How to Grow and Care for Petunias, Which Thrive in Everything from Containers to Garden Beds
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Petunias come in a full spectrum of colors and shapes, making these unfussy blooms a reliable choice for summer containers. And since these blossoms are easy to cultivate and grow quickly, once their seeds germinate, you can expect to see full-size plants in just 12 weeks.
A petunia's scent makes these plants even more special, injecting the air with a lovely fragrance—a trait that makes them quite "attractive to pollinators, especially when paired with species such as verbena bonariensis and lantana," says Eric Groft, CEO, and Director of OvS and author of Beyond Bold.
How to Grow Petunias
To grow petunias from seed, get started at least 10 to 12 weeks before your last spring frost and follow these steps, courtesy of Groft.
- Spread seeds on top of a moist-starting mix; avoid submerging them in soil.
- Cover your container to keep seedlings warm. Since petunia seeds need light to germinate, it's best to cover your container with a clear material (plastic wrap, for example, will keep them moist enough to ensure sprouting).
- Place your container in a warm area, but avoid direct sunlight.
- You should notice sprouting in a week to 10 days, which is when you should remove your plastic cover. Once you see true leaves, transplant your petunias outdoors into pots.
"[It's more] common to buy young petunia plants from a nursery," says Groft, adding, that it is key to "plant them outdoors after the last spring frost—but be mindful of the weather and protect young plants from late-season frosts."
How to Care for Petunias
As for where to plant your mature plants after you bring them home? Be mindful of your garden's light when choosing a plot. Some varieties require a minimum of six hours of direct sunlight per day—but most varieties thrive without strong sun, especially during the peak of summer when UV rays become too strong.
Petunias only bloom well under the correct watering conditions and will not thrive in soggy soil (which can also cause root rot) or when left too dry for extended periods of time. Typically, giving petunias 1 to 2 inches of water (when you don't have rainfall) is sufficient, but "plants growing in containers with limited soil or direct sunlight will require more water than plants growing in garden beds," says Groft.
It's best to feed petunias with a balanced liquid fertilizer as soon as you start planting. Adding a layer of compost for more nutrient-rich soil is also beneficial. For maintenance, use a liquid fertilizer every 2 to 3 weeks to keep your plants thriving. Still, depending on your petunia type, it's always best to check your individual plant's care instructions to ensure you apply the correct amount of fertilizer.
Pruning and Deadheading
To grow a fuller, lusher plant, pinch back your stems. How much you pinch depends on the petunia you are planting. Whatever variety you pick, "most petunias also require and benefit from deadheading, which will encourage more growth and keep your plant looking fresh," adds Megh Wingenfeld, a home and garden expert.
Always transplant your petunias on a cloudy, cool morning. This will stress the plants less than planting on a hot, sunny day.
- To start, add a layer of gravel to the bottom of your container (for proper drainage) and potting mix until 2/3 of your pot is filled.
- Next, create a shallow hole in your soil and remove your petunia from its container. Detangle its root ball and spread out its roots into your soil.
- While holding the plant in place, add soil around the roots until they are fully covered. Keep adding soil until your pot is filled, about 2 inches below the edge.
- Before placing your pot in full sun, let it acclimate to its new pot in light shade first.
The Most Common Petunia Varieties
Petunias have been around since the 19th century and are one of the most popular flowering plants in every nursery. They fall into six main categories, nearly all of which come in a myriad of patterns and colorways and can be grown as annuals in most zones; petunias might thrive as perennials in more temperate regions, including zones 9 through 11.
As the name indicates, spreading or wave petunia plants can reach up to 4 feet; they bloom profusely, making them an exceptional ground cover option. Fairly drought tolerant, spreading petunias also boast long-lasting blossoms, so long as you nourish them with a water-soluble fertilizer every two weeks (and give them at least six hours of sunlight per day).
A more compact iteration, multiflora petunias work extremely well in windy climates due to their strong stems. But, this variety also tends to produce the tiniest blooms. A few common types of multiflora petunias include, mirage, primetime, and carpet. The best part? Multiflora varieties require virtually no deadheading. Thanks to their "self-cleaning" ability, their flowers fall off automatically when spent.
One iteration especially, dubbed supertunia, is considered the "elite" version of petunias. Larger than wave species, petunias that fall into this camp like to spill or spread out (which makes them winners in containers or baskets!). This hybrid, says Wingenfeld, makes growing and caring for petunias even easier; like multifloras, they do not require deadheading and bloom all season long, she adds.
Although grandiflora petunias produce fewer blooms than other varieties, each bloom grows quite large, ranging between 3 to 5 inches in diameter. These petunias do require deadheading and will not flourish in extreme weather conditions like hard rain, humidity, and high heat.
Another aptly-named iteration, these bright, bold, trailing petunias, which often have crisp white edges, cascade beautifully over baskets, planters, and window boxes—but they work in the ground, too (they'll provide ample ground cover). They thrive in full sun and require moderately moist soil.
These petunias are defined by their bell-shaped blossoms and are most often used as draping plants—they look lovely tumbling over a railing or basket. They grow up to 20 inches tall and prefer full or partial sun and nutrient-rich soil. Expect to see blooms from spring through late fall.
How to Identify and Treat Disease in Petunias
Rots and Blight
A few common diseases that can overwhelm petunias include stem, crown, and root rot, which all occur due to poor drainage (and over-watering). Botrytis blight can also occur, which may cause discoloration of your petunia leaves and flowers—another condition caused by excess watering. In these cases, it's best to prune any infected sections and dry your bed out to prevent re-infestation, Groft says.
Powdery mildew is also common, however, this disease can occur sans wet conditions. This type of mildew forms when plants are spaced too tightly together and lack proper airflow. To spot this mildew, look for powdery white-colored spores spread out on your leaves and flowers. Neem oil can help, but correcting the plant's spatial conditions is equally as important.
How to Identify and Fix Signs of Distress in Petunias
If you notice any sagging petunias, the cause is usually due to insufficient water reaching its roots. Inadequate watering can also cause root binding. If this occurs, cut your plants back by half their height, encouraging new, healthier growth.