棵 may be used for measuring plants. Normally, the plants are slim and tall and only have leafs. Flowers cannot be measured by 棵.
一棵树 yì kē shù
一棵草 yì kē cǎo
'a blade of grass'
一颗菜 yì kē cài
小偷躲在一棵树后。 Xiǎotōu duǒ zài yì kē shù hòu.
'The thief lurked behind a tree.'