Home HouseplantsCare & How To When To Start Zinnia Seeds Indoors

When To Start Zinnia Seeds Indoors

by a Friendly Gardener
Zinnia flower plant petals

With their gorgeous, colorful broad blooms, zinnias are popular additions to any home garden. These versatile and easy-to-grow flowering annuals can be used to completely transform patches of bare soil or to fill gaps in borders.

Zinnias come in a wide range of vibrant colors, heights, and shapes and there is a huge variety of zinnias right from small button blooms to tight beehive-shaped flowers, daisy-like blooms, and large floppy blooms that you can choose from.

These flowering annuals, which grow well in USDA Hardiness Zones 3-10 are summer flowers that can tolerate heat and bloom all through the summer months until the first frost.

The best way to start your zinnia garden is from zinnia seeds and we will discuss in-depth how and when to start zinnia seeds indoors.


How to Start Your Zinnias Indoors

Plants sowing tomatoes gardening

Zinnias are typically easy and quick-growing plants and are best grown by sowing seeds indoors or directly outdoors. Zinnias are extremely sensitive to frost and so, if you’re planting the seeds in the garden, it is best to wait until the last frost has passed in the area where you live.

If you’re starting zinnia seeds indoors, then you can do it during spring around 5-6 weeks before the last frost. You can start the seeds in 2.5-4-inch biodegradable peat pots with a starter potting mix.

Using sterile quick-draining potting soil containing perlite and vermiculite is the best for zinnias. The soil offers sufficient stability for the seedlings and also allows air pockets so that the roots get ample oxygen.

Harvest zinnia seeds

Once the soil gets warmer, you can plant the entire pot in the ground. This way the pots will break down in the soil, reducing the transplant shock and giving the zinnias a greater chance to become established.

To plant the zinnia seeds, first, sprinkle water on the soil to ensure that it is moistened uniformly. You can do this by simply setting the pots in water and allowing the water to seep into the soil.

Once the soil is moistened properly, remove the pots from the water and allow the water to drain. Sow around 2-3 zinnia seeds in each compartment and sow them around ½-inch deep.

Place a plastic wrap on the soil loosely or place the pots into a plastic bag. Don’t seal the plastic bag. Place the pots in a bright place that does not have direct sunlight. Mist the soil to ensure that it is moist until germination but make sure that the soil is not soggy.

Maintain temperatures between 75°F and 80°F. So, when to start zinnia seeds indoors in a greenhouse? If you’re growing zinnias in your greenhouse, then you must sow the zinnia seeds at the same time, using the same process we have described above.

If you’re using grow lights, you can place the pots under the fluorescent grow lights for around 10 days as soon as the zinnia seeds germinate. As the plants grow, raise the lights, keeping them around 3-4 inches above the plants.

You can alternatively grow the zinnias in a heated greenhouse.



When the temperature is maintained constantly between 75°F and 80°F, the zinnia seeds will germinate in around 5-7 days.

You can maintain a constant temperature without any fluctuations and moisture loss from the soil by placing the pots away from any cooling or heating vents. Water the seeds when the top ¼ inch of the soil turns dry and the color becomes light.

Add water to the pots very slowly to prevent the seeds from getting washed out. Drain any water that is standing in the tray under the pots to prevent the soil from becoming soggy and causing root rot or mold from growing.


Zinnia Seedling Care

Little plants sow grow

When the zinnia seedlings appear, you can remove the plastic cover. The seedlings need bright sunlight for around 6-8 hours a day, with nighttime temperatures of around 65°F-68°F. Watering must be done when the top 1 inch of the soil turns dry.

Once the seedlings are around 1 inch high, keep just one plant per pot. This will ensure that when the plants grow, they have adequate spacing.

Once the seedlings are around 3-4 weeks old, you can apply a liquid, water-soluble 14-0-14 or 5-10-5 potassium, phosphorus, and nitrogen fertilizer at 1/4th strength, once in 2 weeks.


Transplanting the Zinnia Plants

Zinnia flowers summer garden

When the last of the spring frost disappears and the soil temperatures are above 50°F, the seedlings can be transplanted outdoors into the garden.

But before you transplant the seedlings into the ground, to get the plants acclimatized to the outdoor conditions, first set the pots in an outdoor area like a covered patio, porch, or carport for a minimum of one week.

Transplant your zinnia plants in the garden in an area that gets plenty of direct sunlight. This will help to promote healthy and strong blooms. Make sure that there is adequate air circulation. This will help to prevent foliar diseases like powdery mildew.

Zinnias are quite adaptable and do well in most types of soil conditions, but the best soil for these plants is well-draining soil that is rich in organic matter. Ideally, the pH of the soil must be between 5.5 and 7.5.

It takes around 60-70 days for the zinnias to grow from seeds to flowers. Once the plants start producing flowers and dead-heading them frequently, this will help to produce more blooms.

All through the growing season, ensure that the soil is moist and water as needed to keep it so. Adding compost to the soil will help the zinnias to grow more quickly and produce more blooms.

Beatiful zinnia flowers

Once your zinnia plants begin to produce blooms, you can enjoy them all through the summer months right until fall when the first hard frost arrives. And, when the zinnia plant finishes producing blooms after the growing season, remove them to make space for other plants.

If you want to plant zinnias again next year, then you can allow the flowers to go to seed and collect and store them.


Wrapping Up

In conclusion, gorgeous, vibrant zinnias can turn your summer garden into a riot of colors. Although zinnias are versatile, fast-growing plants, starting the zinnia seeds indoors can help you ‌jumpstart the growing season earlier.

These plants are very sensitive to frost and so you must ensure that you plant them indoors well before the last spring frost. With good, well-draining soil and plenty of sunlight, soon you should have a garden full of beautiful zinnias.

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