What to do now
Mildest step first.
- Move trays off cold windowsills and floors; seedlings grow best around 60–70°F.
- If they're still in the seed tray with true leaves, pot them up into fresh mix.
- Water with a half-strength balanced soluble fertilizer once the second set of true leaves opens.
- Watch the new leaves: if they come in green, the problem is passing.
Why it happens
Purple coloring on seedlings usually shows on the undersides of leaves, along the veins, and on the stems. In tomatoes, peppers and cabbage-family plants it most often points to phosphorus, but rarely because the mix lacks it. Texas A&M AgriLife Extension notes that tomato roots growing in cold soil take up phosphorus poorly, and that a cool root zone is the most likely cause; a true shortage is more likely in containers, hydroponics and some sandy soils. University of Maryland Extension adds that nutrient deficiencies in seedlings are common in early spring because the roots are small and cool mix slows the release of nutrients.
When purple is normal
Some purple is normal. Maryland notes that cabbage-family plants, including broccoli, cabbage and kale, often show red to purple color on leaves and stems, and that it gets stronger when they're stressed, especially transplants left in the same container for more than 5 or 6 weeks. Maryland's Ask Extension experts also say some purpling of stems and leaf undersides is very common in tomato transplants grown under lights, and that plants typically grow out of it after they're planted out. If the seedlings are otherwise stocky and growing, there's little to fix.
Warm the roots
Start with temperature. Iowa State suggests keeping seedlings around 60 to 70°F after they sprout. Windowsills are a common trouble spot: University of Maine Extension notes that seedlings there face very cold nighttime temperatures that stress the plants, so move trays away from the glass at night or onto a shelf under lights. Concrete floors and unheated rooms have the same problem. A helps with germination, but take seedlings off it once they're up.
Feed and pot up
Then look at food and space. Many seed-starting mixes carry little or no fertilizer, and what's there runs low after several weeks. Maine suggests watering with a half-strength water-soluble fertilizer once the second set of appears, and Texas A&M says a balanced liquid feed usually clears phosphorus purpling in pots. Seedlings that have filled their cells with roots also benefit from into fresh mix. Don't overdo it: strong fertilizer on small seedlings can burn roots and cause soft, growth.
If it doesn't clear
Purpling that stays after the seedlings are warm and fed, or that comes with stunting, yellowing or spots, is worth a closer look. Check whether the whole plant is small and dark, which fits phosphorus, or yellowing from the bottom up, which points to nitrogen. Transplanting purple seedlings into cold spring soil can bring the color back, so harden them off and wait for warmer soil before planting out.
- TomatoThe warm-season anchor of the summer garden.
- PepperA tropical perennial grown as an annual — patient, slow, and particular about warmth.
- EggplantA heat-demanding nightshade that needs more warmth than the tomato next to it.
- BroccoliA cool-season brassica that heads up for a short window and then turns on you.
- CabbageA long-maturing head that splits if you water it wrong at the wrong time.
- Why do my seedlings or young plants have purple or reddish undersides on their leaves?Purple or red coloration on leaf undersides and stems is a common sign of phosphorus deficiency, and cold soil in early spring is often the trigger even when phosphorus is present.
- Why are the first leaves on my seedlings turning yellow?The first two leaves (cotyledons) yellow and drop naturally once true leaves take over — this is normal unless the yellowing is moving up into newer growth.
- When should I pot up seedlings?Soon after the first true leaves open, or when leaves from neighboring cells start to overlap and roots show at the drainage holes. Move tomatoes, peppers and eggplant into pots about 3 to 4 inches across. Squash, cucumbers and melons are better sown directly in their own pots and not moved until planting out.
- Why are my seedlings tall, pale, and floppy?Seedlings stretch toward inadequate light — the usual fix is moving the light closer, to about 2–6 inches above the leaves, and running it 14–16 hours a day.
- How do I harden off seedlings before transplanting?Gradually expose indoor seedlings to outdoor conditions over 7–10 days, starting with an hour of shade and building up to full sun and overnight temperatures before planting.
- How close should grow lights be to seedlings?Fluorescent tubes work best 2 to 4 inches above the tops of the seedlings. LED fixtures can usually hang farther away, often 8 to 12 inches, but follow the maker's guidance for yours. Run the lights 14 to 16 hours a day and raise them as the seedlings grow.