We planted eight of them five or six years ago and they have done very well for us; they are blooming away as we speak! They are a Southern Indica with cold tolerances of 0-10 degrees. Zone seven is at their northern limit but should still do consistently well for you.

Why wait until fall to plant them? With the spring rains, this is a good time to plant azaleas.

They are lower and more spreading than some other azaleas like Snow. They bloom after the kurumes and the same time as Pink Ruffles. They do not hold their spent flowers like Snow or turn brown.
I have mine intermingled primarily with Pink Ruffles and it's nice.

'Delaware Valley White' is a Kurume hybrid and is hardy in USDA Zone 6. I have a few of them in the garden and they have experienced winter temperatures below 0°on several occasions. Considered a dwarf or semi-dwarf cultivar, they remain less than 5ft. tall, but spread about twice that, over the years.
Yesterday they were picture perfect, but an overnight TS, with small hail, took away most of my Azalea blooms, except for a few Southern Indica's, that were sheltered by evergreen trees and the Satsuki hybrids, which are still in bud. A large R. canescens, still in full bloom, also looks extremely sad today! On the other hand, one late blooming R. austrinum and R. flammeum, R. periclymenoides, R alabamense and R. x 'Admiral Semmes' weren't affected, nor was R. chapmanii, just beginning to flower. R. colemanii and R. cumberlandense have had buds for weeks, but are shy about opening! Lots of strange happenings in the garden this spring, after a very long and colder that normal winter, which continued well into spring.
I have planted Azaleas, both evergreen and deciduous in the springtime, but have had to pay close attention to water needs during the hot, dry summertime. I will plant Southern Indicas only in the fall, as I've experienced too many failures with Spring planting of those.
MO
Rb

RB, FWIW, I think you are mistaken in your designation of Delaware Valley Whites as Kurumes. I just looked them up to make sure and they are, indeed, Southern Indicas. They are a sport of Indica Alba, also reported to be hardy to around 0 degrees F. Planting them in zone six would be iffy. You must have them in a protected microclimate to not have had any damage during sub zero temperatures.

Further, all spring plantings must be watered during dry spells for the first year, Indicas are no exception to the rule. I have some ten or twelve Indicas that are still in their original pots from a couple of years ago (I am ashamed to say), Mrs. G.G. Gerbings and Pride of Mobiles. They are no more susceptible to drought than anything else in the azalea line.

They bloom, for me, just at the fade of (most of) the Kurumes (Coral Bells and Christmas Cheer) and just into the commencement of the Indicas (G.G. Gerbing, Formosa, etc.) so they form a nice bridge between the early and mid season azaleas. This also has the unfortunate effect of their sometimes being subject to frosts, but in most years this is not a real problem.

nippersdad...
I suppose that the true origin of the cultivar, 'Delaware Valley White' is so obscure that you can probably find many different hybrid associations for it. The species name Azalea indica, var. alba (Lindley)(Non Pub), was once assigned to R. mucronatum, a Japanese species.
R. mucronatum is considered by some to be a parent of DVW. But the published description (Latin diagnosis) described a cultivated plant, so it's anyone's guess as to which plant or hybrid was being described.
DVW was also once listed as a Glenn Dale (National Arboretum) hybrid, but Galle (1985) did not include it in his list of Glenn Dale hybrids. It was probably named and subsequently introduced by the National Arboretum.
The cultivar has been grown in Japanese gardens for at least 300 years and there are no record of it's origin.
But it cannot be a Southern Indica, as it is much too cold tolerant. Kemper Center (MOBOT) rates it hardy to -15°F. (Zone 5) and it's grown there in St. Louis (Zone 6).

My Southern Indicas, 'Formosa', 'Pride of Mobile', 'President Clay', etc., surely didn't survive the -12°F in January 1985, but all other Azalea cultivars did.
Rb

According to Frederic P. Lee, Delaware Valley White is a Rhododendron mucronatum cultivar propagated by Delaware Valley Nursery.

Fred C. Galle agrees, classifying it as a Southern Indian Hybrid.

These were the two ultimate authorities on azaleas. I would never disagree with either.

RB: I think your wrap up pretty much covers everything known about them (in my books, anyway). Kudos on your research! As you say, they are hybrids and no one really knows who all of the chinese/japanese parents the original European hybridizers of Indicas utilized in their production to begin with. One, R. Simsii, appears to have been from China/Korea, thus lending it their general ability to shrug off frosts (?). As you say, who knows?

They may very well have a stronger tolerance of cold than the general category and were published with the others as a convenience. At any rate, Jay's apparent concern that they are comparatively unsuited to warmer temperatures has now been allayed.