Root grafting (also called whole-root or piece-root grafting) uses only a root piece as the rootstock compared to other types of grafts where the rootstock contains both root and stem tissue.
At one time, this was a common graft for apples.
Root grafting is done as bench grafting when the scion is dormant.
They are commonly tied with nursery tape that decomposes naturally when the graft is planted in the soil.
Saddle grafts were once a common graft for rhododendrons using a root piece as the rootstock.
Tree peonies are side wedge or cleft grafted to herbaceous peony tuberous roots.
It is an interesting graft performed between a woody and herbacous perennial.
The graft is done in the fall when herbaceous peonies are lifted from the field.
The healed graft is planted in the spring.