It sounds as if you are trying to grow Ipomoea carnea aka bush morning glory.

I found this info., but I am in 8b and it is NOT hardy for me - it has to go in the greenhouse. Humidity does't seem to be a factor in it growing here in Panama City, Florida. Take a look at the information I found:

Category:
Shrubs
Tropicals and Tender Perennials

Height:
6-8 ft. (1.8-2.4 m)
8-10 ft. (2.4-3 m)
10-12 ft. (3-3.6 m)

Spacing:
24-36 in. (60-90 cm)

Hardiness:
USDA Zone 8b: to -9.4 ðC (15 ðF)
USDA Zone 9a: to -6.6 ðC (20 ðF)
USDA Zone 9b: to -3.8 ðC (25 ðF)
USDA Zone 10a: to -1.1 ðC (30 ðF)
USDA Zone 10b: to 1.7 ðC (35 ðF)
USDA Zone 11: above 4.5 ðC (40 ðF)

Sun Exposure:
Full Sun
Sun to Partial Shade

Danger:
Seed is poisonous if ingested

Bloom Color:
Pink
Violet/Lavender
White/Near White

Bloom Time:
Blooms all year
Blooms repeatedly

Foliage:
Herbaceous
Smooth-Textured

Other details:
This plant is attractive to bees, butterflies and/or birds
Drought-tolerant; suitable for xeriscaping

Soil pH requirements:
6.6 to 7.5 (neutral)

Patent Information:
Non-patented

Propagation Methods:
From herbaceous stem cuttings
Allow cut surface to callous over before planting

Seed Collecting:
Allow pods to dry on plant; break open to collect seeds.