Growing Guides

How to Grow Petunias

How to Grow Petunias

Petunias are a popular choice for many gardeners. The numerous petunia varieties with vibrant shades and patterns make the options endless. Their blooms have a long flowering season, allowing you to enjoy the fresh, beautiful flowers for months.

Once you decide you want petunias in your garden, you must learn how to care for them. This guide will review everything you need to know about petunia care and maintenance. We will detail the steps for how to grow petunias from seed, seedling care, maintenance, common pests and diseases and popular varieties and answer frequently asked questions.

Reasons to Grow Petunias

Petunias are beautiful and functional, so there are many reasons to grow them. Many people choose to grow them to add color to their gardens with a low-maintenance flower. Other reasons to grow petunias include: 

  • Heat and drought resistance: Petunias are heat-and drought-resistant. They might be easier to grow than other flowers in hot climates.
  • Pollinators: Petunias attract pollinators, like hummingbirds and butterflies.
  • Vibrancy: Since there are many color and pattern options, you can mix and match the flowers to create a vibrant effect in your garden.

About Petunias

Before you grow petunias, you should learn basic information about the plant. This can help you decide if petunias are right for you. A few facts about petunias include the following: 

  • Description: Petunias add color to gardens. They are hardy plants that can produce blooms all season. Most petunias available today are hybrids.
  • Plant type: Petunias are annuals
  • Light: They need full sun.
  • Height: Petunias can reach 6-22 inches tall.
  • Width: They can reach widths of 1-4 feet wide.
  • Flower color: The most popular colors are yellow, red, white, blue, pink and purple.
  • Foliage color: Foliage grows in blue or green.
  • Season features: These flowers bloom in spring, summer and fall.
  • Special features: They have a pleasant fragrance, are suitable for containers or baskets and require little maintenance.
  • Zones: The plants can be grown as annuals in all zones.

Types of Petunias

Types of Petunias

A wide variety of petunias exist, so breeders separate them by type based on flower size and quantity: 

  • Hedge Multiflora: Considered a "hedgiflora" petunia, this series is an aggressive grower ideal for landscapes. It exhibits high Botrytis tolerance, and an abundance of 2” blooms that recover quickly after rain and are great performers all summer long. Great for landscapes as a great groundcover when spaced at 24” between the plants, or as an upright dense mounded hedge when spaced at 12” between the plants. 
  • Spreading/Trailing Milliflora: Milliflora petunias are one of the earliest to flower in the spreading petunia class, and they yield a proliferation of 1½-2" blooms that blanket the mounded plants, making them superb for hanging baskets and containers. The Shock Wave petunia series has found its niche as the only small-flowered spreading series from seed, that is a great substitute for calibrachoa that continue to be difficult to produce from seed.
  • Spreading/Trailing Multiflora: These low-growing petunias are primarily multiflora types with medium blooms, but unlike the upright types the are great spreading plants that cover the ground. You can also plant them in window boxes, hanging baskets, and mixed containers. The Authentic Trailing Petunia™, Wave® has been a favorite brand of this type for more than 25 years.
  • Upright Grandiflora: The Grandiflora petunias have large, wavy single or double flowers. These petunias are susceptible to rain damage, so they are best grown in containers or hanging baskets. During dry, humid summers, they may rot, making them not fare as well in the south.Growing Petunias From Seeds
  • Upright Multiflora: These types of petunias are the most durable and prolific. They have medium, abundant flowers. Since they are more tolerant of wet weather, they are ideal for summer.

Now that you have learned more about petunias, here is a guide to help you grow petunias from seed. This guide covers everything from ideal growing conditions, sowing the seeds, caring for seedlings and transplanting petunias outside.

Ideal Growing Conditions

As with any plant, petunias thrive in specific environments. The ideal growing conditions for petunias are:

  • Sun: Petunias love sunlight, so aim for full sun. They prefer six to eight hours of direct sunlight daily.
  • Soil: They need neutral or slightly acidic, well-drained soil.
  • Water: Spreading petunias don't require much water, and other petunias only require watering once a week. Petunias in hanging baskets or containers may need daily watering.

Sowing Petunia Seeds

A good idea is to sow the petunia seeds indoors 10-12 weeks before the last frost date in your area. This gives the petunia time to sprout before transplanting outside. When seeding petunias, you can sow them in soil blocks, trays or small containers. Trays are best if you are starting many petunias from seed. 

The steps for petunia seed planting are as follows: 

  1. Fill a tray or container with seed starting mix: Moisten the soil mix to fill the containers or trays, this allows the mix to have consistent moisture throughout the container.
  2. Plant the seeds: With any seeds, review the directions on the seed packet for proper planting. Petunia seed should be placed on top of the soil. If you use trays, you should only plant one seed per cell.
  3. Press the seeds firmly into the soil: Since petunias prefer light for germination, it's unnecessary to cover the seed with soil. Many growers and gardeners will use a light vermiculite coating to help maintain moisture levels.
  4. Water from above: With any pelleted seed it is recommended you water after sowing. A heavy misting right at the seed will help the pellet melt, allowing the seed to germinate. Any drying out at this stage, can lower germination as the pellet can reharden. For any of the multi-pelleted petunias do not attempt to dissolve the pellet within a single watering. Instead, maintain a moisture level of four to five (medium wet to saturated) from sowing until the pellet is fully dissolved. 

