Why roots have their own target
Ideal medium: About 70 F (20 C).
Cold floors: Containers on a cold slab slow germination and growth.
Sex skew: Cold roots on seed plants encourage more males.
Trade: Warm roots let you run the canopy a bit cooler without the same damage.
How to warm the pot
Air gap
Raise containers off the floor. Foam, wood, newspaper or a pallet all insulate
Push heat down
Overhead fans can send lamp heat toward pots on pallets
Mats and cables
Heat mats put warmth directly under the root zone
Hydro heater
An aquarium heater on a thermostat warms recirculating solution
Balance water against air
Warm air
If the room is over 75 F (22 C), keep solution at or below 70 F (20 C)
Hot air
If air passes 90 F (30 C), drop water toward 65 to 70 F (18 to 20 C) to ease canopy stress
Cool air hydro
In 45 to 60 F air, water can be heated toward 80 F (27 C)
Oxygen limit
Water above 80 F holds little oxygen. Add aeration or hydrogen peroxide if you run that warm
Field heat and frost moves
Ice in a heat wave
Bags of ice at the pot can cool soil that is far above the low-70s target and melt slowly enough to soak in
Weight
One gallon of ice is about eight pounds, so plan the carry
Frost commute
Moving flowering containers indoors at night protects ripening from freezes that stall buds
Same schedule
Bring plants out and in at a consistent time so they still get as much sun as the shortening days allow