After repeatedly being informed that they were prolific, I planted 1,000 periwinkle leads on a 300 ft. wide embankment. In two years, 30% have actually taken. They are healthy enough, but are not spreading.

There are many publications and forums warning of the potential of unchecked weed-like growth and address how to get rid of the invasive.

But I want it to grow. And I want it to spread quickly! Thus far, fertilizer has made the plants healthy, but they are still not much longer than the 12â to 18â length they were when I purchased them.

Suggestions?

Details:
North facing embankment 300 w x 40 h.
4 to 6 hours of generally overcast sunlight. (Smoky Mountains)
USDA Zone 6
Good Soil (grows weeds easily)