Germinating Petunia Seeds

Germinating petunia seeds indoors takes 7-10 days and is like starting any other plant indoors before transplanting. You want to:

  • Cover trays or containers with plastic domes: Large growers will use a mist bench to raise the humidity around the seed until the seed germinates. Growers and gardeners not utilizing a mist bench can achieve high humidity using plastic domes and misting daily until germination. 
  • Maintain the soil moisture: Provide uniform moisture level of 5 (saturated) until germination. With a level 5 saturation soil is dark brown and shiny, free water is present at the surface of the soil, water drips freely from the bottom of the tray, and trays are heavy with a visible bend in the middle.
  • Keep the planting media warm: heat mat for seedlings can keep your planting media warm, please remember air temperature and the temperature in the tray will not be the same. When germinating petunia seeds, the planting media temperature must remain between 72-76 degrees Fahrenheit. Using warmer water where possible will help keep the temperature from fluctuating too much.
  • Provide supplemental lighting: grow light should be placed 4-6 inches above the plants for 16 to 18 hours daily.

Caring for Petunia Seedlings

Now that the seeds have germinated and you see true leaves on your plants, they require different care for another 4-5 weeks.

  • Remove the heat mat: Petunia seedlings grow well at lower air temperatures ranging from 65-75 degrees Fahrenheit during the day and 62-68 degrees Fahrenheit at night. 
  • Continue to move the lights: Ensure you keep the grow lights and raise them as needed so they stay 4-6” above the plants.
  • Fertilize the plants: You can use an organic fertilizer to feed the plants every two weeks.
  • Keep the media moist: The planting media should be allowed to dry between watering. Alternate between moisture level 2 and 4.
    • 2 - MEDIUM: Soil is light brown in color, no water can be extracted from soil, and soil will crumble apart. 
    • 4 - WET: Soil is dark brown but not shiny, no free water is seen at the surface of the soil, when pressed or squeezed water drips easily, and trays are heavy with a visible bend in the middle.

Transplanting Petunia Plants

Healthy seedlings will outgrow germination trays in 5-6 weeks. At that stage, you should transplant them to the larger containers you intend to display the plants in, or into packs to grow for another 5-6 weeks until you are able to transplant or move them outdoors. 

They must be kept in a warm, frost-free area until you can plant them outside. Remember to place the petunias in direct sunlight and about 1 foot apart when you plant them in your garden.

  • Maintain temperatures: Keep plants in a frost free area with air temperatures ranging from 60-75 degrees Fahrenheit during the day and 55-65 degrees Fahrenheit at night. 
  • Continue to move the lights: Ensure you keep the grow lights and raise them as needed so they stay 4-6 above the plants.
  • Fertilize the plants: You can use an organic fertilizer  to feed the plants every two weeks.
  • Harden the plants: Place them outside on warm, sunny days to harden them and prevent shock and prevent shock when transplanting.
  • Move pots outdoors or transplant into the garden after to place the petunias in direct sunlight and about 1 foot apart when you plant them in your garden.

Petunia Maintenance Tips

Once you've relocated your petunias to their permanent home, follow these tips to maintain their healthy, bold blooms:

  • Water: Thoroughly water your petunias about once a week in the ground, and as needed in pots and baskets, potentially daily.
  • Fertilize: Fertilize the petunias monthly to keep them at their peak.
  • Prune: Remove faded, old or dead blossoms from the plants. Cut back any petunias that become leggy, producing blooms at the ends of long, leafless stems.
  • Mulch: Spread a layer of organic material or mulch around the petunia plants. Mulch keeps the soil cool and moist and improves its texture.

Petunia Insects and Diseases

Petunia Insects and Diseases

Various insects and diseases can affect petunias. Some of the most common ones are: 

