Coffee Grounds as a Planting Medium
I have LOTS of coffee grounds. I use them for lawn fertilizer, in compost, for in situ composted beds. I still have lots of grounds. Albert_135 has reported good results withusing coffee grounds as a planting medium, so I decided to do a little experiment.
I made three planting media: commercial potting soil, straight coffee grounds, and a half and half mixture of the two. In one test, I took some marigold seedlings that I had started in a paper egg carton and transplanted them into 4 inch pots containing each of the planting media. In another test, I started seeds of: tomato, kale, broccoli, chard, and lettuce in paper egg cartons, using each of the planting media.
The marigold seedlings in the various planting media showed a similar growth rate. The plants in the coffee grounds had a purple color to the outer (older) leaves. Those planted in commercial potting soil had green leaves, and those planted in 50:50 coffee grounds:potting soil were almost exactly intermediate between the other two.
Of the seeds, only tomatoes showed 100% germination. In every case, the seeds planted in the commercial potting soil germinated first, followed by those in the 50:50 mix, and then by those in the coffee grounds. In the tomatoes, the differences in germination times were within a day. Other plants showed more variation. The broccoli, kale, and lettuce showed little difference in germination success between the potting soil and the 50:50 mix with 91% and 89% of the seeds germinating. Only 61% of the seeds planted in coffee grounds germinated. The chard is slower, but seems to show the same pattern. The seedlings in each of the planting media appear equally healthy as the first true leaves appear.
Not surprisingly, commercial potting soil works better than coffee grounds in supporting seed germination. However, mixing this half and half with coffee grounds has little effect on the germination rate. Tomatoes germinate as well in coffee grounds as in potting soil.