Full sun is best for darker foliage and increase flowers. Will tolerate some shade.
Moisture
average
Soil & Site
average
Flowers
pink funnel shaped
Leaves
simple, opposite, color will vary with the growing site, more sunlight the darker purple, in shaded sites color will change to a green/purple, new emerging leaves will have a green tinge
Dimensions
2-3 feet by 2-3 feet, dense mounded form, I find it to be rather slow growing
Maintenance
If needed prune after flowering.
Propagation
softwood cuttings, Plant Patent PP14,381
Cultivar Origin
The seed parent of this cultivar is W. ‘Victoria’ and the pollen parent is W. ‘Foliis Purpureus’. Dark Horse is a cross between W. Victoria and W. Foliss Purpurea. Breed by Peter Moore, England in 1997.
Author's Notes
I find Dark Horse to be rather slow growing. May need some pruning to get a dense plant. For good color definitely needs strong sunlight.
Notes & Reference
#01-Manual of Woody Landscape Plants (Michael Dirr)