Actually (and being away from home I haven't the reference) there is an excellent book written in Britain in the early 20th century by a scholar of early Anglo Saxon herbals which has much old lore in it about the magical aspects of herbalism (and goes into the old Germanic lore as well). The woman who wrote it was chiefly interested in the medicinal healing of the pre-Christian and early Christian Brits, but she makes a very good case that they worked with the plants both for their physical and magical properties, which were well known. As I recall there were hundreds of plants used in this way. It is a fascinating read. I share Broadwood's opinion.

Because we have lost so much of our connection with nature and the plants used by our ancestors, as well as the lore(medical and magical) that every child grew up with in previous generations, this interpretation is beyond our ken. But this kind of lore is very common in cultures who are still rooted in their agrarian past.

I found this reference, to one of the orginal manuscripts she used in her work:
Anglo-Saxon Remedies, Charms, and Prayers from British Library MS Harley 585: The Lacnunga : Commentary and Bibliography VOLUME II
The Anglo-Saxon "Lacnunga" is a miscellaneous collection of almost 200 mainly herbal remedies, charms, and prayers found only in a mostly 10th-11th century manuscript in the British Library. The collection is written mainly in Old English and Latin, and includes a version of a remarkable 7th-century Hiberno-Latin prayer known as the "Lorica of Laidcenn". There are also corrupt passages in Old Irish, Greek, Hebrew, and Aramaic. It is one of the oldest extant medical collections in Northern Europe. Study of it sheds light on the dissemination, understanding, and translation in Anglo-Saxon England of remedies from classical and classical-derived collections such as the "Historia Naturalis" of Pliny, the "Medicina Plinii", and the "Plinii". The collection also includes a large number of "magical" charms which offer an insight into native beliefs in elves, spirits, witches, and sentient plants. This two-volume edition includes: a detailed discussion of the nature of the collection and its status in Anglo-Saxon England; discussions of the collection's palaeography and codicology, sources, analogues, and language.