How to Grow and Maintain Petunias: A Comprehensive Guide

Petunias are among the most cherished and easily recognizable garden flowers, loved for their vibrant colors, abundant blooms, and versatility in garden design. Whether you’re aiming to enhance your garden beds, containers, or hanging baskets, Petunias can add a splash of color that lasts from spring until the first frost. This comprehensive guide will provide you with all the information you need to successfully grow and maintain healthy, lush, colorful petunias throughout the season.

What Are Petunias?

Petunias are herbaceous annuals known for their trumpet-shaped flowers and a huge spectrum of available colors, including pink, purple, red, white, and many bi-colored options. They belong to the Solanaceae family, which also includes tomatoes and potatoes. Petunias thrive in sunny locations and are much appreciated for their long blooming season, making them a favorite among gardeners of all levels.

History and Origin

Native to South America, Petunias were first discovered in the early 19th century. The name “Petunias” is derived from the Tupian language word ’petun’ meaning tobacco, this language being indigenous to South America. The same word was later adopted by the French and is so named due to the Petunia’s botanical relation to the tobacco plant. Extensive hybridization over the years has resulted in the myriad of diverse varieties available today, providing gardeners a plethora of choices for their landscapes.

Choosing the Right Variety

Selecting the appropriate Petunia variety is crucial for achieving the desired effect in your garden. Here are the most common varietals:

Grandiflora: Features large blooms up to 4 inches wide. Ideal for containers and hanging baskets, they make a striking statement.
Multiflora: Produces smaller, more abundant flowers that are more weather-tolerant. Suitable for garden beds and mass plantings.
Wave (Spreading): Known for their vigorous, trailing growth, this variety is perfect for ground cover, large containers, window boxes, and hanging baskets.
Supertunia: A type of hybrid Petunia that offers vigorous growth and abundant flowering without the need for deadheading.
Cascades and Surfinia: Trailing varieties that are excellent for hanging baskets and window boxes.
Milliflora: Compact plants with bountiful miniature flowers. Great for borders, window boxes, and small pots.

When Choosing, Consider This:

Climate Suitability: Choose varieties that perform well in your local climatic conditions.
Color Scheme: Select colors that complement your garden design, while being in keeping with your personal preferences.
Growth Habit: Decide between upright or trailing growth habits, based upon your planting location and desired aesthetic.

Planting Options

Petunias can be planted from seeds or from mature plants purchased from nurseries or garden centers.

From Seeds

When to Start: Begin sowing seeds indoors 8 to 10 weeks before the last expected frost date for your area. Thin out the smaller plants, keeping the larger and more vigorous seedlings; this process will prevent overcrowding as your Petunia seedlings grow.
Benefits: Seeds are more cost-effective and may allow for a wider selection of varieties.

From Starter Plants

When to Plant: Transplant mature Petunia plants into the garden after the danger of frost has passed and soil temperatures are consistently 60°F (15.5°C), or above.
Benefits: Immediate colorful, floral impact in the garden and less time to reach full bloom.

Proper Soil Preparation

Having the correct soil conditions will ensure your ultimate success.

Type: Use well-draining fertile soil to prevent root rot and to promote healthy growth.
pH Level: Aim for slightly acidic to neutral soil (pH 6.0 to 7.0).
Amendments: Incorporate compost or aged manure to enrich your soil with organic matter.

Sunlight & Temperature Requirements

Full Sun: Petunias require a minimum of six hours of direct sunlight daily for optimal blooming.
Partial Shade: These annual plants can tolerate light shade but may produce fewer flowers.
Optimal Temperatures: Daytime temperatures between 70-85°F are ideal.
Frost Sensitivity: Petunias are sensitive to frost. Plant them outdoors only after the last expected frost date in your area. If a late frost is forecast after you have planted, cover your plants overnight, uncovering as soon as the temperature is warming with the heat of the sun.

Starting Petunias from Seeds

Starting petunias from seed is a cost-effective way to produce a very large number of plants with a single seed packet. Petunia seed packets often contain between 1,000 and 1,500 tiny seeds. Here’s how to do it:

When to Sow

Timing: If sowing indoors, sow seeds about 8–10 weeks before the last expected frost date for your area.
Temperature: Maintain soil temperatures around 75°F (24°C) for optimal germination indoors and sow your Petunia seeds outdoors when soil temperatures are consistently 75°F (24°C).
Indoor Sowing: In cooler climates, start seeds indoors; using a propagator will provide the necessary warmth and humidity for more consistent germination results.

