פירוש על ערלה 1:4

ונשתייר בו שורש – it is attached to the ground and was not uprooted [completely].

Introduction This mishnah deals with when a tree is considered to have been uprooted.

כמחט של מתוח – is our reading, meaning to say, the measurement of the thickness of the root is like the needle that the weavers stretch the cloth after he weaved it to stretch it out and to widen it.

If a tree was uprooted and one root was left [in the ground], it is exempt. As long as one root is still attached to the ground, the tree is considered to still be planted and, if the tree is older than three years, it is exempt from the rules of orlah.

How much must the [thickness of the] root be? Rabban Shimon ben Gamaliel said in the name of Rabb Eliezer ben Judah a man of Bartota: as [thick as] a pin [used for] stretching. In order for the root to count as still attaching the tree to ground, it must be as thick as a pin that weavers used for stretching the cloth. Others explain that this is a pin used by launderers. In any case, if the root is thinner than that, it would not be sufficient to nourish the tree, so we would have to consider the tree as having been detached and then replanted, which means that for the next three years its fruit will be prohibited.