The double cup method is a popular miniaturized version of global buckets, passive hydroponics, or Kratky hydroponics. It’s popular for many reasons, the top being the expense. Set up for double cupping and Kratky can be nearly free other than the cost of nutrients. seeds, and potting soil.
Net cups are relatively inexpensive but my goal for this winter is to do my indoor growing with as many recycled materials as possible. You can scale this up to use other containers as larger net cups. In this case I’m demonstrating with used k-cups.
Start with a used k-cup. Use a sharp knife and slice away the foil on the top of the pod. A sharp craft knife or utility knife will do this well. I slide my knife in along the side and turn the cup to get a nice clean slice.
Peel the foil off, and use a spoon to remove the spent coffee grounds. This can take a bit of work, as they seem to be quite packed in. Once the grounds are mostly removed. The filter is left. I poke this with my spoon and pul it out of the cup. Grabbing this and pulling slowly and evenly seems to get the cleanest results. If the filter tears, pull and scrape that out of the cup as needed. This doesn’t need to be perfect. Wash the cups with soap and water. Rinse well.
I recommend gathering a bunch of the pods before starting. Doing a single pod at a time seems like a waste. You don’t want to wait too long, as the coffee can mold. If you can’t process and remove the coffee, at the very least remove the foil so the coffee can dry out.
At this point you can decide what sort of pattern you want to cut into the cup. You can use a very sharp craft knife or scissors. My preference is to cut 2 or 3 shallow slices at the base- the bottom corner. Then I cup slashes along the sides. I’m not sure if the wider cuts are needed or if a single slash is okay. I’m testing all of them so we’ll see. I suspect that roots will push out of any available crack.
After cutting I line mine with a bit of newsprint. This isn’t totally needed, but I like to have something in there to keep the soil contained while the roots are growing.
Each of these cups holds about two tablespoons full of soil. This is enough to get seeds started for transplant or for use in an into hydro setup. You could also use Rockwool, coco coir, or moss for starting. These would work like any other net cup.
To use in a DIY hydro set up, you’d cut a 1.5inch hole in your container of choice and drop the cup in as you would any commercially available net cup.