How To Grow Petunias From Seed: 9 Expert Tips
Petunias overflow with bountiful, colorful blooms to delight in summertime containers and borders. Growing them from seed lets us choose our favorites and grow them in numbers. Garden expert Katherine Rowe explores how to grow petunias from seed for bright blooms throughout the summer.
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Petunias are among the most popular summer annuals. They energize the display in a rainbow of colors, quickly filling hanging baskets, pots, and borders with cascading stems full of trumpet flowers. From bubblegum pink to velvety black, with single, double, or ruffled flowers, there’s a petunia for every planting combination.
In the right site, petunias are low-maintenance. They flower reliability, are easy to grow, and come in an array of colors and forms. Growing them from seed gives us the opportunity to select unique varieties, including top performers and heirlooms, and to sow them in large numbers economically.
Here’s how to grow petunias from seed to enjoy the greatest diversity and quantity in your garden!
Growing Petunias From Seed
Petunia seeds are teeny tiny and are challenging to distribute evenly. They’re available bare or pelletized with a protective coating that makes them larger and easier to work with. The coating naturally breaks down with regular moisture after sowing.
Start petunias from seed indoors to promote development before transplanting after the final frost. Disinfect previously used seed-starting containers with a bath in soapy water, followed by a dip in a bleach or alcohol solution. Sanitizing supplies helps ward off lingering fungal spores and pests. For a bleach solution, use one part bleach to nine parts water. Give a rinse with clean water following the dip. For alcohol, use a spray bottle or cloth with isopropyl alcohol (70%), with no need for rinsing or water dilution.
Use a moistened seed-starting mix to fill cell packs, trays, soil blocks, or other creative containers that hold soil and have drainage holes.
Plant Selection
Petunias are quintessential “spiller” plants. Their bloom-packed stems mound and cascade from containers and hanging baskets. The tubular blooms emerge from summer through frost.
Petunias fall into classes based on bloom type and growth habit. Grandiflora has the largest flowers, while multiflora blooms are smaller but more prolific. Milliflora are dwarf with miniature flowers. Cascading types are fast-growing spreaders that quickly fill in an area.
In warm climates, try an improved variety that tolerates heat and humidity, like those in the SupertuniaⓇ series. WaveⓇ varieties bring a cascading form and perform well in a range of climates.
Garden Placement
When choosing your planting site, opt for one with rich, well-draining soils and full sun. Five to six hours of sunlight daily ensures the best vigor and flowering. They tolerate partial shade, but too much leads to a lack of flowers.
In hot, southern climates, petunias benefit from protection from the direct midday sun. Morning sun is ideal.
Start Petunias From Seed Indoors
Petunias grown from seed do best when started indoors 10 to 12 weeks before spring’s final anticipated frost date. Starting them early in protective conditions gives them a headstart on strong roots and leafy upper growth. They’ll be ready to grow and flower in spring’s mild conditions.
In addition to indoor starting, you can also give winter sowing a try by sowing the annuals in protective containers outdoors. They’ll germinate as seasonal conditions present the right temperatures for growth. Winter sowing makes for easy transplanting without the need for hardening off. But, starting petunia seeds indoors promotes earlier development and flowering.
Sow a group of three seeds in a cell, pressing lightly for contact. Petunia seeds need light exposure to germinate, so there’s no need to cover them with soil. Moisten thoroughly after sowing. Ideal soil temperatures for germination are between 68-85°F (20-29°C).
Keep the media moist and place the tray in a warm, bright spot out of direct sun. Expect to see sprouts in a week to ten days (but as many as 30 days), depending on the setting and variety.
Tending and Thinning Seedlings
When sprouts emerge, move the tray to a sunny spot or place them under a grow light. Position the grow light four to six inches above the sprouts for 12 to 16 hours a day. When three true leaves appear, pot the seedlings up to larger cell packs or pots. Let them continue to develop in a cool, sunny location (60-65°F or 16-18°C). Allow soils to dry slightly between waterings.
When seedlings reach one inch tall, thin them to the strongest seedling per cell. Thinning improves airflow and reduces competition for sunlight, nutrients, moisture, and root space. Clip the excess seedlings at their base (rather than pulling) to avoid root disruption.
While thinning may seem like we’re removing viable plants, it promotes seedling health and reduces the chance of damping off. Damping off is a common fungal and mold problem for seedlings. It often results from overwatering and soggy conditions, but it can impact seedlings in well-draining situations, too. It carries over in growing supplies, splashes in water, travels on the wind, and through pests. The fungal pathogens thrive in cool, moist situations, making seed-starting setups vulnerable. Thinning opens the crown for leafy development without close contact, leaving space between seedlings.
Hardening Off
Tender seedlings benefit from gradual acclimation from the warmth of indoor growing situations to garden conditions. Before transplanting the young plants, place them in a protected outdoor area for a few days to a week. Slowly move them to conditions mirroring their new garden location.
Protect them from strong winds, cold nights, and afternoon sun in the process. Hardening off sets plants up for success at transplanting.
Transplanting
Transplant petunia seedlings into their garden location or container after the last frost passes. Follow spacing guidelines to allow for growth and airflow between plants (usually 8-12 inches apart).
Ideal growing temperatures are around 55 to 80°F (13-27°C). While the annuals flourish in mild summer temperatures, high temperatures cause stress and result in fewer flowers. Hot spells cause growth and production to slow.
Petunias are frost-sensitive and late spring frosts, chilly nights, and early fall cold snaps inhibit flowering. Growth slows when nighttime temperatures fall below 40°F (4°C). Overwinter potted petunias until the next growing season.
Irrigate and Fertilize
Whether grown from seed or nursery start, petunias prefer even moisture and soils that aren’t too wet or too dry. Young seedlings and transplants have small root systems, and consistent soil moisture throughout the growing season is best. Well-draining soils are essential.
Once established, the adaptable annuals tolerate short periods of drought and can dry out between waterings. If soil is dry to the depth of an inch (one knuckle deep), it’s time to water. Containers dry out quickly in the summer heat, and depending on your climate and pot size, they may need water daily.
Because petunias do all their growing and flowering in a single season, they’re heavy feeders that need plenty of nutrients and minerals to flourish. Apply an organic balanced fertilizer like 8-8-8 or 10-10-10 at planting. A slow-release granular works well to sustain plants for a couple of months as it distributes nutrition over time.
A liquid feed later in the season gives a boost. Petunias benefit from applications starting in June (every few weeks) and increasing in July to every two weeks. Use a formula specific to flowering plants. Higher phosphorus (P) is more beneficial than high nitrogen (N) in the NPK ratio for boosting blooms.
Trimming and Deadheading
Petunias may become spindly as the season wears on, and their long stems grow leggy. To give them a refresh, opt for a light prune in midsummer. Repeat the trim in late summer or early fall as needed. Clipping the stems promotes new growth and flowering.
Many petunias, including smaller-flowered and newer cultivars, are self-cleaning and don’t require deadheading to promote blooming. Some petunias benefit from it, including old-fashioned grandiflora varieties with large or double blooms. Deadheading the blossoms and sepals prevents the plants from going to seed. It channels energy into flower production instead.
In addition to growing petunias from seed, they also propagate through vegetative cuttings. Let the bounty of blooms continue!
