Grow Light for Zinnia Seedlings Before Last Frost
A grow light for zinnia seedlings does most of the work before last frost. University of Minnesota Extension suggests hanging lights no more than 4 inches above seedlings, with 2 inches ideal, for 12 to 16 hours a day. It lists zinnias among flowers started indoors in mid April, 3 to 4 weeks before planting out.
The short answer
Hang a fluorescent or LED grow light 2 to 4 inches above zinnia seedlings plus run it 12 to 16 hours a day with a dark period each night. Start seeds about 3 to 4 weeks before planting outside. Harden seedlings off over two weeks, then plant them in full sun after the last frost date.
When should zinnias be started indoors?
University of Minnesota Extension lists zinnias with cosmos plus sweet peas as flowers started indoors in mid April for Minnesota, about 3 to 4 weeks before planting outside. NC State Extension notes zinnias can also be sown directly in the ground after the last frost date.
| Step | Figure | Source note |
|---|---|---|
| Start indoors | 3 to 4 weeks before planting out | Mid April in Minnesota |
| Light distance | 2 to 4 inches above seedlings | University of Minnesota Extension |
| Light hours | 12 to 16 hours daily | Leave a dark period each night |
| Upper limit | 18 hours | University of Illinois Extension |
| Air temperature | Above 60 F | University of Minnesota Extension |
| Hardening off | Two weeks before planting | Start in shade |
Source: University of Minnesota Extension plus University of Illinois Extension.
How close should lights be to zinnia seedlings?
University of Minnesota Extension suggests lights no more than 4 inches above seedling tops, with 2 inches ideal. Hang lights from chains so they can rise as plants grow. University of Illinois Extension notes fluorescent tubes produce very little heat, so they can sit a few inches above the plants.
How do you move zinnias outside without setback?
Harden them off. University of Minnesota Extension suggests moving seedlings outside two weeks before planting, starting with a few hours of afternoon shade out of the wind, then adding a little more time plus direct sun each day. UF/IFAS suggests planting after the last frost date with enough space, since crowding encourages disease.
How to: step by step
- Sow zinnia seeds indoors 3 to 4 weeks before planting out.
- Hang a fluorescent or LED light 2 to 4 inches over the seedlings.
- Program a timer for 12 to 16 hours of light.
- Lift the light as the seedlings grow.
- Harden off seedlings across two weeks.
- Plant in full sun after the last frost.
Troubleshooting
Leggy, floppy seedlings: too little light. University of Illinois Extension links elongated stems to low light intensity.
Scorched leaves after moving out: no hardening off. University of Minnesota Extension suggests two weeks of gradual sun.
Leaf spots or mildew: wet foliage. UF/IFAS suggests watering soil rather than zinnia leaves.
For related reading see grow lights for cannabis, cannabis seedling light schedule plus hardening off cannabis seedlings. Kind Seed Co also stocks cannabis seeds.
Sources
- Zinnia, UF/IFAS Gardening Solutions
- Zinnia elegans (zinnia), NC State Extension Gardener Plant Toolbox
- Starting seeds indoors, University of Minnesota Extension
- Starting a garden: supplemental lighting for indoor seed starting, University of Illinois Extension
FAQ
How many hours of light do zinnia seedlings need?
University of Minnesota Extension notes seedlings need 12 to 16 hours of light daily plus some dark period each night to develop properly, so avoid leaving lights on nonstop. University of Illinois Extension uses 16 hours of supplemental light, with 18 hours as the maximum for seedlings. A simple timer handles the schedule.
Can zinnias be sown straight outside?
Yes. NC State Extension notes zinnias are propagated by seed sown directly in the ground after the last frost date, in full sun plus loam or soil high in organic matter with good drainage. UF/IFAS notes zinnias are very easy to grow from seed plus also sold as small plants.
Is a windowsill enough for zinnia seedlings?
Usually not. University of Minnesota Extension notes seedlings do much better under fluorescent or LED lights than with natural light alone, even in a greenhouse. University of Illinois Extension notes indoor light cannot match the sun, plus low light gives leggy plants with stretched stems between leaves.
Which lights work for zinnia seedlings?
University of Minnesota Extension notes fixtures with two cool white fluorescent tubes give adequate light plus cost little, plus grow lights target ranges plants need. University of Illinois Extension favors T5 fluorescent tubes for efficiency plus notes LEDs convert electricity to light very efficiently. Tube output fades with use.
What temperature do zinnia seedlings need indoors?
University of Minnesota Extension suggests air above 60 F while seedlings grow, in a spot away from cold drafts plus excess heat. It notes a heat mat under the trays can help seeds germinate. Zinnias are warm season annuals planted out after the last frost.
