How to Grow Petunias from Seed in 7 Simple Steps

With a wide range of colors, bloom sizes, and plant sizes, petunias are a top choice for many gardeners. They grow incredibly well in containers, flower beds, landscapes, and hanging baskets. No matter where you choose to grow petunias, they will provide endless displays of blooms, even in the heat of summer.

It’s inevitably easier to plant potted petunias than to start from seed. Purchasing four cells or potted petunias works if you are growing smaller amounts. But if you need to plant mass amounts of petunias, planting them by seed is a preferred method.

Garden centers and nurseries only carry so many petunia varieties in containers. If you are after a specific variety that is less popular or garden centers don’t carry, planting by seed is your best option.

If you need to plant a large number, are looking for a specific variety, or you simply love growing something from start to finish, consider growing petunias by seed. Follow the simple instructions outlined in this article to get started!

First, a Bit About Petunias

Petunias are a cult classic. They provide floods of blooms even on the hottest days. They are tender perennials but are more commonly grown as an annual in cooler regions. Petunias bloom from spring to the late fall.

There are five categories of petunias, and each one has a unique quality about them. The five different petunia categories are grandiflora, multiflora, milliflora, flordibunda, and trailing or spreading varieties. Even though there are five different categories, they do have a few things in common.

They love full-sun, and they need well-draining soils. If given these few conditions, they will reward you with their beautiful blooms all season long.

Step 1: Choose Your Variety

The hardest part of growing petunias is choosing a variety to grow. There are hundreds of varieties to choose from, and this can be overwhelming, especially for new gardeners. You will first have to decide if you want a mounding variety or trailing variety.

Trailing varieties are great in hanging baskets and containers. As an added bonus, they are self-cleaning. These petunias will cascade down the sides of the container, rock walls, or even trail up a trellis. You can keep trailing varieties shorter by pruning during the season.

Mound varieties are excellent choices in flower beds, containers, landscapes, and vegetable gardens. They don’t spread like trailing varieties and stay in a nice compact mound for their life cycle. Mounding varieties look great when planted in masses and blanket areas very nicely.

Another thing to consider is you can find seeds in pelleted and non-pelleted form. The seeds are very, very tiny and can be difficult to handle. Pelleted petunias are easier to see and handle during planting. The coating on the pellet will dissolve over time.

Step 2: Gather Materials for Planting

Before you begin planting, you will want to gather the necessary materials. Starting petunias by seed is rewarding but also takes time and money. You need the proper materials to ensure you have successful germination.

Begin by identifying the number of petunia plants you want to achieve. This will help you determine how much of each material you will need. You will need to gather potting soil, pots, cells or trays, water, seeds, and labels.

They need large amounts of light when beginning to grow. A growing light system or greenhouse is ideal for large amounts of seedlings. If you are growing a few plants by seed, you can place them in your home’s windows for proper lighting.

Step 3: Planting Seeds

Once you have gathered your materials, you can begin to plant your seeds. You should start planting seeds 10 to 12 weeks before the anticipated last frost of the spring. Begin by filling pots or cells with potting soil. Once they are filled, lightly press the seeds into the soil. They should be just below the soil surface.

Once the seeds are planted, lightly water until the soil is moist. Misting the soil works best because pouring water over the seeds can cause them to shift. Label each container, so you know the variety of each container.

Once watered and labeled, cover the seeds with a plastic dome or plastic wrap. This will help keep temperatures high so seeds can germinate. We want temperatures during germination to be around 75 F to 85 F. If you have a heat mate, this will be beneficial to keep temperatures high during germination.

Place containers in indirect sunlight for at least 6 hours a day. If you are placing your petunias under grow lights, be sure the lights are 4 to 6 inches above the containers. You can move these lights up as the seedlings grow.

Step 4: Germination and Thinning

You will begin to see seedlings emerge within 7 to 10 days of planting. Remove the seedlings from the heat mat and remove the plastic dome or plastic wrap. Place the seedlings in a cooler area, somewhere between 65 F to 75 F.

If you haven’t seen seedlings within 14 days, dig around the area you planted and see if anything is sprouting. If seedlings haven’t sprouted, consider planting another round of petunias, and be sure not to allow the soils to dry out. Make sure to provide proper light.

Once seedlings have germinated, you may have to begin thinning them out. Since their seeds are small, you may have planted many into one pot or tray. Once you believe all the seedlings are large enough to handle, begin thinning.

Simply remove all but 1 or 2 seedlings, depending on the size of your pot or cell. If you have planted in trays, be sure there is at least 6 inches of space between each seedling. If you are lucky and didn’t damage your seedlings, you can transplant thinned petunias to extra containers, cells, or trays.

Step 5: Seedling Care

Check seedlings daily to determine if they need watering. Seedlings can easily suffer if allowed to dry out. Soil should always be moist but never have standing water.

Soggy soils will kill seedlings very quickly. As the seedlings grow, remember to adjust your lights. They should always be 4 to 6 inches above the seedlings.

If you need to transplant into a larger container, be careful not to damage the plant or roots. Continue to water and provide at least 6 hours of sunlight per day until seedlings have 2 to 3 true leaves. Once they have their true leaves, you can begin the process of transferring them outdoors.

Step 6: Hardening Off

A few weeks before you want to place your petunias outdoors, it’s time to harden them off to prepare for outdoor planting. This will allow the seedling to accumulate to the outdoor conditions without going into shock. If plants are moved outdoors without being acclimated, they may go into shock and not survive.

Start by placing your seedlings outdoors for a couple of hours per day, slowly increasing the time outdoors until they are outside for the majority of the day. Hardening off isn’t always necessary if you grow your seedlings in a greenhouse.

Conditions in a greenhouse are very close to conditions outdoors, and you can consider skipping this step if they are grown in a greenhouse. Hardening off is a good idea if your plants are grown indoors, and they aren’t exposed to fluctuating temperatures and light.

Step 7: Planting Outdoors

Once your petunias have gone through the hardening-off phase, you are ready to plant them outdoors. Identify a sunny location that has well-draining soils. Petunias are versatile and can be grown anywhere that has proper light and soil.

Once you have identified a good location, prep the soil in the area. If you have poor draining soils, add organic matter at this time. If you are planting in containers, fill them with loose potting soil.

Before you plant your petunias, consider adding fertilizer into the soil. Petunias are heavy feeders and will need extra nutrients now and throughout the season or year. A slow-release fertilizer is an excellent choice, because, over time, the nutrients leach into the soil. Liquid fertilizers are good but tend to leach out of the soil rapidly.

Transplant the seedlings into your desired location. Once placed into the soil. Water thoroughly to establish good roots. Check your petunias daily to ensure their soil doesn’t dry out and that your petunias are thriving.