Coffee Grounds and Super Plants: True or False?

Plants grown in a mixture of potting soil and used coffee grounds grow better than those raised in plain soil: true or false?

Because my wife is a gardener, I had heard this story long ago and reconciled myself to accept the aroma of coffee permeating from the many bags of used Starbuck’s grounds coming from our car’s trunk. But it was only after my activities were severely restricted following surgery that I put the story to the test. Material in parentheses is scientific jargon for colleagues who have forgotten colloquial English.

THE EXPERIMENT (Materials and Methods)

In April and May 2009, I set up an experiment to test the story. Six clear plastic delicatessen boxes measuring 8” by 8” by 2 ½” were set up on a patio table where each would be exposed to equal amounts of natural sunlight and shade. Six boxes were filled with 2” of commercially available potting soil, and six were filled with a combination of 75% of the same potting soil and 25% used coffee grounds obtained from a local Starbuck’s coffee shop. Seeds were obtained from a single bad of wild birdseed. These were chosen because they contain a variety of six seed types that I knew grew well for me under circumstances that gardeners would likely consider highly negligent. They should, therefore, grow under the more reasonable conditions of this experiment.

Seeds were sprayed with one ounce of water each evening; though they were sprayed at the same time, the hour of the day varied from 4 PM to 8:15 PM. The seeds were thus grown under identical conditions except for the soil contents.

(I used three experimental mixtures and three control groups.)

WHAT HAPPENED (Results)

On the 16^th day after planting, sprouts appeared only in the boxes that contained the soil and coffee grounds mixture. By day 20, the plants in the mixed soil were growing well, while only a very few tiny and yellowish seeds had sprouted in the soil-only containers (see photo).

(Experimental mixture yielded robust plants, while control groups produced very few sprouts, none of which survived past 20 days.)

HUH? (Discussion)

This was a very preliminary test based on a small number sampling. For the types of seeds used, coffee grounds added to the soil produced profound positive observable differential growth.

However, these results should not be interpreted to mean that coffee grounds will benefit all, or even most, plants. As gardeners know, plants vary in their pH toleration, such that acidophilus plants would be expected to thrive in a coffee grounds medium. Plants that prefer higher soil pH might do poorly in a medium containing acidic coffee grounds.

(The results were clear, distinct, and unambiguous for the samples tested. Huge amounts of money should now be poured into decades of graduate-student funding to, first, confirm these results about 350 times; second, to test other seed types; and three, to explain why Tully’s Coffee grounds might fare better or worse that Starbuck’s grounds.)