Planting Petunias

Petunias have been a popular plant to grow for years now.

Let’s jump into how to plant petunias.

There are many varieties of petunias, so some of the details of this article will vary depending on the specific kind of petunia that you are growing.

When to plant petunias

There are a few factors to consider when determining when to plant petunias. First, the answer will be different if you are planting small plants/plugs you bought from a greenhouse or petunia seeds.

When to plant petunia outside

Plant petunias outside any time after the last of spring’s cold snaps are over. This will ensure that the petunia plants will have a long season to grow to their full beautiful potential and not die from a late frost.

Depending on where you grow your petunias, the average last frost date will vary. It should also be noted that there is always going to be risk involved since the last frost date is an “average.” There’s always the chance of a late frost or an early spring that jumpstarts your planting timeframe.

In the Central Pennsylvania area, the last frost date is typically before the second Sunday in May, so a general rule of thumb is to wait to plant your petunias outside until after Mother’s Day. However, many times you are fine to plant annuals outside before Mother’s Day — you’ll just want to be wary that a late frost will likely stunt the growth or kill your petunias if they are unprotected outside.

When to plant petunia seeds

If you’re planting petunia seeds and not plugs/small plants, it’s recommended to start them inside first before planting them in their final destination. You’ll want to start petunia seeds at least a few weeks before you the time you think you can plant them outside.

Where to plant petunias

When choosing where to plant your petunias, you’ll want to consider the plant’s needs and growing habits.

If the plant gets too little or too much sun it likely won’t die. However, you may see less growth or not as many blooms throughout the season.

You also want to consider the plant’s specific growing habits. If it’s going to spread, make sure to give it enough space. If it’s going to get tall, make sure to plant it behind other plants so that all plants are visible and get the sunlight they need.

Choosing whether to plant your petunias in the landscape, in a hanging basket, or in a pot will also need to be a decision you make. In the case of petunias, you can go any direction here and still be ok. 

How to plant petunias

Step 1: Plant petunia seeds

*Skip to step 2 if you are transplanting petunia plugs/young plants

Typically petunia plants will germinate the best when they are planted in a tray or container with potting soil. Place your container in a spot where it’ll receive sunlight. Make sure to keep the soil moist where you planted the petunia seed, but don’t overwater it.

You’ll soon start seeing a sprout emerging from the soil. Continue to water it and keep it in the sun to ensure continual growth and maturing.

If you germinated the petunia seeds in a tray, you’ll want to gently pull the sprouts apart and transplant them into packs or pots at this point. If each petunia sprout already has its only space to grow, you’ll be fine until the petunia plant matures some more.

Plants that are kept too long in a small container will get “root-bound” which hinders their growth potential. This means that the petunia’s roots will fill up the space they are given and form a tightly wound root “cube” (or whatever shape the container is). When this happens, they’ll need to be watered more frequently.

For this reason, it’s important to transplant your seedling into a larger container once it has outgrown the original container.

Step 2: Transplant the petunia plugs/young plants

If you grew your own petunia plant from seed and it has now grown to a young plant or if you have purchased some young petunia plants at a greenhouse/garden center, you are now ready to transplant it into a larger container.

To do this, gently pull the petunia out of its container. If you were growing it in a small pack, the plant will come out like a plug, with a root structure that keeps the soil of the container bound together. 

If your plant is root-bound, gently pull the roots apart. It’s ok if it feels like you are ripping the roots. This is healthy for the plant if done in moderation, and signals to the petunia plant that it’s free to grow outside of its original bounds with its old container. It now will have more space to grow and you want to get it to expand its root structure to fill that space.

If your petunia plant is large enough, you may be able to skip to step 3. If it’s not mature yet, or if it’s too early to plant outside, then you’ll want to plant it in a larger pot and continue to water it moderately and give it enough sun.

Now, find a larger pot or container and fill it up with potting soil to plant your young plant in. Place the young petunia plant in the soil so that the top of the root cube matches the top of the soil line. If you plant it too deep in the new container and cover up the plant’s “heart” (where new growth stems from), you could kill the plant or hinder its growth.

Once planted in its new home, you can water it lightly to further jumpstart the petunia’s root system.

Step 3: Plant the petunias in their final spot

When your petunia plant is fully matured and it’s warm enough outside, it’s ready to plant in its final spot. There are many great options for where to plant your petunias. They will grow well in your landscape, flower beds, hanging baskets, or a flower pot.

Whichever you choose, first you’ll want to pull the plant out of its container. The best way to do this is to squeeze on the plastic pot/pack that it’s in and pull the plant gently out.

Then use a trowel or your hand to move some dirt or potting soil away where you’re going to plant the petunias. Place the plant in the hole and cover it up with the soil you removed.

Again, don’t plant the petunia too deep, as this may hinder its growth or end up killing it. 

You also don’t want to plant it too high to avoid having a mound of dirt above the surface. Mounds of dirt cause any water you give the petunia plant to run off. This can cause soil to be washed away from the surface and uncovers the plant.

Now that your petunia plant is planted, you can water it lightly. A sprinkling can or hose with a spray nozzle setting that provides a gentle stream is advised for this. 

You can also give the plant some fertilizer so that is has something to feed off in its new home.

Finally, if you planted the petunia plant in a landscape or even a container, it does not hurt to mulch it. This will help with keeping moisture in its root and you won’t have to water as much throughout the summer. Bark mulch, wood chips, or many kinds of landscaping mulches work for this. Make sure you don’t spread the mulch too close around the petunia, as the sun can heat up the mulch to a point where it burns the young plant.

Step 4: Caring for petunias

Now that your petunia is planted you’re all ready to care for it throughout the summer and watch it grow and thrive.

Continue to water your petunias.

For care instructions specific to the variety of petunia you have, see the plant’s tag or seed packet.