Paul has made an important point about the western approach to names - most of us really don't know or care what our names or other peoples' names mean. It's all about the sound of the name, and other associations, such as whether it's fashionable or popular at a particular time, or whether there are family members or famous people with this name. Some names also have associations with regions, ethnicities, or occasionally even social class. Very few names actually have a meaning that anyone is concerned about.

But if your niece really wants a name associated with nature, though, there are many flower names she could choose from: Rose, Lily, Daisy, Poppy, Heather and Jasmine are popular names. There's also Marigold,  Iris, Lavender,  Violet, Hyacinth, Bryony, Petunia and Pansy which are a little more unusual.

Or if she particularly wants something green, how about Ivy, Willow, Holly or Fern?

Well, the reason you are having difficulties is because English naming customs are completely different to Chinese ones. Names do not 'mean' anything. When you see these websites with big lists of name 'meanings' what you are actually seeing are etymologies. There are a few exceptions, mainly the handful of girls names which are also the names of flowers like 'Violet', 'Lily', 'Rose' and so on. Also there are names like 'May' and 'June' which are the names of months. But basically, native English speakers are completely unaware of the meaning of names, with the possible exception of their own or those of other close family members, and do not attach any great importance to them.
So my advice to your niece? Pick a name that she likes the sound of. That is all that matters. And maybe run it past a native speaker or two to check that it isn't really old-fashioned and doesn't have any unwanted connotations. 

My opion is that you should stick with your original name. Why pick an English name? I think it is a strange habit. At school in my teenage years we all learnt three languages, and I did one extra. Should I have adopted an English, German, French and Spanish name for each class? No way! I would go schizophrenic.

I agree with Paul, that English names do not have a particular meaning.  The last point about checking that the name is still relevant, is a good idea.

English names for kids are different from English names for adults, and different again from English names for elderly people.  There are some popular names that last for generations, and those are quite safe, but most baby/kids names go out of fashion after 10 or 20 years.  Sometimes it is possible to roughly guess someone's age, just from their name alone.

There is also a trend lately for parents to invent new names to make their child seem unique and different from everyone else.  So the number of kids names (and spellings of those names) seems to have increased dramatically in recent years.

How about a name of a flower ?  Many are quite pretty.

Daisy, Rose, Lily.