If you were wondering how to grow petunias from seed, you’ve come to the right place! Petunias are beautiful garden staples you can find spilling out of window boxes and rambling in flower beds, blooming prolifically from summer on with little care.
General Sowing
When to Sow Outside: Not recommended.
When to Start Inside: RECOMMENDED. 10 to 12 weeks before your average last frost date. Ideal soil temperature for germination is 68°–85°F. Transplant outdoors after your average last frost date.
Indoor Sowing
Use a lightweight seed starting mix/medium (sterile, and lighter than potting mix), and press seeds into the media surface (light aids germination). Sow 3 seeds per pot, thinning to the strongest plant once leaves appear (clip extra plants at the soil level using scissors).
The strongest plant may not be the tallest; look for thick, strong stems and deep color. By thinning early, you minimize the negative impact of crowding, like stretching for light. Read about more indoor sowing tips.
Containers
Containers should be clean, sanitized, and have drainage holes. If you choose to sow in cell packs, you may need to up-pot seedlings once into larger containers before transplanting outside
Transplanting
After hardening off, transplant seedlings spacing them appropriately (see packet).
Outdoor Sowing
Water
Keep petunias evenly moist for best performance.
Fertilization
Petunias grow best in rich, well-drained soil; amend soil as needed. Additionally, incorporate slow-release, balanced fertilizer (equal parts nitrogen, phosphorus, and potassium) at transplanting. Fertilize with a liquid bloom formula fertilizer (higher phosphorus) every 2 to 4 weeks.
Weeding
Keep plants well weeded. Weeds compete with crops for light, nutrients, and water, and can harbor insects and diseases.
Special care
Avoid watering foliage in the evening to help prevent disease. Deadheading (removing spent blooms) keeps petunias blooming profusely.
Common Pests and Diseases
Tobacco budworms attack a variety of ornamental plants, including petunia, nicotiana, and geranium, primarily burrowing into flower buds and eating petals. They overwinter in the soil and can build up in an area over time.
The caterpillars have white stripes along the abdomen but vary in color from green to purple. They are most active at dusk and can be handpicked or plants can be treated with Spinosad or Bacillus thuringiensis (Bt); both are organic insecticides. Bt spray or powder only works on young caterpillars if applied to a surface that they are actively eating.
If caterpillars are more mature or primarily in the flower buds, Spinosad is recommended. Spraying Spinosad after dusk maximizes exposure to the pest at a time when most pollinators are less likely to be impacted.
