Well, in the ballad "The Cruel Mother", in many versions she "leans her back against a thorn", possibly to aid birthing and possibly to shock her body into starting early labor and miscarrying?

I have done some research on abortifacient herbs, and "green willow" doesn't seem to be one of them. There is a really good site at http://www.sisterzeus.com/Abortif.htm, for great herbal advice and analysis. Some of the herbs I found that can cause or help cause abortion are: "American Mandrake, Angelica, Black and Blue Cohosh, Cotton Root Bark, Evening Primrose, Mayapple, Parsley, Pennyroyal,
Pineapple, Pokeroot, Rue, Tansy (very toxic!), Vitamin C (in huge amounts), Wild Cherry, and Wormwood". Very few of these are mentioned in the love songs and ballads that I know that have herbal choruses.

But in one of the most beautiful versions of "Come all ye Fair and Tender Maidens", or "Rue", Dave Van Ronk's version, the betrayed girl laments that in May, when the "primrose fair" comes in, she will pull it up, and, instead, plant a "green willow tree". In other versions, where her "thyme" grew once, now "rue" is growing (both a pun and the name of an herb of mild abortifacient powers.)