Maybe the humble ivy is not as well known as I have supposed.
If we had a puzzle that allowed 3-letter words, I'm sure
ivy would be accepted. But
ivy is mostly used as an "uncountable" noun. We don't usually talk about "one ivy", "two ivies", "many ivies". Rather, we talk about "some ivy", "a lot of ivy", etc. Where the word
ivies might be used is when referring to different varieties of ivy, say in a plant nursery: "The ivies are over there."
There are other words like this. We accept
poetries as a rare word, while
poetry, of course, is common. The plural word is rarely used, but could be appropriate when talking about the poetic traditions in different countries: "Ostashevsky explores Russian and American poetries..." (
The Bookseller, 13 May).