I built it using an open bottom redwood frame, with a thin layer of sand, double layer of 6 mil poly, another layer of sand (to protect the poly. It has a 4" slotted ag pipe wrapped in weed fabric, 90'd to a feeder neck that sits on top of the sand.
Next I added 7" of compacted western red cedar mulch...the nice stringy stuff. On top of that, a layer of weed fabric, and then topped off the whole thing with 7" of garden soil. There is an overflow pipe located just between the cedar mulch and garden soil. This is also going to be my first go at square foot gardening, albeit modified with the sub surface irrigation.

My question is about the cedar mulch I used as the reservoir wicking media. The lion's share of internet info on wicking beds seems to originate from Australia. Most of them are using gravel for the wicking media, but others prefer organic materials. To me, organic made more sense as a wick, but the specific types of mulches recommended for Australians aren't readily available here...the one that comes to mind is a sugar cane process byproduct, which is fibrous, and takes a while to break down.
Anyway, the closest thing I could find quickly and easily was the cedar, so I went with it. It's an experiment, so I have no delusions of getting everything just right on the first pass. I should mention at this point that my gardening experience is very limited, so if I'm making a glaringly obvious mistake, please let me know!

Should I be concerned about PH, nitrogen take up, anything like that? By the way, I also plan to incorporate a worm feeder in this bed, so hopefully the cedar isn't a problem. Considering this material is widely used for top mulching, it seems like it should be safe as far as leachate goes...just not sure if that holds true when burried subsurface.

Any ideas are much appreciated. Thanks!

Thanks.