How and When to Plant Zinnia Seeds Indoors or Out

Zinnias make fantastic cut flowers and companion plants, and their rainbow-hued blooms fit right into garden borders, flower beds, and patio planters. These annual flowers need to be replanted every spring, and knowing when to plant zinnia seeds—either indoors or out in the garden—is your key to success. This step-by-step guide on how and when to plant zinnia seeds will help you time your sowing right so you can grow healthy plants that will bloom for months.

When to Plant Zinnias

There are about 20 species of zinnias and countless cultivars that vary in shape, size, color, and bloom time. But despite all that variety, most zinnias have the same basic care instructions and flower about 60 to 70 days after planting.

Seed catalogs usually recommend direct sowing zinnia seeds outdoors in garden beds or containers after your last frost date of spring, which usually occurs in April or May. But zinnia seeds can be started indoors about 4 to 6 weeks before your last frost date if you want to get flowers a little earlier. You can also succession plant zinnia seeds every few weeks from spring through mid-summer to extend your bloom season even longer.

How to Plant Zinnias

Whether you start zinnia seeds outdoors or indoors will depend on where you live and your personal planting style. Outdoor sowing is generally easier, but indoor sowing can come in handy if you live in a cold climate and want zinnias to bloom earlier. Just keep in mind that zinnias don’t like to have their roots disturbed, so you’ll need to handle indoor-started zinnias carefully when transplanting.

Starting Zinnia Seeds Indoors

Follow these simple seps to start your zinnia seeds indoors so you can get a head start on the growing season.

1. Prep Your Containers
   To start, fill up seed starting trays, pots, or even a cardboard egg carton with a pre-moistened seed starting mix. If possible, choose biodegradable growing containers that can be planted with your zinnias.
2. Sow the Seeds
   Plant 1 to 2 zinnia seeds per pot or seedling cell and lightly scatter a fine layer of vermiculite or seed starting mix over the seeds. Zinnias need light to germinate and they should not be buried deeply.
3. Add Light and Water
   Cover the pots or seedling trays with a clear plastic cover or humidity dome, move the seeds under a grow light, and water regularly to keep the soil evenly moist. Adding a heating mat beneath your pots can help zinnias sprout faster, but most zinnia seeds should generally germinate in about 4 to 7 days.
4. Thin Out Seedlings
   As soon as your seeds sprout, remove the humidity dome, turn off the heating mat, and continue to water regularly. If you planted more than one zinnia seed per pot, thin the seedlings out when they have true leaves and are a few inches tall by pinching off the weakest plants.
5. Harden Off Seedlings
   When the weather warms and the danger of frost has passed, harden off your zinnias for 1 to 2 weeks before transplanting them into beds or pots.
6. Transplant Seedlings
   Bury zinnia root balls at the same depth the plants were growing in their indoor containers and bury biodegradable pots along with your plants to avoid disturbing delicate zinnia roots.

Sowing Zinnia Seeds Outdoors

Zinnias are among the easiest annual flowers to sow outdoors. Use the steps below to ensure your success.

1. Prep Your Soil
   Before sowing zinnia seeds, prepare your garden by removing weeds and working compost or aged manure into the soil. If you’re growing zinnias in pots, choose a well-draining container that’s at least 8 to 12 inches wide and fill it with a rich potting mix.
2. Plant the Seeds
   When the weather warms and temperatures are consistently above 70°F, scatter zinnia seeds over prepared garden soil or plant them in rows or pots. Then, lightly cover the seeds with no more than ¼ inch of earth. If you’re growing zinnias in rows, space the rows about 2 feet from each other.
3. Add Water
   Water zinnias well after planting and continue to water regularly until the seeds sprout.
4. Thin Seedlings Out
   When the seedlings are about 3 inches tall, thin out the weakest seedlings so that the remaining plants are properly spaced for the type of zinnia you’re growing. This is a great time to add a light layer of mulch too.

Tips to Encourage Zinnias to Bloom More

Planting zinnias at the right time and in the right way is step one. But if you want zinnias to bloom their best, here are a few simple zinnia care tips to keep in mind.

• Choose a full sun location: Zinnias need at least 6 to 8 hours of sun per day to bloom. If they don’t receive enough light, they won’t flower as well, or possibly at all.
• Water well: Provide zinnias with about 1 inch of water per week. Water the plants at the soil line if possible, as the plants can develop root rot in soggy soil or powdery mildew if their leaves stay wet.
• Add fertilizer: Working compost into the soil before planting helps zinnias grow better. These plants also appreciate light applications of low nitrogen fertilizer applied throughout the growing season.
• Stake tall plants: Adding bamboo stakes or tomato cages around tall varieties of zinnias can keep their slender stems from flopping over.
• Prune wisely: If you want to boost blooming, wait until zinnias produce their first flower buds, and then pinch all the buds off your plants before they open. Pruning in this manner and deadheading old zinnia flowers when they fade encourages zinnias to branch and bloom more.