Quote from above:
a) Sunken cactus - Grubani Peninsula (183,101)       - Time 0:00 - 0:20
b) Yellow Rose of Izet - Tahrahji Desert (235,216)    - Time 1:20 - 1:40
c) Eruptive dandelion - NRM near volcano (78,173)   - Time 2:40 - 3:00
d) Twilight ogre toes - PV near KJ exit (60,36)         - Time 3:00 - 3:20
e) Lime nightshade - Arius (276,244)                       - Time 4:10 - 4:30
f) Glittering turquoise - Trassian PK cave (645,87)   - Time 5:40 - 0:00

I can confirm that 
... the nightshade can be harvested from 4:10 onward exactly and not anymore at 4:45
... the yellow rose cannot be harvested anymore at 1:48
... the disruptive dandelion can be harvested from 2:40 onward exactly
... the glittering turquoise cannot yet be harvest at 5:37, can be harvested at 5:43 and not anymore at 0:18
... the sunken cactus can be harvested from exactly 0:00 onward.
... the yellow rose can be harvested from exactly 1:20 onward.