How Much Light Do Petunia Seeds Need to Germinate?

Blooming Petunias are a sure sign that summer is just around the corner. Petunias come in various colors and bloom continuously, so they’re great for outdoor planters and hanging baskets. 

Part of their popularity also comes from Petunias being super easy to grow. You can get a head start on your summer garden by germinating Petunia seeds indoors in the winter. But since winters can be dark, ample light is a major consideration. 

Unlike cucumbers and marigolds that can germinate in the dark, Petunia seeds need light to germinate. Petunia seeds need 12 hours of indirect sunlight or light from a grow light per day and to be placed into soil but not covered, in order to germinate.

Do Petunia Seeds Need Light To Germinate?

Some seeds only need contact with the soil for germination. The seeds can either sit on top of the soil or get planted underneath. Energy from light is not necessary because the seeds rely on the energy stored inside the seed to grow. Seeds that germinate like this include cucumber seeds, melon seeds, and marigold seeds. 

Other seeds require a constant light source to produce enough energy to sprout. If these seeds are covered, they will stay dormant until they receive enough light. Plants that fall into this category include Geraniums, Poppies, and Petunias. 

How To Germinate Petunia Seeds

You can always buy Petunias from the garden center at the start of the season. But growing your own Petunias from seed can be very rewarding! All you need is some soil, seeds, and a light source.

Firstly, choose a growing container with drainage holes. Fill it with soil but leave a 1-inch gap from the top. Submerge the pot in water to moisten the soil, then let the excess drain off. 

Next, sow the Petunia seeds into the soil, but don’t cover them since they need exposure to light. Petunia seeds are very small and delicate, so all they need is a slight nudge with the end of a pencil to push them into the soil. 

Petunia seeds need warmth to germinate successfully, so keep the temperature at 75-80°F (23-26°C). Direct sunlight can inhibit germination, so a warm, bright room in your home is the best location for your seeds. Avoid window sills as they tend to be cold. 

But what if you’re starting your seeds in the winter when the daylight is sparse, and the weather is overcast? In this case, a grow light can help give your Petunia seeds the light they need. Set up your grow light 3-6 inches above the seed containers

Whether you use sunlight or a grow light, ensure that the seeds are exposed to at least 12 hours of light a day.

How Often Should I Water Petunia Seeds?

Watering seeds
Petunia seeds need water during germination to help absorb nutrients from the soil. Without these nutrients, the seed won’t develop. Once a seed absorbs enough water, the embryo becomes too big, and the shell bursts open. This is the seed turning into a seedling. 

The warmth from the sun or grow lights will dry out the seeds, so constant moisture from a spray bottle is essential. You can even cover your containers with a plastic bag or saran wrap to lock the moisture, just like a greenhouse. 

How Long Does it Take to Grow Petunias From Seeds?

The time it takes for your tiny Petunia seeds to germinate (sprout) can be anywhere from 7 to 10 days. Germination is only the start of the process, though.

After germination, the seedlings need up to 16 hours of light from the sun, or a grow light. This will help them grow strong stems and eventually flower.

The total time it takes for a Petunia to grow from a seed to sprouting flowers is 10 to 12 weeks. After 12 weeks, you can move your Petunia plant outdoors and replant it in a hanging planter. 

What is the Best Month to Grow Petunia Seeds?

Nothing is stopping you from starting your Petunia seeds in the middle of winter if that’s when you want to. As long as you have the correct indoor environment for growing, including enough light and warmth.

Always ensure that the last frost is well over before you plant your Petunias outdoors. The last frost is different depending on where you live. Generally, the last frost can be anywhere from February in warmer climates to early May in cooler climates

So, you can start germination indoors before the last frost; just ensure the Petunias don’t go outside until after the last frost. 

What Do I Do If My Seeds Don’t Germinate? 

If it’s been two weeks and your Petunia seeds haven’t germinated, something has gone wrong. There are a couple of reasons why seeds don’t germinate. 

Water is a big factor in whether or not a seed will germinate. Too little water and the seed will stay dormant; too much and the seeds may rot. 

Seeds will also stay dormant if the temperature is too cold. Seeds should generally be kept above 50°F (10°C). Petunia seeds prefer it even warmer at 75-80°F (23-26°C).

And lastly, without proper light, Petunia seeds will not germinate. Make sure they get at least 12 hours of light daily through a grow light or natural indirect sunlight. 

Other common errors with germination are poor soil quality, old or improperly stored seeds, or the seeds were not planted deep enough in the soil. 


Do Petunias Like Coffee Grounds?

Coffee grounds
Petunias are continuous bloomers, which means that as their flowers die off during the summer, more will bloom. Because of this constant blooming, Petunias need lots of fertilizer to feed from. 

Coffee grounds are an excellent homemade fertilizer. They provide essential plant nutrients such as nitrogen, potassium, and phosphorus. 

But coffee grounds are also very acidic, so they will only benefit plants that like acidic soil. Petunias prefer slightly acidic soil with a pH of around 5.0 to 6.0, so feel free to use those old coffee grounds!

How Long Will Petunias Live Indoors?

Although Petunias are generally planted outdoors for the spring and summer seasons, they can also be grown indoors. This is perfect if you want to keep your Petunias alive until next summer and avoid going through the growing process again. Bringing outdoor plants indoors for the winter is called ‘overwintering.’

The flowers of your Petunia should drop off before you bring your plant inside for the winter. Over time the leaves will likely fall off, but don’t fret! Keep your plant in a sunny spot, and add grow lights if you find you need more light during the darker months. At least 10 hours of light is necessary to keep the plant from going dormant. 

Petunias can survive up to three years when they are kept alive indoors during the winter.

Do Petunias Like Wet Soil?

Petunias are pretty tolerant to heat. As such, they can handle having dry soil for short periods of time. A thorough watering once a week is plenty for a Petunia plant. Ensure you water deeply since shallow watering encourages the roots to grow shallowly too. 

Petunias in planters will require more water than Petunias planted directly in the ground. This is because planters drain more quickly, and there’s less soil to hold in the moisture. 


Petunias are vibrant, colorful flowers and a great choice for outdoor planters. Growing these flowers from seed is easier than you’d think and quick too! To ensure successful germination, you should sow Petunia seeds but not cover them entirely and expose them to indirect sunlight or a grow light for a minimum of 12 hours a day. 

If your green thumb is urging you to get gardening before the last frost, you can start your Petunia seeds indoors. Due to the lack of sunlight in the cooler months, you may need to use a grow light. 

Ensure the seeds aren’t covered by soil so that they can absorb energy from the light. Give the seeds a minimum of 12 hours of light a day, and they should germinate in about 7-10 days.