Mix it in a compost heap with all the other material you can collect from the house and garden. If you haven't got anything else start gathering now.

What are you processing them for? Where will you use them (either now or post-processing)? What I'm getting at is that you could likely apply directly to wherever, either as a mulch or dig in or whatever; coffee grounds are a fine soil amendment as is. Perhaps only avoiding directly around newly planted, sensitive plants or leafy edibles you will eat soon. Perfectly fine directly on a lawn for example (within reason - not a foot deep) . Test apply if any reason for caution of course. Adjust for your own preferences of how it looks.

Wow, that's a lot of UCG! Composting it on its own is not optimal - as its high in nitrogen, it's difficult to wet and as it is a 'fine' material it will compact. A good pile requires a good nitrogen/ carbon mix, moisture and air- it needs to be able to breath. So add carbon - dry leaves, shredded paper/straw/hay, wood chips etc and then wet it and mix it well to get air into it. Then stand back haha!

nito

Thanks for the responses! We would like to use them on newly planted trees, so we don't want to use the grounds directly while the trees are so young and new. I'll try adding more browns and turning the piles for air. But yes, the grounds do compact pretty heavily, so maybe layering would be a good option.

Obviously don't know how young or sensitive the trees are, but a thin layer around them as a mulch, say, an inch - perhaps lightly raked into soil - isn't likely to hurt your trees. Just don't pile up the grounds to touch the base of the trees. Composting first as you've described (turning being the main thing) fine too of course, but within reason, the composting step is almost unnecessary with coffee grounds (although you do have a lot). Experiment and test first if you're concerned.

I've heard different claims about the amount of nitrogen in coffee grounds - but they're definitely not super high in nitrogen (to a level that would 'burn') like concentrated fertilizers. (They're also not magic soil amendments or fertilizers like some want to believe - that works both ways)

Used coffee grounds make an excellent soil amendment with a Nitrogen level of about 2.28 %, Phosphorus about 0.06%, Potash about 0.6%, and a slightly acidic pH of about 6.2, according to a study done by Sunset Magazine on Starbucks coffee grounds.

To compost them you need a mix of other material, or you could use them as mulch but at a depth much greater than an inch.

https://www.sunset.com/garden/earth-friendly/starbucks-coffee-compost-test

All you need to do is mix in some fibrous stuff - dead leaves, shredded paper, or similar - and keep it moist.

To be clear, was not suggesting thin mulch was optimal in all cases, nor that coffee grounds contain no nitrogen - but allowing for OP's concern that his young and new trees would be sensitive to UCGs and reassuring that some could be used directly, just to be cautious if concerned.

I'll pass along a method that has worked well for me. YMMV. Drill air holes in a large Rubbermaid garbage can. Alternate layers of spent coffee grounds and brown material such as untreated sawdust, microshredded cardboard, dried leaves. Wet each layer as you add it (it takes more than a sprinkle to saturate the coffee grounds). Repeat until you've filled the container. Return the lid and let the microbes work their magic. The first time I tried this approach, I set it up in February and neglected it until July. When I removed the lid I had to jump back to avoid the column of steam. The surface was roiling like a volcanic landscape. Within a few weeks I was able to use the entire batch as a top dressing without having to screen it for weed stems etc. Thank you, Starbucks!

Irving Ragweed has the right answer.

You do not want to add a layer greater than 1/8" of pure coffee grounds. It will cake up and prevent water from easily penetrating. If you mix it into the soil it is not too bad. But, a one inch layer is way too thick.

I personally use coffee ground in a worm farm. I mix coffee grounds, shredded cardboard, and ground up leafs as food. You can mix in other vegetable scraps.

If you are going to compost it do so in layers with cardboard or other carbon sources. Keep the pile moist and turn it often.

I haven't tried to mulch with layers of grounds on the surface, but as I recall, other users have reported that the crusting problem is alleviated by adding some other type of mulch over top. This keeps the grounds more moist so they don't crust up, and also provides a spongy layer to absorb water and slowly release it to the grounds layer rather than running off. YMMV

All good ideas here and if you have the space for a compost pile, just adding the brown material and turning is the easy solution.

Regarding mulching - coffee makes a terrible mulch. However, if you mulch with straw, leaves, or wood chips, add some coffee grounds on top and it will eventually help compost these materials down for next near. That way this year's mulch becomes next year's compost with no work on your part. Win-win! And the earthworms will love you for it.

I think the question of what makes a good mulch depends on what one expects a mulch to do. I've used coffee grounds without composting in a very simple form - adding to other mulches (roughly as RichardBrennan suggests) or almost as a light dusting (a thin layer that is)/additive/amendment in areas where soil is a little bare, patchy lawn areas, etc. Sort of like a thin top dressing of compost. I find it becomes indistinguishable from 'soil' in short order, and lots of signs that worms and other soil life appreciates the snack. I've never tried a six inch layer or anything remotely like it though. Smallish amounts raked lightly disappear into a lawn, grounds don't interfere with grass growth really at all.

Anyway, overall my point was that coffee grounds are already in a form that composting may not be necessary as a separate step, depending how one wants to use it. If I just had the occasional bucket, I'd probably dump it in a part of the yard and rake it to a thin spread as just an addition of organic matter, and skip the composting step. To each his own and YMMV, FWIW, etc.

That is a lot of coffee grounds. If you have them sitting outside in a pile they probably already are heating up and are safe to add around the garden. If you want you can also add them to a worm bin with equal parts cardboard or newspaper.

If you want to add worms to this pile that can work but I would really recommend adding the cardboard or paper along with crushed egg shells in case the grounds are acidic.

I have a video series about running a small worm bin with just coffee grounds, cardboard, and egg shells.

Coffee Ground YouTube Playlist