How to Sow Petunia Seeds

The petunia is a popular member of the summer flower bed. Long lasting and colorful, these attractive flowers are also deer resistant. At their most fragrant in the evening, a bed or pot of these flowers attracts evening and nighttime pollinators such as night-flying moths to your garden.

Typically purchased as plug plants ready for transplanting into the garden, petunia seeds are actually surprisingly easy to germinate. While it can be a long process, seeds can require up to 10 weeks of steady growth before they are ready for transplanting, growing from seed has many advantages. Firstly, it can be cheaper. It also allows you to get more for your money. Additionally, growing petunia seeds gives you access to a wider range of varieties.

If you have the time and a little space you may want to try germinating your own petunia seeds. If so, this is your complete guide to sowing and growing petunia seeds.

1 Bright and colorful petunias

Bright and colorful, the petunia is a staple of the summer flower garden. They are also pleasingly easy to grow from seeds. 

Different Petunia Varieties

Before we start to discuss how to handle and germinate petunia seeds I will just briefly explain the different petunia varieties currently available.

Modern varieties are derived from Heritage specimens. These are native to South America and have been cultivated since the early 19th century. Pest resistant and fragrant they remain a popular choice. One of the most popular heritage varieties is Old Fashioned Vining. This variety happily reseeds and tolerates both heat and cold temperatures. It reaches around 24 inches and flowers well into the fall.

There are currently 4 main petunia varieties commercially available:

  • Grandiflora
  • Multiflora,
  • Milliflora,
  • Spreading or Wave.

The Grandiflora variety was developed in the 1950s. Typically flowers are 5 inches wide. Preferring moderate conditions they tend to struggle from midsummer onwards. Grandiflora also struggle in extreme heat and moisture.

For something a little different the Frost Series produces bright and showy flowers with a white frosting. Bushy plants, they typically reach 14 inches tall and 16 inches wide.

Double Cascade is another Grandiflora variety which, because of its large carnation-like double flowers, is increasingly popular. This plant is ideal for pots, planters, boxes and hanging baskets.

Popular Grandiflora series plants include:

  • Daddy,
  • Dream,
  • Storm,
  • Supercascade,
  • Supermagic,
  • Ultra.

Multiflora types are smaller than Grandiflora plants but tend to produce more flowers. Varieties can be single or double flowering. Their strong stems mean that the plants cope well in exposed, breezy positions. Multiflora flowers last longer than Grandiflora blooms, especially in wet conditions.

A popular dwarf multiflora, the Mambo series handles difficult conditions well. The large flowers, about 3 and a half inches wide, are both colorful and elegant. Mature plants can reach between 6 and 8 inches in both height and spread. A reliable choice, the Mambo series continues to flower until the first frosts hit.

Evening Scentsation is a good multiflora container choice. Its showy, fragrant blooms fill late afternoons and evenings with a pleasing aroma.

Other popular Multiflora varieties include:

  • Carpe,
  • Celebrity,
  • Horizon,
  • Mirage,
  • Primetime.

2 Different varieties
The many varieties on offer means that you are certain to find something to suit your garden.

Milliflora varieties may be smaller but they are also heavy flowering. Typically flowers are 1 to 1 and half inches wide on 8 inch plants. Milliflora varieties tend to flower early. They are also low maintenance and don’t require deadheading. Milliflora varieties are a popular choice for pots and hanging baskets.

Popular Milliflora varieties include:

  • Fantasy,
  • Picobella.

Spreading or Wave plants are the newest cultivars to hit the market. This means that they are more expensive than other options. However, the plant’s vigorous growth habit and colorful flowers mean that it is worth the cost. They are also long lasting, flowering from early spring until the first frosts hit.

Producing 2 inch wide flowers, the plants have a spread of between 2 to 4 ft. This makes them a good ground cover choice. Like other types they tolerate heat and drought well and don’t require deadheading. Silver Tidal Wave is also a popular draw for hummingbirds.

Popular Spreading varieties include:

  • Avalanche,
  • Easy Wave,
  • Shock Wave.

How to Sow Petunia Seeds

This is a heat loving, summer flower. It struggles in cold temperatures.

In northern areas, start your petunia seeds undercover in early March. In southern areas you can start petunia seeds undercover a couple of weeks earlier.

Petunia seeds are small and difficult to handle. It can also be easy to over plant the seeds meaning that if they all germinate you end up with way more flowers than you actually want. Some strains are sold in pellets. This makes handling easier. Otherwise, why not try sowing with a seed dispenser? The Coolrunner Seed Dispenser is adjustable, enabling you to sow a range of different sized seeds with ease.

Petunia seeds enjoy a long growth period. This means that they are best started undercover in trays or pots filled with moist, seed starting mix. Remember to clean or sterilize your trays before using them. You can also start your petunia seeds in eggshells.

Sow the petunia seeds as thinly as possible. If you are sowing in pots, aim to sow no more than 3 per pot. Lightly cover with a thin layer of potting soil.

