How Long Does It Take for a Dianthus and a Petunia to Germinate?

Timing

• Both dianthus and petunias need a head start before being transplanted into the garden, so they do best when started indoors. Petunias take slightly longer to establish, and should be started 10 to 12 weeks before the last spring frost. Dianthus, on the other hand, requires slightly less time to mature and can be started six to eight weeks before the last frost.

Preparation

• Cleanliness is key to successfully starting dianthus and petunia seeds indoors, both in terms of the seeds themselves and the equipment used to grow them. Single-use peat pots work well, although thoroughly cleaned and dried plastic starter pots also work. Fill the pots with potting soil, but leave the top 1/2 inch of each pot empty. While economical, reusing old potting soil is a bad idea when it comes to starting seeds because it may harbor harmful pathogens, so always use fresh soil or compost to eliminate that risk.

Sowing

• When sowing multiple types of seeds indoors, label the pots so you know which seeds are in which pots. In this case, label the pots with a "D" and "P" or other obvious distinguishing mark. Sow the dianthus seeds at a depth of 1/4 inch. Petunia seeds require light to germinate, so press them onto the surface of the soil without cover. Mist the potting soil liberally, then cover the pots with plastic wrap.

Germination Process

• Both dianthus and petunia seeds require consistent warmth and moisture to germinate, although petunia seeds also need light. Position the pots within 3 feet of a west- or south-facing window where the petunia seeds will receive ample light. Place a heating coil or propagation mat underneath the pots and set the temperature to 70 degrees Fahrenheit. The top inch of soil must be kept slightly moist at all times. Remove the plastic wrap and water with a spray bottle to keep from dislodging the seeds. Healthy dianthus seeds may sprout as early as five days after sowing, although the majority of both the dianthus and petunia seeds will sprout by day 10.

Aftercare and Transplant

• Once the dianthus and petunia seedlings emerge, remove the plastic wrap and the heating coil, but keep the pots in their original position near the window. A potentially lethal fungal infection called damping-off is a significant threat to seedlings, so water carefully and always allow the soil surface to dry out between waterings. The seedlings must be slowly acclimated to outdoor conditions, so move them to a sheltered location with bright, indirect sunlight after the last frost. Gradually expose them to direct sunlight for two weeks before transplanting them into the garden.