Why Tomato Seedlings Turn Purple and How to Fix It

Purple-tinged tomato seedlings are common under lights—and usually temporary.

Common Causes

• Cool temperatures (below 60°F) + high light = anthocyanin buildup (purple pigment).
• Phosphorus deficiency in soilless mixes (especially under high light).
• Genetics: Some varieties (especially dark-fruited types) naturally show purple.

What to Do

1. Warm up: Keep at 65–75°F. Use a heat mat if needed.
2. Feed lightly: Half-strength fish emulsion or balanced liquid fertilizer (e.g., 10-10-10) once true leaves appear.
3. Don't panic: Purple fades as temps rise and roots establish.

Prevention

• Start seeds 6–8 weeks before last frost (not earlier).
• Use a quality seed-starting mix with starter charge.
• Provide 14 hrs light, but keep lights 3–4 inches above plants.
• Transplant to larger cells at first true leaf to avoid nutrient lockout.

Sweet Potato Note (Follow-up)

In Zone 3 (Montana), sweet potatoes are marginal. Try short-season slips (90–95 days) like 'Beauregard' or 'Georgia Jet'. Start slips indoors: half a sweet potato in water, harvest 6-inch sprouts, root in water, plant when soil hits 70°F. Contact Montana State Extension for local trials.

Purple tomatoes? Usually just a phase. Warm them, feed them, and they'll green up.