Feminine. As a rule, they are feminine.
Here are some examples, with corresponding photographs:
rosa, -ae : rose
caltha, -ae : marigold
īlex, īlicis : Holm Oak
hedera, -ae : ivy
pīnus, pīnī : (Italic) pine [yes, still feminine]
There are exceptions to this rule, as one may expect.
- robur, -ōris : oak (n)
- acer, acēris : maple (n)
For a better sense of the gender distribution (largely feminine with some neuters), here’s a list of all the Latin names of ‘British’ foliage. Pay attention to the species name (and adjective) to clarify the gender of third declension nouns.
According to A&G, “many names of plants in -us vary between the second and fourth declensions.” They then give no examples. Can you think of any?
The Essential AG: 32, 32b
Famous Phrase: sub rosā [beneath the rose]
A phrase denoting secrecy. The rose was associated with silence, as was given as the symbol of Harpocrates, the god of silence at Rome. In the Middle Ages, a rose hanging over the entrance chamber of a given committee room represented a call for silence about the content of the committee’s discourse.
I struggled with image formatting here. That final picture is an Italic pine.
Papyrus is a 2nd declension masculine or feminine plant noun.