The soil should drain well and not remain soggy, especially in containers. It should also be moderately fertile to promote best growth. Amend poor soil with ready compost before planting.
When to plant petunias
It is easiest to buy young plants from a nursery that sells petunias in apartments. Look for plants that are short and compact. Leggy petunias with lots of flowers already won’t settle as quickly.
If you want to grow petunias from seed, start the seeds indoors 8 to 10 weeks before your last spring frost date. (See your local frost dates.)
Plant young petunias outdoors after your last spring frost date, but keep a close eye on the weather forecast and protect young plants from late frosts.
How to plant petunias
Petunia seeds are very small (dust-like!) and need a lot of light to germinate.
When the young plants have three leaves, plant them outside.
Space the plants about 1 foot apart.
If planting petunias in containers, use a potting mix that will drain well.
Hanging basket with petunia flowers
Petunias are quite heat tolerant, so you shouldn’t have to worry about watering them often. A thorough watering once a week should be sufficient (unless there are long periods of drought in your area). Avoid shallow watering, as this encourages shallow roots.
Note: The spreading types of petunias and those in containers require more frequent watering than those planted in the ground.
Fertilize petunias monthly with a balanced fertilizer to support their rapid growth and vigorous flowering. Double-flowered varieties enjoy a dose of fertilizer every two weeks.
What to do with leggy petunias
By midsummer, most petunias tend to become leggy, producing flowers at the tips of long, leafless stems. To keep petunias looking nice and blooming, we prune the shoots back to about half their length. This will encourage more branching and more flowers.
After pruning, fertilize and water the plants well to force out new growth and flowers. The plants may look ragged at first but they will come back with more color and flowers.
Older garden petunia plants can be pruned back hard (within a few inches of the base) to reinvigorate vigor, especially in colder climates, but keep the remaining leaves.
Remove faded, old, or dead flowers (a practice called “deadheading”) to improve both blooms and attractiveness, especially for petunias with larger flowers. Deadheading prevents seed pods from competing with flowers for the plant’s food supply. Clippings can be added to a compost pile to be recycled.