used coffee grounds are not very acidic anyway. they are an organic material with a little bit of nitrogen so are good to scatter around your garden.....adding any organic mulch is good for your soil in the long run, just don't expect it to make much difference.

Thanks antipodean!

Yes, in principle, coffee grounds can be decent mulch, and are certainly not acidic. Also true that they really won't make much difference. BUT caffeine has strong allelopathic qualities, and is pretty eco-toxic. At least, caffeine inhibits plant growth. In fact, that's probably why coffee plants make caffeine. To deter competition. There is actually some concern these days about caffeine pollution in bodies of water. In the U.S., there are no natural sources of caffeine. It all comes from us, or through us. With regard to coffee grounds, it's just a question of how much caffeine is left in the grounds. Depends on how you brew it. I believe that composting breaks down caffeine, so composted coffee grounds are better than non-composted coffee grounds for direct application to plants.

FWIW, caffeine is toxic to insects, and so can be used as an insecticide, if used in a way that doesn't threaten the plants you're trying to protect.

If your tomatoes were in the ground with a dry mulch (like straw or leaves) on top, I would say definitely add the coffee grounds to the mulch. They would help the mulch break down over the year. Plus earth worms love coffee grounds - as the rain draws them down into the straw or leaves that will be great worm habitat and they will add worm castings to your composted mulch for next year.

This whole organic chain gets throttled in containers, though.

Gee Dan, I didn't even think of the caffeine aspect! Interesting.........especially how maybe that's what the coffee plants use to deter competition.

Richard....yeah, I understand that containers aren't the best way to grow things. I just couldn't lean over so much anymore, so it was sort of my only option. I do still grow pole beans, squash and cucumbers in the ground though.

Thanks Dan and Richard.

I guess if you're desperate to use uncomposted coffee, I'd sprinkle it on top, instead of digging it in where it would be close to the roots. In time, the mulch on top will just compost in situ and the caffeine will disappear.

Have to say that I'd be a little skeptical about worms and caffeine. They'll love composted coffee grounds, but I'm not sure I'd give them a lot of uncomposted grounds. There is actually a lot of controversy in the literature about whether worms like coffee grounds, and it may depend on how long the stuff was laying around before giving it to them.

But coffee grounds make for terrific compost. The microbes are thrilled with it. I've never seen an over-caffeinated bacterium!

Thanks Dan. Lots to think about!

The year I tried using a big handful of coffee grounds in planting holes, my plants were sickly and stunted for a while until, presumably, the coffee grounds decomposed somewhat. So I don't recommend that. I sprinkle my used grounds around the garden when I have too many grounds for my worm bin, and that's fine. I mix a SMALL amount of coffee grounds into planting holes now, because they supposedly nurture beneficial organisms that outcompete disease organisms in soil. That doesn't seem to negatively affect the plants, so whether or not the science actually pans out re: coffee as a medium to encourage beneficial Trichoderma strains, it doesn't hurt and adds organic matter to my soil.

The worms in my worm bin like coffee grounds fine. But I suspect they wait a while before they really dig in to them. Fresh grounds, like fresh kitchen waste (unless it's high in sugars and starches), isn't as attractive to them as waste that's had time to start to decompose. So I think coffee grounds are probably not that special in this regard.