Soil and air temperature
Transplants
Afternoon soil at 3 inches (7.5 cm) about 65 F (18 C), nights not under about 50 F (10 C)
Direct seed
Night soil preferably above about 60 F (15 C) or the seed sits wet without sprouting
Growth rate
Below about 60 F (15 C) air, growth is slow. From 60 to 80 F (15 to 26 C) with enough light, growth can roughly quadruple
Roots
Ground under about 65 F (18 C) limits water and nutrient flow to the canopy
Variety and calendar
April to July sets
Often finish September to November, variety depending, if sun is strong
Indicas
Early. Some flip as soon as they go outside because summer nights are already long enough
Sativas
Late. Some equatorial lines wait until late September and finish in December or January
Low latitudes
Day length changes less. You can plant more of the year, but indicas may flower immediately
Keep them in veg
Night-break
A 400w HPS can cover about 600 sq ft. Or use about one 24w CFL per square yard
Pulse
About 10 minutes in the middle of the night, at least once, on a timer
Spectrum
HPS and warm-white fluorescent are rich in red, which stops flower. Incandescent fights you with far-red
Spring
After the March equinox, nights shorten toward the June solstice. Before that, many plants need a night-break or they flower
Light in autumn
Weak sun
September and October light, and UV, are a fraction of June
Shade shift
A bed that was sunny in June can sit in shadow as the sun angle drops
Clouds
Overcast delays ripening even in a south-facing garden
Supplement
About 10 to 30 watts per square foot of HID, CFL, or HO T-5 during the brightest hours can finish buds that would otherwise stall