Clematis seeds are notoriously slow to germinate. Once in the ground, many of the seeds will germinate within six months to one year, but some will take up to three years to produce foliage. However, you can do two things to increase the germination rate of your clematis seeds: Use fresh clematis seeds harvested in fall — they more likely to germinate quickly — and break the seeds’ dormancy by exposing them to cold temperatures.
Fill a deep planting tray to within 3/4 inch of its rim with moistened seed-starting medium.
Sprinkle the clematis seeds over the surface of the medium so that each seed is roughly 1/2 inch apart.
Sprinkle a thin layer of horticultural grade sand over the seed layer so that the seeds are just covered.
Firm the seed, soil and sand mixture with your hands to ensure that the seeds make adequate soil contact.
Moisten the sand layer with water from your spray bottle.
Place the lid over the planting tray.
Place the planting tray outdoors in a shady spot to weather the winter if you live in USDA zones 4 through 7 where winter temperatures fall at or below freezing. In warmer growing zones, place the planting tray in the refrigerator. Remove the lid two to three times weekly to aerate the seed and check the soil’s moisture level. It must remain consistently moist (not soaking) to ensure germination. Remoisten it with your spray bottle if necessary.
Move the tray to a position where it will receive full sunlight in spring when temperatures warm.
Remove the lid once the seeds germinate in late spring or early summer.
Pot growing seedlings once they develop their first set of leaves. You may leave the ungerminated seed in the tray. If you keep the soil moist, reintroduce it to cold temperatures in winter and warmer sunny weather during the growing season. They may germinate next year.