"I'm delighted that Ben Morgan's decided he wants to play for England. That's fantastic news for us," said Rowntree.
"He's a 22-year-old, born in Bristol and we've been looking at him for a long time. He's a young English guy and he played at Dursley and Cinderford before playing for Merthyr Tydfil and the Scarlets.
"We've been impressed with his performances in the Heineken Cup and in the RaboDirect and he played very well in the recent game against the Ospreys.
"I'm personally really looking forward to working with him. He's an athletic, big, ball-carrying No8 who's good at lots of things. He's a good lineout forward as well and he's got good collision skills also.
"We're delighted he's on board and, without pre-empting selection, we'll be looking to get him involved very quickly."
Morgan would have qualified for Warren Gatland's World Cup semi-finalists on residency grounds later this month and has been at the centre of a long-running battle between the two nations.
But despite previously turning down a chance to represent England's second-string Saxons, Morgan admits he is now fully focused on building an international career with the country of his birth.
"I feel privileged and honoured that both Wales and England have been following me," said Morgan, who has been a standout performer for the Scarlets since ousting Wallaby David Lyons from the side last season.
"It has taken me a while to think over and discuss with my family before I came to the decision of where I'd like to focus my ambitions for the future.
"But I'm English, I grew up supporting England and as a youngster I always dreamt of playing for England one day.
"Ultimately, if given the opportunity at international level, I want to try and take things forward with my home country."