Learn exactly how far to hang grow lights above seedlings and how many hours a day to run them, so you get sturdy, stocky transplants instead of pale, leggy ones. Includes the one mistake most first-year seed starters make with timing.
Light intensity drops fast the farther it gets from the bulb, which is why distance matters more than most new gardeners expect. For standard LED grow lights, keep the bulbs 2 to 4 inches above the tops of your seedlings' leaves. If you're using fluorescent shop lights (T5 or T8 tubes), aim for 3 to 4 inches since they run cooler and less intense. Check every few days and raise the fixture as plants grow, since seedlings growing into hot bulbs can scorch, while light that's too far away gives you thin, pale, stretched-out stems reaching for the source.
Run your lights 14 to 16 hours a day, then give seedlings 6 to 8 hours of real darkness every night. A cheap mechanical or digital outlet timer costs under $15 and takes the guesswork out of this, which matters because plants actually need that dark period to process the sugars they made during the day. Leaving lights on around the clock doesn't speed things up; it stresses seedlings and can make them weaker, not stronger, so resist the urge to treat 'more hours' as automatically better.
The most common beginner mistake isn't the light itself, it's the setup around it. A sunny windowsill alone almost never provides enough intensity or duration for strong seedlings, even in a south-facing window, because winter and early-spring daylight is both too weak and too short. If your seedlings are leaning hard toward one direction or look thin and tall with wide gaps between leaves, that's legginess, and it means the light needs to come down closer or stay on longer, not that the plant needs more water or fertilizer.
Time your seed starting around your last frost date, not a fixed calendar day, since that date varies widely by zone. Most tomatoes, peppers, and other warm-season vegetables get started indoors 6 to 8 weeks before your local last frost date, which you can look up for your specific zip code. Flip the lights on as soon as seeds germinate, not days later, since that early stretch under weak light is when legginess sets in and it's hard to fully correct afterward. If you're starting tomatoes, keep in mind tomato leaves and stems are toxic to cats and dogs, so keep trays off the floor or anywhere a curious pet can reach.
Key Takeaways
- ✅ Hang LED lights 2-4 inches above seedling leaf tops, fluorescent 3-4 inches
- ✅ Run lights 14-16 hours daily on a cheap outlet timer
- ✅ Give seedlings 6-8 hours of real darkness every night, no exceptions
- ✅ Raise the fixture weekly as seedlings grow taller
- ✅ Pale, stretched, leaning seedlings mean the light is too far or too weak
- ✅ Start seeds indoors 6-8 weeks before your local last frost date
Timing in this guide is counted from the average last frost date. Frost dates are averages, so check your local date before you plant.