How to Avoid Leggy Seedlings When Starting Indoors Under Lights

ByStephen Albert

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Leggy seedlings—those tall, stretched, weak stems that flop over—are one of the most common challenges for indoor seed starters. Fortunately, they’re easy to prevent once you understand what causes them. Seedlings stretch when they’re reaching for more light, growing too warm, or crowded by neighboring sprouts. Here’s how to keep your seedlings sturdy, compact, and ready for transplanting outdoors.

Give Seedlings Strong, Close Light

Light intensity is the number one factor in preventing legginess.

  • Position lights 2–4 inches above seedlings (for LEDs); lower or raise as needed.
  • Use full-spectrum grow lights that run 14–16 hours per day.
  • Raise the fixture as seedlings grow to maintain the same distance.
  • Avoid weak window light, which causes immediate stretching.

If seedlings lean toward the light, rotate trays every day or two.

Keep Temperatures Cool After Germination

Warmth helps seeds sprout, but once they emerge, cooler temperatures keep stems short and strong.

  • Ideal range: 60–70°F (16–21°C) for most vegetables.
  • Turn off or remove heat mats once seedlings have emerged.
  • Avoid placing trays near heaters, sunny windows, or warm appliances.

Cooler temperatures slow stem elongation and encourage root development.

Start with Proper Spacing

Crowded seedlings stretch as they compete for light and airflow.

  • Sow thinly—just one or two seeds per cell or pot.
  • If too many sprout, snip extra seedlings at the soil line.
  • Use larger cells or soil blocks if plants tend to outgrow small trays.

Good spacing means each seedling gets adequate light from all sides.

Provide Gentle Airflow

A light breeze strengthens stems and reduces humidity.

  • Run a small fan on low for an hour or two per day.
  • Make sure air movement is gentle—seedlings shouldn’t whip around.
  • Ensure vents and humidity domes are opened or removed once sprouted.

Airflow encourages sturdiness and prevents damping-off disease.

Water Wisely and Feed Lightly

Overwatering contributes to weak, spindly growth.

  • Water from the bottom so seedlings can wick up moisture.
  • Keep soil just moist, never soggy.
  • After the first true leaves appear, give a dilute organic fertilizer every 10–14 days.

Healthy roots support strong stems.

Pot Up Before Seedlings Outgrow Their Space

If roots fill the cell or seedlings begin crowding each other, transplant them to larger pots to keep them thriving indoors. Light, airflow, and spacing all improve when plants have room to grow.

Conclusion

Leggy seedlings are preventable with good light, cooler temperatures, smart spacing, and steady airflow. With these simple practices, your indoor seedlings will grow short, sturdy, and ready for a successful transition to the garden.

Stephen Albert is a horticulturist, master gardener, and certified nurseryman who has taught at the University of California for more than 25 years. He holds graduate degrees from the University of California and the University of Iowa. His books include Vegetable Garden Grower’s Guide, Vegetable Garden Almanac & Planner, Tomato Grower’s Answer Book, and Kitchen Garden Grower’s Guide. His Vegetable Garden Grower’s Masterclass is available online. Harvesttotable.com has more than 10 million visitors each year.

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