How to grow petunias from seed

Petunias are a garden staple, known for their colorful flowers and great for gardens or containers. They come in more colors and types than you could count! They do great in full sun but because they tolerate shade well they are often placed close to homes where they spent part of the day in the shadow of the house. 

Petunias growing from seed

I have been growing petunias for many years. I like using them in the garden but they're also great in planters. Since they spread out so well, they can be used as a pot filler. Their colorful blooms attract lots of pollinators like butterflies and bees. Since your vegetables need pollinators, it's a good idea to welcome add to the garden!

Petunias are tender perennials though can grow year round if the weather stays warm enough! They can actually live for 2 - 3 years! They are from the the Solanaceae family which is the same family as tobacco, chili pepper, tomato, potato and deadly nightshade. They have slightly fuzzy leaves with tubular flowers and grow from 6-18 inches tall!

Why grow petunias from seed?

Petunias are available in all plant nurseries, garden centers and big box stores so why would you want to make the effort of growing them from seed? Because growing from seed is sometimes the only option for uncommon varieties. Well, unless you want to spend a lot of money and time tracking them down!

Different colors and varieties are being invented constantly and saving petunia seeds is super easy to do. The newer, different varieties are often harder to find locally and since seeds can be ordered from all over the world, starting petunias from seed is the answer to growing some rare varieties. 

If you're on a budget then growing any plant from seed is cheaper then buying started plants from a store. Petunias are common enough that you can find packets of seeds in almost any store. It's one of the seeds I mention in 15 garden supplies from the dollar store

My new favorite is a purple petunia with white spots that almost look like stars! This one is called Night Sky. 

Purple and white petunias, Night sky

How to grow Petunias from seed

Petunia seeds are super small so it can be difficult to spread them out evenly. Because of this you will want to overseed and thin the seedlings once they germinate. Of course if you try to separate seedlings and pull apart tangled roots you run the risk of losing some...or most, of the seedlings. 

If you plant more seeds than you need though, you can simply trim off some of the seedlings at root level leaving the ones you want to grow.

If you decide to start your seeds indoors, start them about 4-6 weeks before you'll be moving them to the garden. I like starting petunias indoors or using a cover over them outdoors. Since you don't bury the seeds, birds tend to eat them if you don't protect them!

When starting inside, use a commercial potting mix, as garden soil can compact and make it difficult for the tiny seed roots to grow. Prepare your planting pots (or garden area) with soil and water well, then just drop the seeds where you want them. Do not cover with soil! Petunia seeds need light to germinate. Mist lightly with a little more water or use a sprinkler.

Water and place on a heat mat with a clear cover. You can use plastic wrap if your pots don't have a cover. It should take about 7 days for germination. Remove clear cover when sprouts appear. Water lightly every day till germination, then switch to every few days till well established. Transplant seedlings to their final spot in the garden when they are a few inches tall and all danger of frost has passed. 

Thin when plants are about 1" tall. You don't want to wait till they're larger because the plant will struggle after transplanting and you might lose more that way. Best to do it when they're small and haven't yet begun to tangle their roots with the next plant!

In the garden, space petunias about 8 to 10 inches from other plants. Petunias only take about 8 weeks from starting seed to blooming.

Petunia seedlings, sprouting from seed

Planting Petunias

Petunias grow best in full sun. They can tolerate partial shade but will bloom best in full sun. Choose a sunny location with a minimum of 5-6 hours of direct sun.

You do not need to deadhead petunias but they will certainly do better if you do! Also, the flowers close and dry up and the base of the flower is where the seeds are produced. So if you don't deadhead them, they will put energy into making seeds.

Petunia plants are heavy feeders and when you do fertilize them, you have to be careful not to give too much. This will cause the plants to grow taller, but not produce as many flowers. I add some compost at planting time and a weak fertilizer every 2 weeks or so during blooming.

If your plants are not producing many flowers, cut back the fertilizer for a few weeks and see if they start producing more.

Once established, the petunia will need thoroughly watered about once a week. Do not allow them to stand in wet soil for too long. They are heat resistant mildly drought tolerant. They can grow in almost any kind of soil, though they prefer well drained soil. 

This is one of the attributes that makes petunias such a popular addition to flower beds and gardens. They adapt well to almost any growing condition. They will need watered  a bit more in containers and raised beds though, as these tend to dry out quicker.

Petunias are a heavy feeder and will require weekly feeding while blooming to continue producing flowers. I like a balanced fertilizer, I believe my current one is a 10-10-10. You can also mulch with compost or feed compost tea weekly. Be careful not to overdo it as too much fertilizer will cause the leaves to turn yellow or brown.

Petunias are mostly an annual and will die off at the very first frost of the year. You'll want to pull out the plants once they start to die off. I have had pretty good luck growing petunias indoors over winter, so if you get some sprouts popping up towards fall, transplant them into pots to bring inside! 

If you happen to live in zones 10 or 11 though, your petunias will continue to grow all through winter!

Quick tips for growing Petunias

  • Petunias do not like being watered from above, they prefer to be watered at the base of the plant. Try not to get the foliage wet when watering.
  • Petunias are mostly pest free except for the occasional issue with whiteflies, thrips and aphids.
  • Mulch around the base of the plant when plants are small to keep soil moist. 
  • The most prevalent issue with petunias is caused by incorrect watering. Allowing the plant to have wet roots for too long can cause root rot or crown rot.
  • They do not require deadheading but it encourages more blooms, prevents self seeding and keeps the plant from getting too leggy.

How to save seeds from the Petunia plant

The seeds are formed in the base of the flower, so you'll want to forgo dead heading the flowers if you plan on saving the seeds. Since I prefer less common varieties of petunias, I always save the seed from my favorite plants. Most of these new colors are not easily available as seeds. By purchasing the plant once and saving the seeds, I can keep growing the same flowers every year! 

The base of the flower will turn brown and brittle and a small green pod will form. Once that pod turns brown and dried out, the seeds are ready for harvest. Simply pluck off the seed pod and crush it between your fingers releasing the seeds.

Seeds can be saved for next year. Allow seeds to dry for several days before storing, and store in paper envelopes (not plastic containers) to discourage mold. 

If you are looking for ways to make money off your garden,  petunias will produce a ton of seeds to sell. Of course if you start with a rare color or variety they will sell better.


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