In houseplants, you may see a buildup of salts on the soil surface. Try to use a coarser grind of coffee – your neighbourhood coffeehouse may give away grounds.
One reference I consulted noted that fruit fries may be attracted to coffee grounds – so if you see an increase in population of these critters once you start using the grounds, you may want to reconsider using the grounds.
Another source recommends adding 250 ml (1 cup) of coffee grounds per 1 litre (1 quart) of potting soil when you repot indoor plants.
The following may be of interest:
- Better homes and gardens. Can I Put Coffee Grounds or Cooking Water on My Houseplants?
- SF Gate. Fertilizing Plants With Coffee Grounds and Eggshells.
- Purdue Extension. Grounds for gardening.
- National gardening association. Coffee Grounds – a Miracle!