Has anyone had success growing Spring's Promise camellia in a cold zone 6a climate?
Please keeping in mind, a plants zone hardiness if only what the roots will survive, not whether the leaves, bud or flowers will survive. A good example of this is Hydrangea macrophyllum often rated Zone 4 even Zone 3. In those zones the plant will be killed down to the roots. The plant will survive and grow in spring, but most often fails to produce any blooms in summer. (There are some newer cultivars that claim they can, but the truth is hit and miss.)
To answer this question more specifically I will be quoting the website of the Missouri Botanical;
‘Spring’s Promise’ is a Japanese camellia introduced by Dr. Clifford Parks. It is temperature rated to USDA Zone 6, but is best sited in areas where winter temperatures are unlikely to dip below zero degrees F'
'Camellia japonica and most of its cultivars are considered to be winter hardy to USDA Zones 7-9. Even where winter hardy, unusually cold temperatures in winter (below 10 degrees F), particularly when occurring as a sudden temperature change, can damage or sometimes kill these plants. Japanese camellias may not be grown outdoors year round in the St. Louis area (USDA Zone 6) except for a very limited number of recently developed cultivars which have displayed exceptional hardiness.'
'Excellent flowering shrub for lawns, shrub borders, backgrounds, informal hedges and around homes in mild winter locations. When grown outside their specified hardiness range, camellias are used primarily as conservatory plants, and they perform quite well in this environment. Since they react badly to change (e.g., drop their buds), it is usually not recommended that they be moved outside in the summer months and then returned inside in the fall. If possible, camellias should be grown in a brightly lit space that can accommodate their large spreading habit and be kept cool in the fall and winter to induce flowering.'