How many hours of light, and how high should the light hang?

Run 18 hours on and 6 off while the plant grows. To flower a photoperiod plant, switch to 12 hours on and 12 off, with a dark period nothing interrupts. Then set the height by a light reading, not a ruler.

The light feeds her. The dark tells her when.

The hours

• Seedlings and veg: 18 hours on and 6 off is the standard. Anything from 16 to 20 hours of light works.

• Flower: 12 hours on and 12 off, for plants that flower by the length of the night.

• Autoflowers: they flower by age, not by the night, so they can stay on their veg hours start to finish.

When you switch to 12 and 12, she isn't counting the hours of light. She's measuring the night. Most photoperiod plants need a dark period longer than about 10 to 11 hours before they start flowering, and the exact number depends on the plant. Twelve hours clears it with room to spare.

Will a longer night finish her faster?

A grower asked us this one: if the dark runs 13 or 14 hours instead of 12, will she finish faster?

Not by much. 12 hours already clears the switch. Piling on more dark doesn't flip it any harder. What it mostly does is take away light she builds with. Any time a longer night saves is likely small, and it comes out of her light.

Some growers do shorten the light to about 11 hours in the last weeks of flower. That's their call, and some growers swear by it. Just don't expect it to rescue a slow finish. If she's running late, check the room first.

Keep the dark dark

A leak can matter as much as the hours. Light partway through her night can make it read as two short nights instead of one long one, and flowering can start late or stall. Even dim light left on all night can hold her back.

Here's the check. During the day, with the grow light off and the room lights on, close yourself inside the tent and give your eyes a minute. You're looking for room light getting in. Total dark is the goal. Any pinpoint, glow or line of light is a leak.

• Look at the zipper, especially at the top.

• Look at the vent and cord ports and the corner seams.

• Look for bright status lights on gear inside the tent.

• Make sure the timer still switches at the times you set. A timer can drift or reset after a power cut.

Tape or patch what you find, and do your tent work in her lights on hours.

How high to hang it

Inches depend on the light. Every fixture spreads its light differently, so a number someone posted for their light means little for yours. Set it by the reading at the tops of the plants:

• Seedlings: about 200 to 300.

• Veg: about 300 to 600, the low end early and the high end late.

• Flower: about 600 to 900, for a room with no added CO2.

Those are PPFD readings. A light meter gives you the number. A phone app gives a rough reading, good for comparing spots and changes, not an exact figure. Most LEDs end up about 12 to 24 inches above the plants. The maker's hanging chart comes in the box, and it's a good place to start. Read the edges of the canopy too, not just the middle under the light.

If the tops look stressed

Check the room before you touch the light. Heat, dry air and still air can make a light look like the problem when it isn't. A strong LED a hand's width off the tops is almost always too close, though.

If the room checks out, make one change: raise the light 4 to 6 inches or dim it 10 to 20%. Then give it 2 to 3 days and judge the new growth, not the leaves that already took the hit.

How many lights do I need?

Usually one, sized to your space. Pick a light its maker rates for your tent's footprint in the stage you'll run longest. A light that's a little strong for the space is fine, because you can hang it higher or dim it. Watts on the box won't tell you much. The reading at the canopy will.

When it's something else

• Bleached or curling tops: see Tacoing or bleached tops: is my light too close?

• Deciding when to switch to 12 and 12: see When should I flip to flower?

Keep going

Picking or sizing a light: see our grow lights shelf guide.

Watch the episode: Is Your Grow Light Too Close? Read Your Plant's Tops Before You Touch the Dimmer on MyRemy Live.

Steady light, a dark night, one change at a time. The whole system for a steady grow is written down in the free Stability Playbook. Get the free Stability Playbook

More answers: all grow guides. Every shelf guide and maker feed chart: Grower Resources.