Natsu = summer
Koyu = peculiar or characteristic
Ki = tree

The plant in question is a compact white Cattleya, smaller than standard size.
According to Bill Valavanis, natsu means summer, as you said, but he divided
the rest of the word differently. Yuki means snow and ko may mean small, and
that explanation makes the most sense. Many white flowers are given names
relating to "snow in summer", so the name means something like a little snow in
summer or snow in summer on something small. Can't be more definite unless the
Royal Horticultural Society starts publishing its database in Kanji.
If you're curious, you can find it on the Web as Cattleya Angelwalker x
Hawaiian Wedding Song 'Snow White.'

Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)