Grow Lights for Broccoli Seedlings: Stocky Starts
Grow lights for broccoli seedlings hold starts short plus stocky until they go into the garden. University of Minnesota Extension advises lights no more than 4 inches above seedling tops plus raising them as plants grow. Run lights 12 to 16 hours a day plus harden plants off before transplanting.
The short answer
Turn on bright grow lights once seedlings emerge plus hold them 2 to 4 inches above the tops for 12 to 16 hours a day, as University of Minnesota Extension advises. Iowa State University Extension advises sowing 4 to 5 weeks before planting out plus hardening transplants off first.
Why do broccoli seedlings get leggy?
University of Illinois Extension describes leggy seedlings as having elongated internodes caused by absent or low intensity light. University of Minnesota Extension notes lack of light is the major cause of elongated, skinny stems plus that windowsill seedlings tend to be too tall with thin, bent stems.
How close should lights hang?
University of Minnesota Extension advises no more than 4 inches above seedling tops, with 2 inches ideal. University of Illinois Extension lowers a T5 fluorescent fixture so tubes sit a few inches from plant tops plus runs it 16 hours a day, with 18 hours the maximum for seedlings.
| Stage | Light | Temperature |
|---|---|---|
| Germination | Turn on bright lights once seedlings emerge | Warm mix speeds germination |
| Seedling growth | 2 to 4 inches above tops for 12 to 16 hours | Air above 60°F |
| Four or five true leaves | Same setup with less watering | Cole crops grow well at 60°F to 70°F |
| Hardening off | Gradually more sun plus wind outdoors | Withstands light frosts |
Source: University of Minnesota Extension plus Iowa State University Extension.
When are seedlings ready to transplant?
Iowa State University Extension advises sowing seed indoors 4 to 5 weeks before planting out plus notes large, older rootbound transplants are more likely to form small heads too early, called buttoning. University of Minnesota Extension notes plants reach four or five true leaves after about four weeks.
How to: step by step
- Sow seed one fourth to one half inch deep in cells 4 to 5 weeks before planting.
- Switch on bright lights once sprouts appear.
- Hang lights 2 to 4 inches above the tops.
- Run lights 12 to 16 hours a day on a timer.
- Pot up seedlings too big for their cells.
- Harden off before transplanting.
Troubleshooting
Tall thin stems: low light. Drop the lights to 2 to 4 inches.
Small early heads after planting: old rootbound transplants. Transplant at 4 to 5 weeks.
Stalled starts: crowded cells. Move up to pots 1 to 2 inches wider.
For related reading see grow lights for cannabis, leggy cannabis seedlings plus hardening off cannabis seedlings. Kind Seed Co also stocks seeds for indoor growers.
Sources
- Starting seeds indoors, University of Minnesota Extension
- Growing broccoli in home gardens, University of Minnesota Extension
- Growing broccoli in the home garden, Iowa State University Extension
- Starting a garden: supplemental lighting for indoor seed starting, University of Illinois Extension
FAQ
How long do broccoli seedlings stay indoors?
Iowa State University Extension advises sowing broccoli indoors 4 to 5 weeks before planting outdoors. University of Minnesota Extension notes seedlings reach four or five true leaves after about four weeks, at which point it advises reducing watering plus starting to move plants outside.
Can broccoli seedlings handle frost?
Iowa State University Extension notes cole crops such as broccoli can withstand light frosts without injury. University of Minnesota Extension notes broccoli continues to grow after the first light fall frosts. Harden seedlings off gradually first, since plants raised indoors have not been exposed to wind or full sun.
Do brassica seedlings need fertilizer?
University of Minnesota Extension advises a diluted starter solution once a week for broccoli seedlings, then fertilizer twice a week after two true leaves appear. Its general seed starting guide suggests diluted fertilizer once a week for seedlings in soilless mix. Avoid overfeeding.
Can I bury leggy stems?
University of Minnesota Extension advises setting broccoli transplants so the plant sits at the same level it was in its pot, so the sources here do not support deep planting. Prevent legginess instead by holding lights 2 to 4 inches above the seedlings plus raising them as plants grow.
How long should grow lights run for broccoli seedlings?
University of Minnesota Extension advises 12 to 16 hours of light daily plus a dark period each night. University of Illinois Extension runs seedling lights 16 hours a day plus gives 18 hours as the maximum for seedlings. A timer holds the schedule steady from day to day.
