Coffee Grounds As A Herbicide

Coffea robusta, flanks of Kelud Volcano, Java.

Coffea robusta, flanks of Kelud Volcano, Java.

“Can I grow blueberries without peat?” Ask Alys Fowler is a gardening Q&A column in The Guardian newspaper. How misleading could a gardening Q&A response be?

Alys answers: “A maximum pH of 5.5 makes for ideal growing conditions for blueberries to thrive, so mulching around your blueberry bushes with a thick layer of pine needles will also help keep the soil acidic”. This part is fine. Strawberries also benefit from occasional mulching with pine needles.

Alys continues: “Peat-free soil for acid-loving plants does exist, but you might not be able to get it from every garden

centre you visit”. Also good advice. Lime-free or ‘ericaceous’ potting mix may not contain peat, but you need to check the label to guarantee this. Some do, some don’t. Coco Pro does.

Coffea arabica 'First Fleet', Wynnum.

Coffea arabica ‘First Fleet’, Wynnum.

Peat has been unsustainably extracted worldwide, including in Australia. When I managed Sydney Botanic Gardens, we moved away from peat to coir (coconut, cocopeat) based alternatives in the 1990’s for two reasons. First, coir has similar properties, it can be produced sustainably and its use in horticulture adds value to coconut plantations.

Second, peat extraction in Australia is a key threatening process for the extinction of our endemic gentian, Gentiana wingecarribiensis. Extraction is destabilising its habitat, which was never large, so alternatives to peat in horticulture are vital to the species recovery programme.

So how did Fowler muck up her response in The Guardian? Fowler claims “I’ve also had good results with blueberries

Coffea arabica, North Vietnam, 1962.

Coffea arabica, North Vietnam, 1962.

grown in tubs by regularly adding the ends of the coffee pot to the surface of the compost. Blueberries love nitrogen, which coffee grounds are high in, and their acidic nature helps gently to keep the soil at the lower end of the pH scale”.

And this is where Fowler flops. Coffee grounds are also a source of caffeine. Coffee plants (Coffea spp.) use caffeine to suppress the growth of competing roots and seedlings of other plant species. Caffeine impairs the ability of plants to absorb water and nutrients and it impairs the growth of soil microbes – it impacts on the biological activity of soil, including beneficial mycorrhizae.

Caffeine is a biochemical with the purpose of affecting the germination, growth, survival, and reproduction of other

competing plants, and that includes blueberry. Over time, the accumulation of caffeine in the soil gradually affects the viability of older coffee plantations (a coffee plant’s productive life is between 7 and 20 years). Extra inputs, like fertiliser and mulch, become extra costs to the grower over time, reducing profit.

Do not add coffee grounds to worm farms. If you reuse coffee grounds regularly, dilute their impact on plants by mixing them in the compost heap, spread them thinly over the soil or spread them around the garden.

Jerry Coleby-Williams
Patron, National Toxics Network Inc.
Patron, Householder’s Options for Protecting the Environment Inc.
Director, Seed Saver’s Network Inc.

10th December 2017