The two-thirds rule
Look down
When plant canopy hides about two thirds of the floor, start flower
Fill-in
The remaining third fills during stretch and early flower
Height
If the room is short, flip earlier. Stretch will still add feet
Do not wait
A veg plant that is already tall will hit the lamps after the flip
How much they grow after flip
Sativas
Often add about 2 to 3 feet (60 to 90 cm), and can roughly double in size before growth stops
Sativa-leaning hybrids
About 1 to 2 feet (30 to 60 cm)
Indica-leaning hybrids
About 1 to 1.5 feet (30 to 45 cm)
Indicas
Up to about 1 foot (30 cm), or about 20 to 25 percent taller and wider
Hybrids
Sit between those poles. Know the cut before you fill the tent
First-week habit change
Slow
Veg can add about 2 inches (5 cm) a day. That pace drops after the long night starts
Females
Stems get stockier, internodes shorten, new leaves have fewer fingers, and phyllotaxy switches from opposite to alternate
Stigmas
The first white hairs appear, usually in week two on a typical indoor run
Males
If any remain, they elongate and lift pollen sacs above the females
Plan the slot
Lamp gap
Leave stretch room plus a safety margin for heat
Screen
If you use netting, set it before the stretch, not after spears punch through
Outdoor move
Indoor plants set outside in summer still stretch. Shade them first so UV does not burn the new growth
Indica shock
Indica-leaning plants grown on 24-hour light can flip hard outdoors. An 18-hour indoor day is a softer handoff