Petunias are one of the most popular flowering plants to grow in window boxes because they’re easy to care for and they put on a stunning display of flowers from early summer until fall.
In this article I’ll tell you how to plant and care for petunia flowers in a window box and give you some tips to extend the bloom time.
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Petunias are beautiful annual flowers that can make any window box look amazing.
They come in a variety of colors, flower shapes and sizes.
Petunia plants are usually grown from seed, but you can also buy petunia seedlings from garden nurseries.
They grow best in warm conditions and need at least six hours of sunlight a day.
Types of petunias for window boxes
Wave petunias are one of the most popular varieties of petunias because they’re easy to grow and produce an abundance of blooms.
Grandiflora petunias are another variety that are easy to grow and produce large, long lasting blooms.
The flowers of Grandiflora petunias are bigger than other types of petunias so they can be planted in window boxes for a stunning display.
Calibrachoa (Million Bells) look like a smaller version of petunias and have a beautiful trailing habit.
They bloom from spring until fall with colorful flowers that can be yellow, red, purple, pink, orange or white.
Trailing petunias are another great option to plant in window boxes.
They produce a beautiful cascade of colorful blooms and can reach heights of 8 to 12 inches (20 to 30 cm) when in bloom.
Trailing petunias can also be planted in hanging baskets and patio planters and look great cascading down the sides of the planter.
How to plant petunia seedlings in window boxes
Petunia seedlings aren’t frost hardy, so wait until the risk of frost has passed before planting them.
1) Fill the window box with good quality potting mix and make a hole deep enough to plant the seedling.
2) Gently press the soil around the roots to ensure there are no air pockets. The soil level should be just below the top edge of the window box.
3) Water the plants thoroughly to help them settle in and to reduce transplant shock.
How to plant petunia seeds
Petunia seeds are best started indoors because they’re quite delicate in the first weeks of life.
Fill a tray with seed starting mix and sprinkle the seeds on top of the mix.
Petunia seeds are tiny so it can be difficult to space them out but you can thin them out after a few weeks if they’re too close together.
Gently moisten the soil with a spray bottle and place it in a bright place out of direct sunlight.
It normally takes between 5 to 15 days for the seeds to germinate.
Once the seedlings are about 2 to 4 inches (5 to 10 cm) tall you can plant them in the window box (as long as there is no chance of frost).
It usually takes about 12 weeks from planting the seeds to flowering.
How to care for petunias in window boxes
The key to growing healthy petunias is regular watering, so be sure to keep them watered every few days during the summer months.
Petunias like bright, direct sunlight and will become droopy if they’re not given enough light.
For large, long lasting flowers, it’s important to fertilize petunia plants every two to three weeks during the summer season with a liquid fertilizer like Bloom Booster Flower Food.
Removing the spent flowers by pinching them off can help to prolong the bloom time.
Petunias can be affected by a number of pests including mites, thrips and caterpillars. 
The good news is that you can easily get rid of these pests with insecticidal soap or organic neem oil.
If leaves turn yellow, this can be caused by not enough light, too much heat or nutrient deficiencies in the soil.
What type of window boxes should you use for petunias?
Window boxes and planters come in many different styles, so it’s important to choose one that will complement the design of your home.
Timber window boxes can match the charm of older homes but they’re also a good option for modern homes with an outdoor deck.
Metal window boxes are sleek and contemporary, while plastic window boxes are lightweight and inexpensive.
You can choose from pre-made window boxes that come in different shapes and sizes or create one on your own if you have DIY skills.
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So there are my tips for growing petunias in window boxes or planters.
Window boxes are an excellent way to maximize your growing space and bring color and life to the front of your home.
Now that you know how to grow petunias in window boxes, you can enjoy their beautiful, colorful blooms all summer long.
Have you tried growing petunias in a window box? Let me know in the comments below.