One of several documents comprising British Library MS. Harley 585 is the Lacnunga. Also a healing book (the word læc can be translated as ‘remedies’) it contains nearly 200 treatments and dates to the late 10th or early 11th century. Interestingly it is written in a mix of Old English and Latin, Aramaic, Hebrew, Greek and Old Irish.
A third source from the 10th century but was penned in Latin by Ælfric, the first Abbot of Eynsham in Oxfordshire is his Glossary (c.995). It is exactly that, but rather than arranged alphabetically the words grouped by topic, one of which is plants.
For lists of plants that appear in the three tomes - together with a whole host of other useful information on early gardens and their plants, visit Wyrtig (the Old English adjective itself means 'garden like, full of plants')