Should You Start Zinnias Seeds Indoors? Yes, And Here’s Why

pink zinnia in foreground, other zinnias in background in garden

Zinnias are one of the most popular annual flowers, prized for their showy blooms and long flowering season. Though they’re not difficult to grow from seed, starting zinnia seeds indoors does have some advantages.

Starting zinnia seeds indoors will give your plants a head start on the growing season and result in more robust plants sooner. Larger transplants can withstand pest damage more easily than newly germinated seeds, and they’ll likely bloom sooner than direct sown zinnias.

Not only will you have stronger, healthier plants, but with transplants, you’ll be that much closer to having a fresh bouquet of zinnia blooms on your kitchen table. What a way to ring in summer!

Zinnia started indoors withstand pest damage more easily

No matter what region you live in, you probably have snails, slugs, or earwigs just waiting for your zinnias to be planted in the garden. These destructive creatures are particularly fond of newly germinated seedlings and can quickly decimate a row or flat of seedlings in just one night.

While zinnia seedlings are still susceptible to pest damage, the larger plants have more of a chance to survive than a tender sprout with just one or two leaves. If a snail or earwig does take a nibble out of a zinnia seedling, it’s less likely to kill the plant outright.

Once you do set out your seedlings, apply an organic bait such as Sluggo to give your zinnias the best chance of avoiding slug, snail, or earwig damage.

Give seedlings the advantage over weeds

If your flower beds tend to become overrun with weeds, then starting your zinnia seeds indoors might be the best way to avoid them getting lost in a sea of weeds.

Weeds are constantly vying for the same resources as your flowers, and they have a few advantages over your seedlings. The weeds have already germinated, and they’re likely to be the same size or larger than your seedlings.

Transplanting zinnia seedlings that have been started indoors puts them on a level playing field with the weeds. The larger, more established plants can better compete for resources, and they’re less likely to be overwhelmed by weeds.

Once you set out your transplants, make sure to apply a layer of mulch about three inches thick around your plants. The mulch will help prevent new weed seeds from germinating as it blocks sunlight from reaching the soil.

Woodchips are my favorite for mulch, but if you have grass trimmings or leftover fall leaves, those make great mulch, too.

Get a headstart on the season by sowing indoors

Another advantage to starting zinnia seeds indoors is getting a jump on the blooming season. By transplanting larger, more mature seedlings, you can have blooms up to up to four weeks sooner than if you direct sow your seeds.

This headstart is especially beneficial if you live in an area with a short growing season or want to extend your zinnias’ blooming period by planting a second crop later in the season.

This method is called succession sowing, and it’s a great way to get blooms earlier in the year and keep them going longer through the season.

Flower farmer Floret talks about sowing zinnias three times through her growing season to maximize the number of blooms she can harvest.

Imagine if you did the same, starting three batches of seeds at the start of the season.

  • Batch 1: indoors, four weeks before your last frost date
  • Batch 2: indoors, one week before your last frost date
  • Batch 3: outdoors, two weeks after your last frost date

This planting schedule would not only give you blooms a month earlier than the direct-sown ones, but it would also give you a side-by-side comparison of zinnias started indoors and out to see if you have a preference for either method.

Not sure what your last frost date is? Check it out here by using your zip code.

Starting zinnias indoors is easy

Last but not least, starting zinnias indoors is no more complicated than direct-sowing them outside. The only special gear you’ll need is a grow light to provide artificial sunlight to the new seedlings, which can be as simple as a garage shop light.

tray of zinnia seedlings
These zinnia seedlings germinated in less than a week, and there are a few more seeds left to pop up.

To start zinnia seeds indoors, fill pots or seed trays with a seed starting mix and sow 1-2 seeds per cell. Cover the seeds with 1/4 inch of soil and gently press them into the soil. Water the seeds well and place them in a warm spot.

Zinnia seeds will germinate quickly if the surrounding temperature is at least 60 degrees, so try putting the seed tray on top of the fridge, in the laundry room, or somewhere else out of the way but warm.

You don’t need to turn on the grow light until they’ve started to germinate, at which point they’ll need about 16 hours per day of light. A timer can control the lights, so you don’t have to remember to turn them on or off.

You can fertilize the seedlings once they get a set of true leaves, but even if you don’t, they’ll grow just fine. The seedlings will only stay indoors for four weeks before you can plant them out, so they’ll have enough resources to grow well even without fertilizer.

If you’re looking for a more detailed guide to starting zinnia seeds, check out this article, Germinate Zinnia Seeds Indoors: Tips and Tricks For Success.

If you start any other seeds indoors already, then try a tray of zinnias to see if it doesn’t pay off with earlier blooms and fewer seedlings lost to pests.

FAQ

Can you bury leggy zinnia seedlings?

If your zinnia seedlings haven’t been under strong enough lights, they may become what is known as “leggy.” Leggy seedlings have tall, thin stems that are noticeably stretching to get closer to a light source.

If your zinnia seedlings have become leggy, go ahead and bury the stem deeply when transplanting them to the garden. The soil around the stem will help support the seedling, preventing it from falling over and possibly breaking, which would kill the seedling.

Should you thin zinnia seedlings when transplanting?

If the zinnia seedlings are spaced too densely in their nursery pots, you should thin them during the transplanting process as long as the seedlings haven’t gotten too big. If the seedlings are under 2-3 inches tall, their root system should be compact enough to allow for separation without damaging the roots.

Should zinnia seedlings be pinched out?

Zinnia seedlings should be pinched, or cut, back when they are 6-10 inches tall. Pinching out the top of the seedling will slow upward growth and encourage side growth through the development of lateral branches, each of which will then produce an abundance of flowers.

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