Zinnias (Zinnia elegans) are some of the easiest annuals to grow, but they die with the first frost. Their growing season is short in many climates, so you must time your plantings carefully. Protect the seedlings from the cold weather that spells doom for warmth-loving zinnias. Planting seeds indoors gives the plants a head start, but you will need to take measures in moving them outdoors because they don't handle transplanting well.
In deciding when to plant your zinnia seeds, check the last frost date for your area. Sow seeds indoors about four to five weeks before the last expected frost. Even if you live in a warm climate where frost is not an issue, zinnias will not thrive outdoors if temperatures fall below 60 degrees Fahrenheit. Starting them indoors will give the fast-growing zinnias time to develop while the weather warms. Seed company Burpee warns against starting plants sooner because it can result in overcrowding and spindly flowers.
Zinnias love warmth and will not grow well at temperatures below 50 F. Zinnias usually take anywhere from five to seven days to germinate but flowers can take up to 70 days after germination. As a result, starting zinnia seeds indoors where you can keep them warm, up to 30 days before you plan to move them outside, may be your best hope of seeing spring flowers. If you wait until later in spring to plant the seeds, your flowers may not bloom until summer. If overnight outdoor temperatures remain below 60 F well into spring in your area, starting indoors, where you can control the temperatures, may be your best chance at zinnia flowers. For a longer supply of colorful flowers, you can plant them in succession, as frequently as every other week.
Soil temperature is a key factor in germination. Zinnias germinate best at 70 to 80 F, so when starting the seeds indoors, it is important to maintain the soil temperature using heating pads, insulators, heaters or grow lights. The seeds should germinate in five to seven days.
It is best to start zinnia seedlings in peat pots because you can plant the peat pot directly into the ground without disturbing the zinnia's roots. Zinnias generally do not like the upset of being transplanted. Keep the zinnia seedlings indoors until overnight temperatures are reliably and regularly above 60 F. Harden the young plants off, starting two weeks before the last frost date, by placing them outside in a cold frame or on a porch, patio or balcony during daytime and bringing them indoors at night or covering with the cold frame sash. By progressively increasing the time outdoors, you help your zinnias adapt to outdoor conditions.