משנה על סוטה 8:1

Gaps in an aris must be eight cubits and somewhat more [in order to be able to sow seeds in the gaps]. [In the case of] all measurements that the sages said in connection with a vineyard, there is no ‘and somewhat more’, except in the case of gaps in an aris. The following count as gaps in an aris: if an aris was razed in its middle and five vines remained on one side and five vines on the other side. Then if the gap is [only] eight cubits, one may not bring seed there. [But if the gap is] eight cubits and somewhat more, they give [the vines] space to work them, and he may sow the rest.

If one has bent a vine into the ground [and then brought it back up elsewhere], then if there is not soil over it to the height of three handbreadths, he may not sow seed above it, even if he bent [and conducted it underground] through a gourd or through a pipe. If he bent it [and conducted] it through rocky soil, then even if there is not soil over it only to the height of three fingerbreadths, it is permitted to sow seed above it. As for a knee-like vine [formed by burying and conducting it underground], they don’t measure [for how much space to work it] except from its second root.