Petunia seeds are very tiny and need to be handled very carefully. It takes a petunia seedling roughly twelve weeks to grow strong enough to be planted outside, so make sure you start them in late February or early March. The following will provide a guide on how to start your petunia seeds.
Step 1 – Germination Trays
Use clean potting compost or soil in germination trays to start of your seeds. Dampen the soil but don’t wet it.
Step 2 – Sprinkle the Seeds
The seeds are too tiny to be handled individually so sprinkle them over the soil in the trays. Use a very fine atomizer to spray the seeds so that they set into the soil as they are watered.
Step 3 – Cover the Germination Tray
Cover the seed tray with plastic wrap to retain warmth and protect the seeds from drying out.
Step 4 – Put the Tray in a Bright, Warm Location
Petunia seeds need both indirect light and heat to germinate. To get the right temperature of around 80ºF, it can be helpful to use a heating mat designed for seed germination. If there is a likelihood that the correct temperature will not be maintained, a heating mat is essential.
Step 5 – Remove Plastic Wrap
Once the seeds germinate, and you can see the sprouts, remove the plastic wrap. The seedlings are very tiny and delicate, so don’t damage them by distorting the tray or disturbing the seedlings.
Step 6 – Cooler Location
The seedlings require a slightly lower temperature so you can dispense with the heating mat if you were using one. An ideal daytime temperature is around 65ºF. Night time temperatures can fall to as low as 55ºF, but the seedlings will not tolerate much colder than that.
Step 7 – Lots of Light
Light is very important to the seedlings, so it is best to provide artificial lighting. A fluorescent tube about six inches above the seedlings is good enough. The light should be on for sixteen hours a day.
Step 8 – Husbandry
Keep the seedlings watered so that they don’t dry out. Protect them from strong drafts that could cool them too much. As the seedlings grow, raise the fluorescent tube so that the gap between the tops of the seedlings and the light stays around six inches.
Step 9 – Transplanting
When the seedlings have three true leaves, transplant them into individual containers. Ideal free containers can be made from compressed paper egg trays. Feed the seedlings with a diluted liquid fertilizer every ten days or so.
Step 10 – Harden the Seedlings
Take the seedlings outside for several days before you finally plant them in the soil to get them used to the new climate.
Step 11 – Plant the Seedlings
Once the soil has warmed up and the risk of frost has passed, the seedlings can be planted outdoors. If you have used egg trays, you can cut the trays and plant the seedling completely so that you do not have to disturb the roots at all. The egg tray will rot away.
To have plants that branch out and become bushy. you can pinch out the growing tip when they are three or four inches tall.