Celosia seed can be planted outside once the weather warms or indoors. While you may think that planting them outside will save you time, the key to the success of this technique is to plant the seeds once your local frost free date had passed and the soil has had a chance to warm. These seeds require warm soil to germinate.
The second way is to start your seeds indoors. Doing this latter approach will allow you to plant seedlings outside, which means the plants will bloom sooner.
To begin this process, starts off with a calendar. You will need to count back 8 weeks from your local frost free date. Once you have that date, you can begin to plan for your planting.
The day of the planting should start with clean and sterilizing your containers. This process starts off with a sink full of water that has had a capful of bleach added. Once you have this prepared, the next step is to place your container(s) into the water and allow the containers to soak for a few minutes. Next, scrub your pots to remove any soil and vegetation. After they are clean, rinse them in a sink full of plain water. Take out of the water and allow to dry.
While you are waiting for the pots to dry, moisten a bucket full of an all purpose potting soil. Using this soil, fill each one of your containers. After the containers are filled, gently tap on a hard surface and add additional soil as needed.
Now you are ready to plant your seeds. Open your seed packet, and sprinkle the seeds on the soil’s surface. Add 1/8 inch of soil on top and mist with water. Move the planted container(s) to a sunny location and keep the soil evenly moist. In 6 to 10 days, you should see little green dots appear on the soil surface. These green dots represent your germinated seeds.
Continue to monitor the soil moisture and when the seedlings reach 2 to 3 inches in height, you will need to either thin out the plants or transplant them into separate pots. If you choose to do the first, you will need thin them so that there is 5 inches of space between each plant.
As soon as your local frost free date has passed, take your celosia seedlings outside to harden them off before planting them in containers and/or the garden space.