at what stage does the bud start swelling up

[jaye13]

at what stage does the bud start swelling up

[Weed420]

The last 3 weeks they usually start building some good bulk.

[Ms.Fox]

^ agreed. They spend a lot of energy still growing in the first 3-4 weeks. A cannabis plant can double or triple in size when switched to 12/12. When they stop growing, they switch their energy source to the buds. ;-)