For germination to be successful the petunia seeds must be kept warm and moist. Cover the pots or trays with a lid or place in a plastic bag. This helps the temperature to remain constant. Ideally, the temperature should average between 64 and 75 ℉. MIXC Seed Trays come with a lid and adjustable vents, enabling you to precisely control the heat and humidity levels around your developing seedlings.

Place your trays in a light position, away from direct sunlight. Germination usually occurs within 7 to 10 days.

Germination only happens if the petunia seeds get the right amount of light. Grow lights or fluorescent lights can be used if you struggle to provide enough natural light. Remember to keep moving the lights upwards as the seedlings grow. Artificial lights should always be 6 inches above growing leaves

Caring for Growing Seedlings

Following germination treat petunia seeds like tomato seedlings, or indeed any Solanaceae seedlings. Once seedlings have emerged, remove the cover and move the tray to a cooler spot. The temperature should be around 65 ℉.

Allow the seedlings to continue developing in position. Apply a water soluble fertilizer once every two weeks. Water when soil dries out. A plant mister is a useful way to keep the soil wet without disturbing developing seedlings. If you have sown a few petunia seeds per pot, or a clump of seedlings are developing too closely together, prick out weaker seedlings when they are large enough to handle.

Once the seedlings have developed two or three true leaves they can be transplanted into small, individual pots. Fill the pots with fresh seedling or potting soil. Be careful when transplanting not to overly disturb the root system. When transplanting I like to use a small spoon or a butter knife to scoop the seedling root system and some of the soil out in one go. Plant to roughly the same level as in the seed tray.

Continue to grow your seedlings on undercover until you are ready to transplant out into the garden.

3 Protect from frosts
Grow on undercover until the last frost has passed.

Transplanting Outside

While these are reliable flowers you should wait until the last frost has passed and temperatures have started to warm up before transplanting outside. Don’t forget that you need to harden off your seedlings before transplanting.

Don’t transplant seedlings too early. These flowers struggle in cold weather and are unlikely to grow or develop.

Petunia seeds develop rosettes of foliage as they develop. When the foliage rosette covers most of the soil move the pot to an outdoor cold frame or unheated greenhouse to start hardening off. As long as they are in a frost free position, the plants will be happy.

As the seedlings harden off, use the time to prepare the planting site, making any necessary amendments. Ideally the soil should be nutritious and well draining.

If you are transplanting into pots, make sure they are clean and filled with fresh, lightweight potting soil. Work a slow release fertilizer into the soil before planting. There should also be lots of room for the flowers to spread and develop. Overcrowding flowers can cause their growth to become stunted and flowering to fail.

Transplant into either a full sun or partial shade position. A partial shade is better in hot climates, because it helps to keep plants cool.

4 Transplant with care
When transplanting try to lift as much of the roots and surrounding soil as possible. 

To transplant, make a hole in the soil large enough to hold the root ball of the plant. Carefully lift the plant from its container. Again you can use a butter knife to help. Plants growing in biodegradable peat pots or eggshells can be planted still in their containers. The container then breaks down as the plant develops. Aim to plant to roughly the same depth as in the pot.

After transplanting water well and apply a petunia fertilizer. This helps plants to settle and also stimulates growth throughout the growing season.

Space the plants 7 to 18 inches apart depending on their variety. The exact spacing for your particular variety will be on the back of the seed packet. If you are planting in pots, try not to overcrowd the plants. 3 plants per 12 inch pot is ideal. The young plants may look small but they soon grow and fill the pot.

If possible, controlling the temperature around your young plants after transplanting helps to encourage bushy, flower filled plants. Temperatures lower than 50 ℉ deter growth of the main center shoot and encourage side shoots to emerge. However, while these cooler temperatures make for a bushy plant, they do delay flowering. Temperatures above 59 ℉ deter bushy growth, instead encouraging a central stem to develop. Plants growing in warmer conditions also tend to flower earlier.

Additional Care Tips

Water your growing flowers regularly, particularly during dry spells. Ground cover varieties require more water than other varieties. In pots and hanging baskets water when the top couple of inches feel dry. In the heat of summer this may be every day. A self watering pot is a great way to keep your flowers hydrated while also maintaining a low maintenance garden. When watering try to water deeply, until excess water begins to trickle out of the drainage holes at the bottom of the pot. Don’t water already wet soil. Overwatering your flowers can cause root rot.

Apply a balanced water soluble fertilizer for flowers regularly. This may be once a week or once every two weeks depending on the product you are using.

Deadhead spent blooms at the base to prevent plants from going to seed. If you allow the plants to go to seed then growth and flowering slows or ceases.

In midsummer cut back plants and apply a high nitrogen liquid fertilizer to encourage more flowers. Leggy plants can be cut back and fertilized. This encourages new, branching growth to emerge.

5 Satisfying flowers from seed
Bright and colorful, growing petunia seeds is a satisfying process. 

A popular colorful bedding plant, germinating petunia seeds is a satisfying process. If you have the time and space it enables you to enjoy a wider range of plants with just a little extra effort.

Petunia seeds 1 Petunia seeds 2