How Long Should Seeds Be Under a Grow Light? A 5-Step Seedling Checklist

Got a tray of seeds and a grow light still in the box? In my role as a grow lighting support specialist at Spider Farmer, I've handled more than 200 rush setups over the last 6 years. The question I hear most is straightforward: how long should seeds be under a grow light? The short answer: once the first seedling breaks the surface, give it 16-18 hours of light per day. The full answer is a checklist, because hours are only half of it.

I won't promise you a yield number. Per FTC guidelines, a claim like that needs evidence (ftc.gov), not a marketing page. What I can give you is the process I use when a customer says, 'Help, my seeds are up and I don't want to ruin this.' As of March 2025, this is the checklist I run through.

Once seedlings break the surface, give them 16-18 hours of light per day. That's the number to start with.

Step 1: Turn the light on only after germination, not before

Seeds don't need light to germinate. They need moisture, oxygen, and warmth. Running a full-spectrum LED 24 hours a day before sprout doesn't speed up germination; it dries out your growing medium. I've seen growers panic because the surface dried into a crust before a single taproot showed. Instead, put the tray on a heat mat and wait. Once 50-75 percent of the seeds have pushed up, turn on the light. That's your true start point.

Step 2: Right-size the fixture (SF600 vs SF4000)

For seedling trays and vegetative growth in a 2x2 area, the Spider Farmer SF600 (74W) veg LED grow light is enough. It's built for leafy growth, draws 74 watts at the wall, and the PPFD map on spider-farmer.com shows decent coverage for a small bench. If you're filling a 4x4 canopy later, the Spider Farmer SF4000 (450W) LED grow light is a better target because it gives you the intensity and spread for flowering.

But buying a 450W fixture for a tray of seedlings is a total-cost mistake. You pay more upfront, use more energy, and have to hang it high just to avoid blasting the babies. I calculate TCO for every lighting purchase: fixture cost, energy over 5 years, replacement risk, and time spent managing heat. The 74W SF600 often wins for seed starting. The 450W SF4000 wins when you're actually running a full tent.

Step 3: Put the light switch on a timer before adjusting height

This is the step I always check first on an urgent call. Set the timer for 18/6 before you hang anything. If you're using a smart controller, schedule the light switch in the app. If you're using a manual mechanical timer, set it to a fixed time and leave it.

I ignored this once. I thought I'd remember to turn the light on and off manually. Then I left town for 72 hours. When I came back, my seedlings were pale, stretched, and leaning like a drunk fence. Seriously, get a timer. It costs $10 and removes you from the equation.

Step 4: Run 18 hours on, 6 off — not 24/7

The exact answer to 'how long should seeds be under a grow light' is, for most seeds, 18 hours. Seedlings don't need 24. They need enough light to reach a reasonable daily light integral, and they need darkness for normal plant processes. If you're running photoperiod plants, 18/6 can take you through veg. If you're growing autoflowers, 20/4 is also common. I wouldn't go below 16 hours unless you're supplementing with intense light, and I wouldn't go to 24.

There's a misconception carried over from the HID era that more hours always mean more growth. That was never a free lunch: it meant more heat, more electricity, and often plants that looked stretched. With LEDs, you can manage intensity instead of just time. If you ask me, 24/0 is a red flag. It wastes energy and stresses plants without a good reason.

Step 5: If you use CO2, sync it to the light schedule via Zigbee

This is the step almost everyone forgets until something beeps at 2 a.m. Plants only take up CO2 when stomata are open, which means when the lights are on. Injecting CO2 during the dark is like leaving the air conditioning on with the window open.

A controller with CO2 Zigbee can prevent that. On the Spider Farmer GGS controller, you can link the CO2 sensor and the grow light switch in the same scene. When the lights switch on, CO2 enrichment starts. When they switch off, CO2 shuts down automatically. I once had a customer call because their CO2 alarm went off every night. The issue wasn't the sensor. It was the schedule. The controller was releasing CO2 according to its own clock, not the light schedule. We changed the scene, and the alarm stopped. If you're not using CO2 yet, skip this step. If you are, don't ignore it.

Three mistakes that turn seed starting into an emergency

Here's what I see most often when a grower calls and says their seedlings don't look right.

1. Hanging the light too high or too close. Distance charts are generic. The PPFD map in the manual tells you the real intensity at each height. For an SF600 over seedlings, start with the manufacturer's recommended veg height and check the seedlings for stretching. If they lean, lower it a few inches. If the leaves look pale or burnt, raise it.
2. Running a 450W fixture in a small tent on full power. If you have an SF4000 in a 2x2, dim it while seedlings establish. Better yet, use the 74W SF600 for the first few weeks and switch to the larger fixture when the canopy actually needs it.
3. Ignoring total cost. A cheaper light with lower PPFD can force you to buy a second fixture, run it longer, and replace it sooner. I ran a comparison for a customer who wanted to save money. The no-name light had a lower upfront price. After a second unit, higher energy draw, and a failure in month 14, the budget option cost more than the SF4000 they were avoiding. Count everything: fixture, electricity, replacement risk, and time.

If your seeds just sprouted, your next move is simple. Turn on the light, set the timer, and resist the urge to run 24/7. If you know someone who just bought their first grow light, send them this. And if you've got a controller question, especially CO2 Zigbee, check the scene schedule before you call support. Most of the time, the light switch is fine. It's the schedule around it that's broken.