Petunias are one of those flowers that make non-gardeners want to learn to grow from seed. They’re the flowers lining lawn edgings, putting beautiful bouts of color in gardens around neighborhoods.
Some are grown in containers perched by the front door steps, and trailing types of petunias are in hanging baskets giving a stunning decorative display. And the fragrance! That’s an added bonus.
Petunias are annual full sun plants and when they flower, well, that’s the summer fragrance. It’s the reason gardeners favor these year on year. And it lasts all summer because petunias bloom more than once. When they do, it can be single bloom, double blooms, some striped, others are a solid color.
The problem with petunias is that their oh-to-eager-to-grow plant parents aren’t always clued in on the petunia care needs for these to flower best. There is a little more involved than taking seeds from a packet, planting it and adding water.
Here’s all you need to know for growing and caring for petunias that are strong, healthy and produce vibrant flowers.
Guide to Petunia Care – For All Varieties
Did you know there’s four types of petunias? Well, two mainly. Multiflora and Grandiflora. Those are the most common. There’s also Milliflora. That’s like the miniature petunia variety that only grow to about 8 inches in height and width. The flowers on millifloras are only 1 to 1.5 inches in diameter.
Grandifloras produce the biggest of bell-shaped flower heads of 3 to 5 inches. Multifloras produce flower heads about 2-inches in diameter.
There’s also Wave Petunias, – a spreading variety – suitable for filling entire plant beds and to use as a trailing plant in hanging baskets to give a spread of 2 to 3 feet of trailing foliage, all with colorful flowering blooms sprouting right along their stems.
The reason the above is explained is because the type of petunia care required differs by the variety.
The Differences in Petunia Care Routines
The amount of water petunias need vary by their type. Spreading petunias will do best with daily watering little and often. They need a drink but they don’t like bathing in water all day in full sun.
Edging plants, summer beds and petunias grown in containers favor a good soaking less frequently.
Caring for Petunias from Seed or Pellets
Petunias are available in most nurseries as seeds or as petunia pellets. The only difference is the size.
For those that have never grown from seed, petunia pellets are the same with an additional coating making them larger, so easier to handle. It doesn’t change how to take care of your petunias, or the watering requirements. Use seeds or pellets. Both need the same care and that starts with the right soil.
Potting soils for seeds are different from regular potting soil mixtures. Specialist seed starting kit soil is used to deliver key nutrients to support root growth. The majority of seed starting mixes have some fertilizer pre-added to the mix.
If you’re reusing a container, wash it out thoroughly first to make sure there’s no lingering bacteria that could hurt the plant.
Planting the Seeds
Before planting seeds, it’s a good idea to sprinkle them into a clear container as petunia seeds are tiny. Use about five to six seeds for each seedling you want to grow.
It’s easier to do this when you can spread them out in a clear container then just dab some onto the end of your finger and gently push them into the soil.
The soil you use for seed germination should be seed starting mix as those have more nutrients in them to encourage healthy plant root growth.
The seeds are going to need to things. More specifically, heat and light, but not direct sunlight. The ideal temperature for seeds is 75oF / 24oC. If you need to use grow lights, have it six inches away.
How Many Seeds Do You Need?
Petunia seeds are tiny and you only need five to a half dozen to produce healthy roots and a few leaves before transplanting to their own containers. As these are used for summer beds, lawn edges, borders, hanging baskets and containers, determine how many seedlings you need.
For transplanting outdoors, in hanging baskets as groups in containers, you’ll want each plant to be at least an inch apart so they aren’t competing for nutrients.
Growing multiple seeds with the same growing conditions is easier done with a seed starting tray (preferably lidded) and a heat mat to place under it. If your trays don’t have a lid, an alternative is to use plastic film to cover the tray until the seeds sprout.
A lot of petunia varieties are slow germinators and do best with a heat mat under the trays. With the right conditions, a couple of weeks should be long enough for small seedlings to spring up with enough roots developed to transplant them into their own containers.
Small yogurt tubs with the same type of starter soil can be used for each plant. Or smaller nursery pots.
When you’re putting the seeds into the soil, don’t push them in too far. Seeds need light to germinate so just gently push the seeds under the finest part of the top layer of soil. Don’t submerge the seeds or they won’t get the light they need.
Also, give the soil a thorough watering and keep watering it daily until the seeds sprout.
Planting Petunia Seedlings
Once the seeds have sprouted, they’re ready to get their own dedicated little growing pot. You’ll know they’re ready when they have three leaves. At this stage, switch from a seed starting soil to a potting soil with a pH between 5.5 to 6.3. Petunias do well with a slightly acidic soil.
As you’re switching to potting soil at this stage, use a nursery pot with drainage holes in it. If you’re using old yogurt tubs or similar, puncture the base to allow for some drainage and put the pots on a saucer (or anything you have) to collect water.
Take care when you’re lifting the seedlings. If you feel you’re heavy handed, use a butter knife or a stick to gently get under the root and pry it up out of the soil, rather than pulling from above.
When putting these into soil, only tap them down gently. You only need the root to be submerged in the soil with as much of the stem left above the soil as possible.
Once each is in, use a gentle sprayer to soak the soil through. At this stage, you won’t need the heat mat, as the temperature needed is around 650F / 18oC – the average temperature in most homes.