How to Sow

Prepare a Fine Seed-Starting Mix—Use a finely-textured, sterile, seed-starting compost to accommodate the itty-bitty seeds.
Pre-Water the Soil—Moisten the compost thoroughly before sowing to prevent seeds from being displaced.
Sow the Seeds—Scatter the seeds thinly across the surface without covering them; Petunia seeds need light to germinate.
Maintain Warmth and Light—Place the tray in a warm, sunny area or in a propagator for 7-14 days. A temperature-controlled propagator ensures consistent warmth and humidity, guaranteeing better germination rates.
Germination Time—Seeds typically germinate in 7-14 days under optimal conditions.

Transplanting and Growing Seedlings

We recommend that you transplant seedlings to a larger pot to attain more maturity before transplanting to the garden or to their final container destination.

Transplant seedlings when they have at least two to three true leaves. (The first set of leaves are not considered true leaves.)
Gently lift seedlings by the roots, avoiding direct contact with stems or leaves to prevent damage. Use a popsicle stick or plastic ‘lifter’ to carefully raise the roots from the soil.

Initial Transplanting:

Place each seedling in a small pot with drainage holes, filled with quality potting mix. Seedling trays and propagators are available with multiple growing cells.
Water each small pot or cell gently to settle the soil around the roots and support further growth. Soil should not be allowed to dry.

Further Transplanting:

After about 4-5 weeks, when the seedlings have outgrown their original pots, carefully transfer them to larger pots (3-4 inches or 7-10 cm) to provide more room for their roots to grow, more adequate moisture, and nutrients. You can skip this step if transplanting directly to the garden. Remember, the soil temperature should consistently be 60°F (15.5°C) or above. Alternatively, you can transplant to whatever container you will be using to display your Petunias.

Hardening Off:

Gradually acclimate seedlings to outdoor conditions over the period of a week before transplanting in your garden or when transplanting to their ultimate container.

Spacing Guidelines

The spacing guidelines below are for garden-planted Petunias. If planting in pots, they can be planted closer together, though you should plant no more than 4 Petunia plants to a 12” diameter pot.

Grandiflora and Multiflora: Space plants about 12 inches apart.
Wave, Cascade and Surfinia Varieties: Space 18-24 inches apart due to their spreading habit.
Milliflora and Supertunia: Can be spaced closer, about 6-8 inches apart.

Planting Depth

Plant both seedlings and mature plants at the same depth they were growing in their pots, ensuring the root ball is level with the soil surface.

Specifics for Container Gardening

Containers: Use pots with adequate drainage holes to prevent waterlogging which may cause root rot. Choose containers that are 12 to 14 inches deep to allow for healthier root growth.
Soil Mix: A high-quality, nutrient-rich, well-draining potting mix is ideal.
Decorative Arrangements: Combine different Petunia varieties or mix your Petunias with other flowering or foliage plants for more visual and textural appeal.
Hanging Baskets and Pots: Consider adding a chain to bring your containers to eye level, allowing you to fully appreciate those cheerful blooms while making their care easier.

Watering and Fertilization

Water Regularly—Water slowly, at the soil level; avoid wetting the leaves and blossoms. Petunias prefer a consistently damp soil, but not soggy. In pots, 1 to 2 inches of water a day should be sufficient. In the garden, water deeply. 6-inches of water once a week is recommended. Drip irrigation on a timer will provide the most consistent moisture and can be adjusted to fit the changing weather conditions.
Mulch—This will help to conserve water by retaining moisture and preventing water evaporation. It will also suppress weed growth and give your garden a well-groomed appearance.
Frequency—In warm, sunny locations, your Petunias may need watering daily. In hot weather, especially for hanging baskets or smaller containers that dry out quickly, you may need to water twice a day.
Type of Fertilizer—When first transplanting, use a balanced liquid fertilizer to give plants a strong start. When flowering, Petunias are heavy feeders and thrive with a high-nitrogen, water-soluble fertilizer like Jack’s Petunia Feed.
How and When to Fertilize—If using a liquid fertilizer, mix one tablespoon to one gallon of water. Follow package directions if using a dry fertilizer.

- Fertilize once a week during the early growing season.
- By midsummer, you may want to increase feedings to twice weekly to support larger plants and more vigorous blooming.

Hot Climate Care—In very hot climates, consider placing your Petunias in an area that receives morning sun and afternoon shade to prevent excessive drying and heat stress.

