We already have "Bloom" as an image type. Since catkins are clusters of inconspicuous blooms, I think adding it to the image types would be redundant. If we start further sub-categorizing the types of images, I fear we'd end up with a long list that would get pretty unwieldy.
For this particular plant: Pignut Hickory (Carya glabra) under bloom color I described it as yellowish-green catkins.
For other plants I think I've also be mentioned in the comment section that the blooms are catkin like panicles etc.
~ I'm an old gal who still loves playing in the dirt!
~ Playing in the dirt is my therapy ... and I'm in therapy a lot!
Hopefully we will get catkins added to the list. 
