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growing petunias from seed

Painting Your Garden With Petunias

Every year, leading home and lifestyle magazines come out with all sorts of "top ten best" lists. If the topic is garden ornamentals, petunias are, without fail, high within the ranks.

If their five, rounded petals were fused—and you have to look closely to see that they aren't—they resemble fancy old-school phonograph horns. The blooms grow at the ends of long, delicate, leafy stems, nearly eclipsing the equally-attractive foliage. Dense, spear-shaped, alternating leaves resemble basil plants, with the same bright, matte green hue. The entire plant takes on a bushy, rounded growth habit, but they can also spill over the edges of retaining walls, baskets, and containers in a cascade of color.

They're extremely popular as border and edging plants, especially among those who are going for that "exploded My Pretty Pony factory" aesthetic...without the messy cleanup. They might look precious and high-maintenance, but they're easy keepers. And any coddling they might need to help them retain their shape is actually kinda satisfying if you like pinching stuff, or you were the kid who liked to give your (or your sister's) Barbies buzz cuts.

Origins and cultural history

Europeans first stumbled upon petunias in South America in the late 18th century, and 1823 is thought to be the year in which amateur botanists began developing showy cultivars from the considerably plain original species. We stumbled upon a blog called "The Promise of Seeds: Magic in a Packet"  by Emma Craib ("Retired elementary school art teacher interested in smiles and robots, insects and botany, kayaking and gardening, researching, and honeybees and pumpkin pies"...OMG...our kind of woman!). Ms. Craib found a selection of 19th-century letters, articles, illustrations, and ponderings on this pretty little plant written by botanists exploring petunia's endless potential for crossbreeding.

Petunias became popular around the same time as phlox, and both are compatible species for hanging baskets, pots, and flower beds. Both plants stole the show at the acclaimed Glasgow Botanic Garden's 1836 spring season; phlox had only arrived from Texas the year before, and its pairing with petunias made them the Fred and Ginger of the cottage garden.

Petunia is derived from the Brazilian Portuguese word for tobacco, "petun." The Petunia and Nicotiana genera are closely related within the nightshade (Solanaceae) family.

Petunias in the garden

The petunia genus is classified as a tender perennial in USDA hardiness zones 10 and 11, and annuals in zones 3 to 9. They're among the most popular bedding annuals, sharing the spotlight with impatiens, phlox, and pansies. All of these look amazing together, especially when inter-planted with species having smaller blooms and foliage: alyssum and lobelia, for example.

Ready for petunia's vital stats?

  • Typical plant height: 8" to 1" tall
  • Typical plant width: 1" to 3" spread
  • Bloom period: April/May until the first frost
  • Flower color: Every color and shade imaginable, including deep, velvety black

You can find more color patterns in petunias than most any other ornamental genus, as well as ruffle-edged petals and double blooms. Flowers are typically around 2" across, with some giant varieties pushing 4".

One of the most frequently asked questions we get about petunia care is, "How do I keep my petunias from getting leggy?" The short answer is to follow our recommendations for choosing the right spot in your garden and mind some basic care guidelines. It's not difficult at all to grow petunias in the tidy mounding shape you see in all those glossy magazines. And, really, even if they do get spindly, they still retain their charm.


While container plants tend to dry out faster than those grown in beds and therefore should be watered more frequently, petunias withstand short periods of drought. Still, we recommend providing them with consistently moist soil. Plan to give bedding plants 1 to 2 inches of water per week, depending on temperatures. An extra half to one inch will keep those hanging baskets and potted petunias happy.

If you notice issues with fungus, focus on deep waterings on a less-frequent basis until the issues clear up. Under-watering your petunias is preferable to over-watering them.

Sunlight and heat

Petunias do best in full sun. If they're in partial shade, they'll "reach" for the light, causing their stems to become spindly. Ceramic containers tend to absorb a lot of heat and expire moisture, so we recommend placing potted petunias where they can be out of the sun in late afternoon. Do keep them out in the open if at all possible.

Peak summer temperatures will halt petunia's bloom vigor, so make note of our maintenance recommendations to keep them chugging along through the season.

Soil preferences

Petunias can handle poor to medium soil as long as it drains well. Prepare your petunia beds with plenty of aged compost to improve drainage and add some extra nutrients. We like to add some Perlite to container-grown plants to prevent compaction. While petunias can handle variances in soil chemistry, they prefer a neutral pH between 6.0 and 7.0.

