[name]Zinnia[/name]'s great, [name]IMO[/name]! The ‘Z’ adds a quirky and fresh zing to it. NNs Zinny/ Zinna/ [name]Nia[/name]/ [name]Zia[/name] etc. potentially makes it more usable in real life. I don’t love [name]Amaryllis[/name] as much as [name]Zinnia[/name]- it’d be great as a mn.
My unusual flower name favorites are [name]Azalea[/name], [name]Bryony[/name], [name]Hyacinth[/name], [name]Lavender[/name], Celandine, [name]Dahlia[/name] and [name]Calla[/name]. As echoed by a previous post, I agree that [name]Hyacinth[/name] (and probably some other names such as Celandine, [name]Orchid[/name], [name]Jonquil[/name]) is a little odd to picture on a real-life girl but it seems beautiful in theory! Still, it might work- sometimes people just grow into their names and you won’t be able to imagine them having any other name…
I think [name_f]Zinnia[/name_f] is a beautiful, fresh name with lots of zing, yet it has a very sweet, soft sound to it also. [name_f]Amaryllis[/name_f] is a very heavy name, yet gorgeous too! But I would definitely go with [name_f]Zinnia[/name_f].
[name_f]Zinnia[/name_f] is a great name, feminine and zesty. I like [name_f]Amaryllis[/name_f] a lot too. 
Here are some of my favorite flower names for girls:
[name_f]Kerria[/name_f]
[name_f]Lilac[/name_f]
[name_f]Begonia[/name_f]
[name_f]Marigold[/name_f]
[name_f]Gladys[/name_f] (for glads)
[name_f]Orchid[/name_f]
[name_f]Peony[/name_f]
Nandina
[name_f]Pansy[/name_f]
[name_f]Geranium[/name_f]
[name_f]Viola[/name_f]
[name_f]Tansy[/name_f]
[name_f]Chrysanthemum[/name_f]
[name_f]Verbena[/name_f]
[name_u]Quince[/name_u]
[name_f]Freesia[/name_f]
Vervain
[name_f]Daphne[/name_f]
[name_f]Violetta[/name_f]
[name_f]Lillian[/name_f] (for lily)
[name_f]Forsythia[/name_f]
[name_f]Gardenia[/name_f]
Hydrangea
[name_f]Fuchsia[/name_f]
[name_f]Calla[/name_f]
[name_f]Camellia[/name_f]
[name_f]Flora[/name_f]
[name_f]Della[/name_f] and [name_f]Robbia[/name_f]
Astrantia
[name_f]Eglantine[/name_f]
[name_f]Zephirine[/name_f] (for a rose)
[name_f]Ghislaine[/name_f] (for a rose)
Gloire (for a rose)
[name_f]Happy[/name_f] Hunting!
[name_u]Leslie[/name_u]
I think [name_f]Zinnia[/name_f] is definitely usable. It fits right in with the trend to use xyz in a name, as well as using floral and word names. [name_f]Amaryllis[/name_f] is also lovely, but it makes me think of the [name_f]Music[/name_f] [name_m]Man[/name_m] and the little boy with a lisp…
[name_f]My[/name_f] favorite unique flower names that I would consider using are: [name_f]Hyacinth[/name_f] (I’ve actually met someone with this name, so it’s not as hard to imagine using it), [name_f]Azalea[/name_f], [name_f]Dahlia[/name_f], [name_f]Lilac[/name_f], [name_f]Lavender[/name_f], [name_f]Primrose[/name_f], [name_f]Magnolia[/name_f] & [name_f]Marigold[/name_f] (I also really like [name_f]Eglantine[/name_f], but wouldn’t consider it due to the ‘eg’ beginning
I like [name_f]Zinnia[/name_f]! I don’t think it’s hard to spell or pronounce and is easy. So while it is extremely uncommon, it doesn’t feel that way!
[name_f]Zinnia[/name_f] [name_f]Jane[/name_f]
[name_f]Zinnia[/name_f] [name_f]Kate[/name_f]
[name_f]Zinnia[/name_f] [name_f]Lucy[/name_f]
[name_f]Zinnia[/name_f] [name_u]June[/name_u]
[name_f]Zinnia[/name_f] [name_f]May[/name_f]
[name_f]Zinnia[/name_f] [name_f]Hope[/name_f]
[name_f]Zinnia[/name_f] [name_f]Sophie[/name_f] ** [name_u]Love[/name_u] this!
[name_f]Zinnia[/name_f] [name_f]Claire[/name_f]
[name_f]Zinnia[/name_f] [name_f]Emme[/name_f]
[name_f]Zinnia[/name_f] [name_f]Juno[/name_f]
[name_f]Zinnia[/name_f] [name_f]Katherine[/name_f]
[name_f]Zinnia[/name_f] [name_f]Emily[/name_f]
[name_f]Zinnia[/name_f] [name_f]Mary[/name_f]/[name_f]Marie[/name_f]
[name_f]Zinnia[/name_f] [name_f]Helene[/name_f]
It’s a YES from me, very workable.
I [name_u]LOVE[/name_u] [name_f]Zinnia[/name_f]. 
Actually, though I’m apparently in the minority so far, I really like [name_f]Amaryllis[/name_f]; I think it’s perfectly usable as a first name. She can always shorten to [name_f]Mari[/name_f]. Other faves of mine are [name_f]Lilac[/name_f], [name_f]Azalea[/name_f], [name_u]Sage[/name_u], [name_u]Aspen[/name_u], and I’ve actually heard of a Trillium too - the sister of a childhood friend whose mother was major into the nature theme for her girls. I’ve never heard that on anyone else but really like it. Trill, Trilly? Food for thought.
I wouldn’t be at all afraid of more adventurous names. We’re not living in the same world our parents were when they were naming us - remember when everyone thought [name_u]Cheyenne[/name_u] and [name_u]Dakota[/name_u] were weird? Besides, as with every other name, common or uncommon, your daughter will color and shape it as she grows into her identity.
I think it’s usable, because yes, it’s uncommon, but easily recognizable. (Zinnias are a commonly known flower, and [name_f]Zinnia[/name_f] is easy to spell and pronounce.)
The name [name_f]Zinnia[/name_f] is unusual in the best possible kind of way!
[name_f]My[/name_f] other flower names are (in order of preference:)
[name_f]Flora[/name_f]
[name_f]Tansy[/name_f]
[name_f]Raisa[/name_f]
[name_f]Camellia[/name_f]
[name_f]Zahara[/name_f]
Iolanth
[name_f]Zinnia[/name_f] is quirky but perfectly usable.
As for my own favourites - there are too many. I’d probably actually use:
[name_f]Hyacinth[/name_f]
[name_f]Iris[/name_f]
[name_f]Lilac[/name_f]
Plumeria
[name_f]Chrysanthemum[/name_f]
[name_f]Delphinia[/name_f]
etc.