Northern Lights, triple in size? No way, NL is a short strain, it always has been, that's why it's genetics have been used in so many indoor strain, you'll be lucky if it doubles in size when you flower it, depending on where you got your seeds. (Some may be further hybridized NL and some may be the 'real' NL#5)Expect them to double or triple in height once flowering is induced.
Totally agree when growing from seed. (Clones will likely be sexually mature when taken)letting the plant get to preflower, that's when she's telling you that she is mature
The height is mostly determined by genetic, different phenotypes amongst one strain show different characteristics, the more hybridized the strain is, and the less stabilized it is, the more likely it is that more variation will show between phenotypes. Other than that, if some of the taller plants were at some stage in low light conditions, they may have stretched to gain more light, thus making them taller. Usually if a plant has stretched for light, it will have longer internodes than the others (internodes being the space between 2 nodes *See Glossary of Terms in my signature for more info on these things if need be), but once again, the length of the internodes is partially determined by the plants genetics, thus, different phenotypes may have different length internodes even if they're grown in the exact same conditions.One of my plants is shorter than the others (it is the oldest). Does it matter?
They all look happy and healthy.
Do I have to do anything to make the shorter one reach the height of the others?
Tru.Yes, you can start 12/12 if they're not all showing sex yet.
Wonder why I cant see the pictures?The height is mostly determined by genetic, different phenotypes amongst one strain show different characteristics, the more hybridized the strain is, and the less stabilized it is, the more likely it is that more variation will show between phenotypes. Other than that, if some of the taller plants were at some stage in low light conditions, they may have stretched to gain more light, thus making them taller. Usually if a plant has stretched for light, it will have longer internodes than the others (internodes being the space between 2 nodes *See Glossary of Terms in my signature for more info on these things if need be), but once again, the length of the internodes is partially determined by the plants genetics, thus, different phenotypes may have different length internodes even if they're grown in the exact same conditions.
Pre-Flowers
As for the pre-flowers, they look like this:
Picture 1: Female pre-flower. (I know the two hairs coming from the calyx are hard to see in the picture, but look closely, the two white hairs are their).
Picture 2: Male pre-flower.
Hope this helps a bit, cheers.