Growing Clematis from Seed
Combine 3 parts seed compost with 1 part sharp sand to create a rich, well-draining starting mixture for the Clematis seeds. Moisten the mixture before filling the rooting pots. Fill several 4-inch pots with the mixture, leaving 1/2 inch of space at the top of each pot.
Sow one Clematis seed per square inch of surface area in each pot. Gently press the seed into the surface of the soil. Spread a 1/4-inch-thick layer of the compost and sand mixture on top of the seeds. Firm the soil.
Set the pots in a cold greenhouse or in light shade outdoors where they are exposed to cold temperatures and bright, indirect light. Keep them outdoors over the winter. Moisten the soil in each pot using a plant mister whenever it feels dry at a depth of 1/2 inch.
Move the pots into partial sun when temperatures reach 60 degrees F in spring. Watch for germination starting two weeks after moving them.
Scoop out and transplant the clematis seedlings when they reach 2 inches in height. Transplant them into individual 4-inch pots filled with 3 parts sterile potting soil and 1 part sharp sand. Take care not to break the long tap root during transplanting.
Keep the Clematis seedlings in their individual pots until they reach 6 inches in height. Place the pots near bright but diffuse light. Keep the soil slightly moist but not soggy as they grow. Plant the Clematis vines outdoors the following spring once soil temperatures reach 68 degrees F.
Place the potted Clematis seeds in the refrigerator for three weeks to cold stratify if outdoor air temperatures stay above 50 degrees F during the winter.