noun
moonflower; Ipomoea alba
A night-blooming species of morning glory. The term is rare and usually written in katakana. Compare with 夜顔 (ヨルガオ), which is a different plant (Ipomoea alba is sometimes called ヨルガオ as well, but ヤカイソウ is a less common name).
See also: 夜顔
ヤカイソウは夕方に花を開く。
The moonflower opens its flowers in the evening.
ヨルガオ is the more common name for moonflower (Ipomoea alba). ヤカイソウ is a rare synonym.
The name 夜開草 literally means 'night-opening plant', from 夜 (night), 開 (open), and 草 (plant). The reading やかいそう is a straightforward on'yomi compound.