How to Grow Petunias from Seed

Get more bang for your buck by growing petunias from seed instead of buying flats at the store! Learn how to grow petunias from seed here!

Every spring, I spend a fortune buying petunias for my containers and flower bed borders. While I love them for their prolific blooms and ability to spread out and cover an area, I don't love having to pay so much for each plant!

While the seeds of varieties like the Proven Winners Supertunias you see here aren't available for purchase, there are plenty of other types, including Wave petunias, that you can grow from seed! Here's how!

This post contains affiliate links for your convenience. Purchases made through these links may earn me a small commission at no additional cost to you.

Growing petunias from seed at a glance

Seed Starting Indoors	Start seeds indoors 8-10 weeks before last frost date.
Germination Conditions	Maintain a temperature of 70-75°F (21-24°C). Seeds need light to germinate.
Germination Time	7-21 days
Light Requirements	Provide bright light for 16 hours a day.
Watering Seedlings	Keep the soil moist but not soggy.
Transplanting Seedlings	Harden off seedlings by gradually acclimating them to outdoor conditions. Transplant after the danger of frost has passed.

Where to buy petunia seeds

Petunia seeds are harder to find than other flower or vegetable seeds. I've never seen them for sale in stores, but I've had good luck with seeds I bought online.

Baker Creek

Baker Creek specializes in rare heirloom seeds, and they have a selection of interesting petunia varieties that you probably have never seen before! I picked up two gorgeous frilly pink and white petunias that originally came from the Czech Republic: Karkulka and Greetings from Jaromere (Petunia superbissima).

Swallowtail Garden

Swallowtail Garden Seeds has become my go-to shop for those hard to find seeds that aren't sold anywhere else! They have a wide variety of mounding petunias and trailing petunias to choose from.

This year, I'm growing Skybox Rose Star and Shock Wave Purple Tie Dye, which I'm hoping will perform like the Supertunias I had in our front garden last summer.

What do petunia seeds look like?

Petunia seeds are tiny, black specks that are difficult to handle. You can see what the Baker Creek petunia seeds look like in the small bowl.

Because petunia seeds are so small, the Swallowtail Gardens seeds came pelleted. This water soluble coating makes them easier to handle, so you don't get too many plants in one spot accidentally.

Keep in mind that pelleted seeds don't save as well as bare seeds, so you'll want to plant them all in the first year to get the best germination rate.

When to start petunia seeds indoors

Petunia seeds should be started indoors 8-10 weeks before your last frost date. This gives your tiny seedlings plenty of time to put on growth before the weather warms up.

I started my petunia seeds this year on January 25. Technically, our last frost date is March 23, but I prefer to play it safe and wait until at least April 1 to plant out warm season flowers like these.

How to start petunias from seed

Seeding petunias in trays is easy, but there are a few things to keep in mind. Let's get growing!

Prepare the soil

Use a good quality seed starting mix, not potting soil. It's specially formulated to be lighter, so the tiny seedlings can push through easily. Mix it with water until it holds its shape when you squeeze it in your hand without any drips.

Fill each cell of a clean and sanitized seed tray with soil, then press down lightly to remove air pockets. Top off the tray with more soil as necessary.

Surface sow the seeds

Petunia seeds need light to germinate, so you can just sprinkle them lightly over the surface of the soil. I prefer to use a toothpick dipped in water to pick up each seed to make it easier to place.

Water the seeds lightly

With seeds this small, you risk washing them all away if you use a watering can to water them! I prefer to use a misting spray bottle to settle in the seeds.

After watering, place a humidity dome over the tray. Keep the dome on in between waterings until most of the petunia seeds have germinated, which usually takes between 7 and 21 days.

Keep your petunia seedlings warm

Petunia seeds need to be kept warm in order to germinate. Use a heat mat to keep the soil temperature between 70 and 75 degrees Fahrenheit. Even after germination, I leave them on the heat mat set to 68 degrees, since our basement is typically cooler.

Fertilize once a week

Typical seed starting soil doesn't have any nutrients, which is fine when your seeds are just starting to germinate. But after they put on their first set of true leaves, your seedlings will need a weekly dose of half strength fertilizer to continue growing strong.

I use a quarter teaspoon of this liquid fertilizer mixed in a half gallon spray bottle. Use the diluted fertilizer in place of your daily watering once a week.

What do petunia seedlings look like?

The two Baker Creek petunia varieties sprouted just four days after planting. The Wave petunias took a little longer, popping up twelve days after I sowed the seeds.

After four weeks, this is what my petunia seedlings look like! They're doing great!

As you can see, I have a little bit of algae growth on the surface of the soil. While it's not a huge issue, I'll be sprinkling vermiculite mixed with cinnamon to the top of each cell when I pot them up to prevent it in the future. This will absorb excess moisture and prevent the algae from growing.

After a few more weeks of growth, I separated out the cells with more than one plant so they don't compete for nutrients and light. I had almost 100% germination on these petunia seeds, and I don't want to lose any of them by thinning them out!

When I potted them up into individual peat pots with regular potting soil, they just exploded with growth! After a week, they started to bloom!

When to transplant petunia seedlings

Wait until after your last frost date to transplant petunias. They don't like the cold, and prefer for their soil to be at least 60 degrees to avoid shocking the roots. Use a soil thermometer to test the temperature in the area you want to plant your petunia seedlings.

I'll add a tutorial on how to transplant petunia seedlings once our weather warms up! Stay tuned!