The root mass here needs to be reduced by at least half before gong on the rock. That can take some time, particularly with nursery material that's been grown in deeper pots.
Moss attracts birds. A lot of moss attracts a lot of birds. Moss/muck should be more muck than moss, should look and act like play-doh. Shallower muck walls, and less moss in the muck=less birdie attention.
I have used this stuff to make muck walls on slab plantings. You can indeed make your own with clay and chopped sphagnum. I used this because I didn't want to futz around with collecting ingredients and storing the excess. It also has moss that isn't long fibered, but chopped small--too small to be of any real interest to birds.
When you really need to muck things up, use Muck-O! Muck-O is ideal for all your Kokedama (moss balls), Kusamono (accent plants) and Seki-Joju (rock plantings). The purpose of Muck-O is to hold it's shape while providing soil for the plant you are working with. Muck-O retains a lot of water and...
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