Maintenance, pests, and diseases

Petunias aren't particularly susceptible to pests and diseases, though they're not immune. Proper spacing, smart watering practices, and planting in full sun will prevent most of these issues.

  • Leaf spot
  • Crown rot
  • Root rot
  • Botrytis blight
  • Powdery mildew
  • Verticillium wilt
  • Caterpillars
  • Leaf miners
  • Mites
  • Thrips

Deer and rabbits tend to leave petunias alone.

If you notice any issues with fungus rot or wilt, or your plants show damage from virus-carrying thrips, clean up and destroy any diseased or dropped plant matter to prevent contagion. Mulch will help retain moisture and moderate soil temperatures, but be sure to keep it away from the plant stems. Petunias tend to respond well when treated with organic pesticides and fungicides.

If your petunias begin to get leggy, or midsummer's high temperatures have caused them to brown up, cut back your plants to where they only have enough leaves to keep them fueled. Your plants will quickly regrow and most likely bloom at least once more before the end of the season.

We're big fans of providing our petunias with balanced slow-release granular fertilizer or diluted 10-10-10 liquid fertilizer once every 10-14 days. Petunias burn up a lot of energy with their rapid growth and prolific flowering, especially if you cut them back once, twice, or even three times during the season. Feeding is especially important for container-grown plants, which have limited soil to pilfer.

Growing petunias from seed

Petunias are easy to grow from seed, whether you start them indoors or directly outside. We like to start them in nursery cells or 3" pots. If they're set in a sunny, south-facing window and kept small via pinching, petunias make agreeable houseplants. (Though we wouldn't be too agreeable if we were getting pinched all the time.)

If you don't have a sunny window for your seedling containers, try starting them in a cold frame or under fluorescent lights.

When direct-sowing, note that petunia seeds are extremely fine. Mix them with sand if you plan on broadcast-seeding, or consider making your own seed tape. Check out our blog post, "The Dirt on Successful Seed Germination" to learn how.

  • Seed Treatment: None required.
  • When to Plant Outdoors: As soon as the soil is consistently 65°F to 70°F.
  • When to Plant Indoors: 6 to 8 weeks prior to last spring frost
  • Seed Depth: Surface sow onto moist, fine soil. DO NOT COVER. Petunias require sunlight to germinate.
  • Seed Spacing: Plant or thin 1' apart in dryer conditions; 2' in humid regions or when planting in consistently-irrigated beds.
  • Days to Germination: 7 to 14 days at 70°F.

Harden off your petunia starts for a few days prior to transplanting. If you used biodegradable pots, score and moisten before placing the entire container into the bed or container. Before you transplant your starts, pinch them just above nodes, above the second set of leaves; this will encourage more branching and, as a result, more flowers.

Be sure to keep your planting beds consistently moist with a mist sprayer. Once the plants have grown a few pairs of true leaves, you can scale back watering.

A note on crowding: Growing petunias from seed creates fantastic mass-planted carpets, but overcrowding will encourage legginess. It will also hold the door open for diseases that thrive when air circulation is poor. While arid regions can get away with "mosh pit" petunias, those in more humid climates should plant at a minimum of 2' intervals.

Floral design with petunias

Petunias, when collected on the brink of opening, will last up to a week in floral arrangements. Be sure to put them in water as soon as you cut them, and then transfer them to a hot water bath (about 100°F) after you've re-cut a few inches from the stem bottom, at an angle. Let the water cool naturally, and set your flowers in a bright area out of direct sunlight.

We recommend using an additive in the water. Here's a popular DIY recipe using stuff you can scrounge from around your own home.

Contact Seed Needs for your fresh petunia seeds!

Fresh seeds are quality seeds, and we source ours from the best genetic stock available. We only order enough seeds that we predict to be able to sell in one season, and we use a climate-controlled storage system to make sure you have the best chances at a productive garden. If your seeds bomb out, so does our business, and our family's been working hard for more than a decade to keep our customers happy.

When you're ready to add enough color to your garden to make the little old lady next door have acid flashbacks, contact us for recommendations. We're always adding to our online catalog, and we currently have some fantastic petunia mixtures and individual varieties to get you started!
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