They’ll still require light, just not additional heating, unless the area you’re growing them in needs it.
How to Plant Petunias for Ground Cover
Petunias are tremendous for adding bright colorful borders to lawns and as summer bedding flowers. By starting the plants from seeds indoors, it gives you the advantage of being able to add color instantly as soon as the better weather appears.
When planting your petunias, give them at least one inch spacing in all directions so they have room to grow without getting crushed against surrounding plants. Use the same spacing for designing a summer bed or if you’re using these in raised garden beds.
Petunia Care for Hanging Baskets
When preparing petunias in hanging baskets, the spacing is the same (1”), but the conditions are more important as is the watering care. One of the largest problems with hanging baskets is watering.
As they’re being watered, a lot of the water may not actually reach the root ball but instead run off the topsoil and down the side of the basket. It can look like the plant has been saturated with water, yet actually received very little.
When hanging baskets are displayed outdoors in a south facing position, they get full sun all day long. That’s some serious heat that’s great for growth but makes the plants dehydrate fast. They need watering everyday throughout the summer.
Maximise Petunia Care with Water Retaining Gel
Have you ever looked at someone’s hanging basket brimmed full of blooming colors, lush petals and lush foliage and wondered how they did it?
Chances are they’ve done something more than watering regularly. Water retaining gel crystals can be put into the soil mix. The gel crystals store the water then release the moisture slowly giving the plant more frequent watering throughout the day, rather than once a day and at a smaller amount.
The same water retaining gel can be used in most types of plants, not just to care for petunias, including those used as ground cover and in containers outdoors.
Proper Petunia Care Calls for a Balanced Fertilizer
Fertilizer labels have three numbers indicating the weight percentage of nitrogen (N), phosphorus (P) and potassium (K), in that order.
A balanced fertilizer means all numbers are equal. A 10-10-10 fertilizer will have 10% of each nutrient, whereas as 24, 8, 16 would have 24% nitrogen, 8% phosphorus and 16% potassium. Petunias need fertilizer to grow best.
Apply a balanced fertilizer to petunias every three weeks to one month to give them the nourishment they need to keep producing colorful blooms while maintaining a healthy root system.
The Importance of Deadheading Petunias
The only type of petunias that don’t need deadheading is Wave Petunias – the spreading type. These take care of themselves. When the blooms die, they fall off and new ones grow in.
With all other types of petunia, that doesn’t happen. The color starts fading, eventually they’ll brown, then die but the dead heads stay attached until you remove them.
That’s what deadheading refers to. Getting rid of spent flowers. Although it is best done before the blooms are completely dead. Ideally, when you spot the color fading, take it off. You’ll notice because you’ll be watering them frequently anyway.
Keep an eye on them when you do. Otherwise, the plant is going to focus its energy on producing seedpods, rather than channeling all its energy into creating the blooms with bursts of vibrant color.
Deadheads remove easily from the stem. Pinch and gently pull it away or use a pair of pruners/shears.
Petunia Care to Avoid Nature’s Pests
Remember the class that explained how butterflies start life as caterpillars? Well, surprise, they need food and lots of it. Butterflies tend to lay their eggs along plant stems because it’s a rich source of food for their soon to hatch eggs.
When those eggs hatch, the caterpillar will continually eat through plants. That can be total destruction depending how many there are on the plant.
If you see a caterpillar in any of your plants, the best thing to do is remove it. Some people will drown them in a bucket of water to make sure they don’t find their way to another plant and start feasting again, while others can’t stomach the idea.
If you don’t like the thought of killing off baby butterflies before they grow their wings, perhaps put an insect house somewhere away from your plants and put them in there instead.
Once you’ve removed the caterpillars from the plants, it’s a good idea to apply Bacillus thuringiensis (Bt) – a bacteria that’s harmful to caterpillars, while being safe to use on plants and around pets and it won’t harm any other wildlife either.
The other two most common pests to petunia plants are mites and thrips. Mites suck on the juices from the plant, while thrips are known virus carriers. Thrips have a tendency to cause white spots on foliage.
Both can be managed with a weekly treatment of Neem oil for plants.
Caring for Petunias Over Winter
If you’ve grown petunias in containers or hanging baskets, you’ll be able to overwinter them. Before bringing them indoors, it’s best to acclimatize them to the shade first.
This is best done at the end of the fall but it needs to be done before the first freeze so an earlier start may be a good idea. Over the course of a week, gradually move the plant into shady locations so it isn’t getting as much sun.
When overwintering petunias, less water is needed because the plant won’t be producing flowers. It only needs its roots maintained and that only needs the soil to be moist. Watering once every few weeks and keeping the temperature around 65oF (18oC) is all petunias need over the winter.
Just be sure that before you bring them indoors you check the entire plant for any signs of insects and treat it for any you do find.
Growing up with a mom who filled her home (inside and out) with all sorts of plants, Lisa got her start in gardening at a young age. Living now on her own with a home and yard full of plants (including an indoor greenhouse), she shares all the gardening tips she’s gained over the years.
Tuesday 15th of June 2021
This was a great article, informative and well written
Thursday 17th of June 2021
Thanks Elaine! I'm glad you liked it.