Coffee grounds for plants

Used grounds contain nitrogen, potassium and phosphorus, in low concentration and in an organically bound form. The plant can do nothing with them until microorganisms have broken the material down.

In open garden soil that breakdown runs along at a decent pace. In a pot in the living room the soil life is far poorer, and the grounds sit on top almost unchanged for a long time.

Used coffee grounds are close to neutral after brewing. Do not rely on them to lower the pH for acid-loving plants.

As a nutrient source, coffee grounds are a soil improver on a small scale and no replacement for fertilizer.

Wet coffee grounds on the surface grow mold. After a few days a white fuzz sits on top, because the fungus breaks down exactly this kind of organic material at a steady dampness.

The second effect is mechanical. The fine grounds dry into a crust that repels water. Water then runs down the side while the root ball underneath stays dry.

The third one is fungus gnats. A surface that stays damp and is rich in organic material is where they lay their eggs. Anyone who scatters coffee grounds on top regularly is building them a nursery.

For those three reasons coffee grounds never go on indoor soil as a thick layer, however well the plant is doing otherwise.

Add grounds to a compost pile with other materials rather than scattering regular doses into indoor pots. Oregon State recommends keeping coffee grounds below 20 percent of the compost volume.

Drying the grounds does not establish a safe dose for a houseplant pot. Residual caffeine can inhibit growth, and uncomposted grounds can temporarily tie up nitrogen.

Use finished compost as an ingredient in a suitable potting mix. Coffee grounds alone do not provide a balanced fertilizer.

A compost heap is where coffee grounds belong. They break down there along with other material, and the nutrients end up in a form that roots can use.

If you have finished compost, mix a small amount of it into the soil when you repot. That is the only form in which coffee really acts as a nutrient source in a houseplant pot.

Without a garden, a worm bin takes coffee grounds too, in small amounts alongside other kitchen scraps.

Composted grounds still replace no liquid fertilizer. They improve the soil, they do not feed the plant on demand.

They do not replace fertilizer. A plant that has sat in the same mix for over a year and grows pale with small new leaves needs a complete fertilizer, not coffee.

They do not drive off fungus gnats. That tip comes around regularly and has the thing backward, because a damp organic layer encourages the larvae.

They do not reliably acidify a mix. A plant that really needs acidic conditions goes into soil blended for it and gets soft water.

They do not keep slugs or cats away. There is no solid evidence for those effects, and indoors the question rarely comes up anyway.

The simplest route is a liquid fertilizer for foliage or flowering plants, dosed by the instruction on the bottle. You feed while the plant is visibly growing, and not through the dark half of the year.

Water with plain water first if the soil has dried out. Concentrated feed on bone dry roots does more harm than good.

Freshly repotted plants get no fertilizer for several weeks, because fresh soil brings its own and disturbed roots react badly to salt.

A white or yellowish crust can come from fertilizer salts or minerals in irrigation water. Brown tips and edges can accompany salt damage, but do not diagnose overfeeding on their own. Check both the fertilizer dose and your water source before changing the care routine.

Azalea, gardenia and camellia need a suitable acidic mix. Used coffee grounds are not a reliable way to create or maintain that acidity.

Choose the potting mix and fertilizer for the species. Do not use repeated coffee-ground doses to correct the soil pH.

For other houseplants too, composting the grounds is preferable to an untested dosing routine in a pot.