I've heard that you can mold coffee grounds into any shape you want, so I was wondering how would I want to mold coffee grounds into edging for a water collection basin to keep all the water for everything I plant inside the garden (think like a seasonal sand castle of coffee grounds).
Is doing something like this even a good idea?
To answer your questions: No, there, is no good way to make a coffee ground water collection barrier.
You can certainly mold damp coffee grounds into any shape you like, and then you can watch them fall apart when they get wet - or dry. Not much use for water collection.
No, "something like this" does not appear to be a particularly good idea. Coffee grounds can be useful in the garden, but not as a construction material in the garden. Compost them, as already advised.
The closest I can even think of "working" would be the "mushroom foam" approach from http://www.ecovativedesign.com but that is based around materials with more cellulose, and I don't think it will hold up well to garden conditions, water, and weathering - but you can evidently get a bag of their stuff and try it out, if you like (not affiliated, had heard of their product sometime in the past year and looked up the site.)
You can get things that have been "molded" out of cow manure, too - they also break down in the rain, but they hold together a bit better than coffee grounds (so I hear - spending money for molded cow manure has never seemed like a great idea to me, but evidently some folks like to do that.)
Gosh, coffee grounds have 2% nitrogen and a bit of potassium. NOT readily available for quite awhile to plants or soil micro/macro organisms until processed by the 'decomposers'...great for organic matter for ACID LOVING plants and the soil but to use coffee grounds for fertilizer is kinda lame. I would only use coffee grounds in a large outdoor garden to thinly spread around...blueberries for instance would love this addition, your lawn would NOT. Otherwise they are fine in your compost pile. To rely on grounds for fertilizer or soil ammendment is not a good idea. Decomposed organic matter is far better to pile on top of your soil. Decomposed is the main word here! Lots of plants love acidic (low pH conditions) but there are other plants that need slightly alkaline such as grasses and cannabis (grins). Far better to test pH with a real soil test from your cooperative extension service (nearest university), know your specific plant needs and rely on decomposed organic mulch (tilled into new garden soils or put on top of the beds...the organisms in the soil come up and eat the decomposed organic matter, go back into the soil profile to poop it out aerating and mixing organic matter into your soils)...using coffee grounds as a 'structure' is a waste of time sweetie. The magic in a garden is knowing as much as you can about botany, soils, waters and specific needs of each plant. Planting plants that have the same needs together for instance is proof of one's gardening know hows. Find those organic fertilizers that add bacteria and mychorrhizae and are slow release. Worth every penny...instead of using synthetic fast fertilizers.
You can form firelogs from coffee grounds with wax, sugar, molasses. See this instructable here. It mentions other recipes but it sounds like a lot of work for little gain. But you'll end up with logs, or other shapes, that might withstand the weather and water for a while to do what you want to do with them. And since you have heaps of coffee grounds, you can keep making them when they fall apart.