Overwintering—Petunias in containers can be moved indoors. If planted in the garden, remove old plants and debris as the season ends.

Drainage—Adding a layer of small rocks or pebbles at the bottom of your pots will improve drainage if your containers do not have adequate drainage holes.

Pruning and Deadheading

Regular Pruning—encourages bushier growth. As it grows, pinch back leggy stems to encourage a fuller, bushier plant. Prune about a third of the plant midsummer if you want promote new growth and denser blooms.

Deadheading—Removing spent flowers prevents the plant from diverting energy into seed production and encourages continuous blooming.

Remove the entire flower stalk, including the small green seed pod beneath the bloom.
If plants become leggy, cut back the entire plant by half to encourage a flush of new blossoms within two weeks.
Prune plants about two weeks before a special occasion to ensure the maximum blooming display during the event.
Continue deadheading and pruning to extend the flowering season into the fall.
As the season nears its end, stop deadheading and allow your Petunias to go to seed. Collect the seeds for next season’s flowers!

Pest and Disease Management

Root Rot, Blight, and Powdery Mildew—Are often caused by poor drainage, overwatering, or overcrowding. To prevent these common Petunia diseases:

Ensure proper spacing to improve air circulation.
Use well-draining soil to prevent waterlogging.
Be careful to not overwater your Petunia plants.
Avoid overhead watering to reduce moisture on foliage.

Earwigs, Budworms, and Fungus Gnats—These pests can affect Petunias, especially in humid conditions.

Troubleshooting Common Petunia Problems

Wilting Plants: Check for underwatering, overwatering, or root rot.
Yellow Leaves: This may indicate nutrient deficiencies or overwatering. Fertilize if it has been a while and/or cut back on watering to allow the soil to dry out. If there is water in the bottom tray, remember to empty it. Root rot is the most common reason for yellowing leaves; remove any yellow leaves to prevent further disease and to keep your plants healthy.
Poor Blooming: Ensure your Petunia plants are receiving adequate sunlight and are properly fertilized. Reduced blooming can also occur if you are not deadheading those varieties that require it.
Gray Moldy Spots: This is usually indicative of blight, a fungal disease that usually occurs in cool, humid conditions. It can spread quickly, so promptly remove any affected flowers and thin your plants to allow adequate air circulation.
White Powdery Substance: This is powdery mildew and will be present on stems, flowers, and leaves. Overcrowding and watering from the top, instead of at the soil level, are the usual causes. Remove affected plants, as it can spread quickly. An alternative would be to apply a fungicide specifically formulated for powdery mildew.
Holes, Chewed Leaves or Flowers, Web-Like Filaments: Any of these signs is the result of some type of garden pest, such as spider mites, thrips, tobacco budworms, aphids, leaf miners, slugs, or snails. Check your plants regularly and identify the pests, taking appropriate measures quickly to control them.

Companion Planting

Companion planting can be aesthetically pleasing, as well as attracting beneficial insects that will naturally control those harmful pests.

Lobelia and Foxglove: These companion plants have complementary, contrasting coloration and flowers. Foxglove is also tall, so both will provide visual and architectural appeal. They also have similar care requirements to Petunias, though Lobelia plants are a perennial and Foxglove is a biennial or an herbaceous, short-lived perennial plant.

Yarrow and Dill: Attracts ladybugs, lacewings, hoverflies, and parasitic wasps. All of these are beneficial insects that will naturally control many of the pests bent on destroying your Petunia plants. Yarrow is a perennial, so will make its appearance every year without replanting. Dill, on the other hand, is an annual herb.

Basil and Lavender: These aromatic plants tend to repel many of the pests that are common to Petunias, while enhancing your garden’s aroma and contributing to its aesthetic appeal. Most varieties of Basil are annual plants, while Lavender is an amazingly fragrant perennial plant.

Growing Petunia plants can be a rewarding and enjoyable experience, adding vibrant color and life to your outdoor spaces, while enhancing your mood and contributing to your physical wellbeing. You have choices when it comes to color selection, form choices, and whether to grow from seed or mature plants.

Petunias are “happy” flowering plants; their presence in your garden or containers is sure to bring at least the hint of a smile to your face and heart, even on a particularly bad day. They offer endless possibilities for creativity and enjoyment in your garden, on your deck or patio, or in window boxes. You can even decorate your sunniest indoor spaces with these joyful blossoming plants! You just can’t lose with Petunia plants!