Rab Judah ruled in the name Of R. Samuel b. Shilath who had it from Rab: An eruv may be prepared with papuin (cress), chalglugot purslane and godgandanit (melilot) but not with lichen or unripe dates.
The requirement is that the items left in the eruv be edible – or at least things that people will eat. That point is disputed with one of the herbs on the list:
Is it, however, permitted to prepare an eruv with melilot seeing that it was taught: “Those who have many children may eat melilot but those who are deprived of children must not eat it; and if it was hardened into seed even those who have many children should not eat it?”
Since it can be harmful, how could it be considered as part of a meal?
Explain it to [refer to melilot] that was not hardened into seed and [that is used for people who] have many children.
Since it is suitable for some it is considered “food.”
And if you prefer I might say: It may in fact refer to [people who] have no children [the use of the plant nevertheless being permitted] because it is fit [for consumption] by those who have many children; for have we not learnt: ‘An eruv may be prepared for a nazirite with wine and for an Israelite with terumah’, from which it is evident that [certain foodstuffs may be used for an eruv because] through they are unsuitable for one person they are suitable for another?
So also here [it may be held that] though [the melilot] is not suitable for one it is suitable for another.
An interesting and useful principle – just because something is not suitable for the individual (wine for a nazirite for example, or the terumah of a priest for an Israelite – both are forbidden to the individual named) does not mean that it cannot be used for their eruv. Rather surprising, in fact.
But let’s not overlook the other point: melilot seems to have been considered a natural contraceptive! Forbidden to one who is childless (since they need to fulfill the mitzvah of “be fruitful and multiply” but permitted to one who has enough children!