by Coty Perry
Starting tomato seeds indoors weeks before the last frost date allows gardeners to get a jump on the growing season. However, leggy, weak seedlings stretching upward toward the light are a common problem when growing tomato transplants inside.
If left unchecked, tall, spindly seedlings will become unstable plants prone to damage once planted in the garden. Fortunately, with some adjustments to light and care, it is possible to cultivate stocky, robust tomato seedlings ready to thrive when transplanted outside.
The Problem
Reader David asks:
“I start my tomato seeds indoors, with fluorescent grow lights about 24 inches above the trays. Every year, the seedlings grow tall and spindly, and they are very fragile. What should I do?”
Fortunately, I can help with this problem. The main reason for tall, spindly tomato seedlings is insufficient light.As the plants reach toward the light source, they elongate and become thin in an effort to reach it. This results in fragile, weak seedlings.
There are several tactics you can try to address this issue.
The Solution
Use the dropdown tabs below to see common solutions to your leggy tomato plant problem:
One of the easiest ways to address leggy tomato seedlings is to provide more light. Here are some methods:
- Lower the grow lights closer to the top of the plants. Try adjusting the height to 18 to 20 inches above the seedlings.
- Raise the seedling trays closer to the light source using blocks or supports.
- Rotate the trays periodically so all sides of the plants get equal light exposure.
- Use grow lights with higher wattage or lumen output. Opt for T5 or T8 fluorescent bulbs or LED grow lights.
While grow lights are helpful, they simply cannot match the intensity and spectrum of natural sunlight. Move the seedlings to a sunny indoor spot near a window — a south- or west-facing window is ideal. The natural light will help the plants grow stronger and prevent legginess.
Pinch or trim one inch of growth off the main stem when seedlings are four to six inches tall. This encourages branching from side shoots rather than upward stretching. Pinching weekly can produce stockier, bushier plants.
When transplanting seedlings to the garden or containers, plant them deeply. Bury the lower leggy stems underground. The underground portion will form extra roots along the buried stem, strengthening the root system. Many gardeners bury leggy tomato plants up to the top two to three leaf sets.
Additional Tips
Circulate air around seedlings with a fan to strengthen stems.
Pick a compact, determinate variety of tomatoes that is less prone to legginess.
Start seeds six to eight weeks before your last expected frost, so plants have time to grow strong before transplanting.
Use a seedling heat mat under the trays to encourage faster, stronger initial growth.
Water carefully, avoiding overwatering, which can weaken growth.
When transplanting, use supports like cages or stakes to minimize legginess.
So, How Do You Prevent Tall and Spindly Tomato Seedlings?
In summary, leggy tomato seedlings usually result from insufficient light exposure during early growth. I’ve found that the best solutions involve increasing light intensity and duration through supplemental grow lights, natural sunlight, and proper positioning.
Encouraging branching by pruning tops and transplanting seedlings deeply can also compensate for early legginess. With proper light and care, it’s possible to get stocky, robust tomato plants off to a strong start indoors.
FAQs About Tomato Seedlings
What causes leggy tomato seedlings?
Leggy tomato seedlings are caused by inadequate light exposure in early growth, leading plants to elongate and stretch upward, seeking light.
How much light do tomato seedlings need?
Tomato seedlings need 12 to 16 hours of bright light daily. Supplement natural sunlight with grow lights as needed to reach this daily light amount.
When should I start tomato seeds indoors?
Start tomato seeds indoors six to eight weeks before your estimated last spring frost date. This provides adequate time for seedlings to grow strong before transplanting outside.
Should tomato seedlings be kept warm?
Yes — consistent warmth between 70°F and 80°F is ideal for starting tomato seeds and growing robust seedlings indoors. Use a heat mat if needed.
How close should grow lights be to seedlings?
Grow lights should be two to three inches above newly sprouted seeds, then gradually adjusted to six to eight inches above developing seedlings.
Further Information
- Tips for Growing Tomatoes (video)
- How to Grow Tomatoes in the Deep South
Editorial Contributors
Coty Perry
Expert Writer & Reviewer
Coty Perry is a lawn and garden writer for Today’s Homeowner. He focuses on providing homeowners with actionable tips that relate to the “Average Joe” who is looking to achieve a healthier and greener lawn. When he isn’t writing he can almost always be found coaching youth football or on some trail in Pennsylvania in search of the next greatest fishing hole.
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Lee Ann Merrill
Chicago-based Lee Ann Merrill has decades of experience writing and editing across a wide range of technical and scientific subjects. Her love of DIY, gardening, and making led her to the realm of creating and honing quality content for homeowners. When she's not working on her craft, you can find her exploring her city by bike and plotting international adventures.
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