  • Botrytis Blight: This fungal disease is most prevalent during periods of cloudy, humid weather. It causes the flowers to develop white spots, turn brown and become covered in fuzzy gray masses. Treatment for this disease includes applying fungicides, removing the affected blossoms and not wetting the flowers when watering.
  • Tobacco Mosaic Virus and others: Tobacco Mosaic Virus causes crinkled leaves, light and dark green mottled areas and mosaics. Other viruses may result in dwarfed, cupped leaves. There is no cure for this virus, so treatment focuses on prevention. To prevent this virus, place petunias away from solanaceous plants, like tomatoes and potatoes, manage insect populations and remove any infected plants.
  • Impatiens Necrotic Spot Virus: Western Flower Thrips transmit this incurable virus. Infected plants develop black ringspots or blotches on the leaves. Manage the thrip population and remove any infected petunias to control this disease.
  • Greenhouse Orthezia: This dark-green or brown scaly insect secretes white wax as it feeds. It damages plants by withdrawing sap from them. Use pesticides as needed to control these pests.
  • Potato Flea Beetle: Potato Flea Beetles are small, black jumping beetles that eat round holes on the underside of plants. The top of the leaf eventually turns brown and dies. These beetles may make their first appearance during spring. You can use pesticides to control this pest.
  • Yellow Woollybear: A white, brown or yellow hairy caterpillar that feeds on petunias. If you notice any on your plants, you can remove them by hand.
  • Aphids: This tiny green insect loves to feed on plants. They suck out the nutrients, causing misshapen or yellow leaves, distorted flowers, leaf drop, a sticky excretion on leaves or black mold. Controlling aphids involves using a pesticide, cutting off the water supply, using slow-release fertilizers, avoiding excess nitrogen and encouraging aphid predators, such as ladybugs and spiders.
  • Slugs or snails: These insects create irregular holes in leaves and flowers, causing seedlings to disappear and leave a slimy secretion on plants or soil. You can pick slugs and snails off your plants and drown them in hot, soapy water. Avoid thick bark mulch and overhead watering, and place boards over the soil in the evening to keep them away. 

Petunia Care FAQs

Here are some of the most frequently asked questions about growing petunias.

Will Petunias Self-Seed?

Petunia plants are self-seeding. Self-seeding annual plants drop their seeds in your garden before they die and return next year. The seeds germinate on their own.

How Big Do Petunias Get?

How big petunias grow depends upon the variety you choose. Grandifloras produce the most prominent flowers, while Millifloras have the smallest. There are many other varieties in between.

Do Petunias Return Each Year?

Petunias are perennials, so in ideal conditions, they come back every year. They can't handle frost, so they become annuals in colder climates.

Do Petunias Like Shade or Sunlight?

Petunias prefer direct sunlight all day long. They won't produce as many flowers if they are in the shade.

How Long Do Petunias Bloom?

Some petunia varieties bloom all year, while others last until the first frost. This also depends on environmental conditions and your hardiness zone.

Are Petunias Toxic?

Petunias are generally not toxic to cats, dogs or humans if your pets or children accidentally consume them.

What Month Is Best for Planting Petunias?

It is best to plant petunias anytime from September to November or late January to mid-March. Remember, petunias don't like frost, so try to plant them after your last frost date. 

Best Petunia Varieties

There are countless varieties of petunias, so there is sure to be one you like. Some of the best petunia seeds include:

Wave Petunias

Classic Wave petunia seeds are at any garden center. They are a spreading variety with abundant blooms that are good for baskets, containers or garden beds because you don't have to trim them. 

The easy Wave seed variety grows up on supports or in a hedge-like shape when planted close together. It requires fewer plant growth regulators (PGRs), spreads less and blooms sooner than the classic Wave petunia. Easy waves are annual plants that grow to 6-12 inches in height and spread 30-39 inches. They bloom in yellow, white, blue, red, burgundy, pink, lavender and many other shades and patterns. 

Tidal Wave petunia seeds, also called Hedgiflora petunias, are aggressive growers and form a shrub-like shape that lasts all year, has a high tolerance to Botrytis Blight and recovers quickly after rain. They produce 2-inch pink or red blossoms and spread 30-60 inches.

Grandiflora Petunias

This Dream variation of the Grandiflora petunia has 3 to 3-1/2-inch flowers and can withstand hot, humid and rainy weather. When you plant the seeds together, they bloom within seven days of each other. Blooms come in pink, red, burgundy, blue and mixed colors. 

The Daddy Grandiflora petunia features extra-large ruffled flowers with prominent veins. This variety blooms early, tolerates weather and is uniform in color. It spreads 10-12 inches and grows 10 inches in height. It's available in pink, blue or a mix of colors. 

The Sophistica Grandiflora petunia is a unique variation that features large flowers in one-of-a-kind colors and patterns. They spread 10-12 inches and grow 10-15 inches in height.

Double Cascade Petunias

The Double Cascade Petunia flower is an annual grower known for producing frilly blooms on the ends of long stems. They grow to 10-15 inches in height and spread 10-12 inches. Blooms come in blue, pink, burgundy or mixed colors.

Shop Petunia Seeds at Harris Seeds

Harris Seeds has provided high-quality flower and vegetable seeds since 1879. We stay current on industry trends and learn about new products. Our facility in Rochester grows and tests the latest varieties of plant seeds, so we can offer the best ones to you.

We have a wide selection of petunias in various colors and patterns — our collection of petunia seeds today to add a splash of color to your garden! With so many types available, we are sure to have what you're looking for.

Shop Petunia Seeds at Harris Seeds

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