I live in Northern Maine. I planted 3 variety\'s of tall phlox about 26 bare roots on May 20th. I planted them in big containers until I had a bed prepared for them. Only 3 of the 26 have sprouted any green. And are only about 2 inches tall. Yesterday I prepared a bed to transfer them to, and was shocked to see all very healthy roots, but absolutely NO green sprouts on them. I have only had to water the containers once as we have had a very rainy season so far. And it has been cooler overall than normal. All my other perennials (ones I planted this spring and older ones) are all thriving. They seem to be loving this weather, including my vegetable gardens. But the Phlox has me baffled.
The soil may have been an issue. If the soil was not light enough to allow good drainage and oxygen to the plant roots it may have stunted the growth. Watering only once in this time period seems to indicate heavy soil that is not draining.
Plant them out and see how they do.