There are 4 possible explanations that I can think of; sun, soil, water, and genetics. Soil includes nutrients, ph, drainage as well as composition (organic matter, for example), which can all really vary within 6'.

You probably know if one of these gets more sun or moisture, and how the soil was prepared when you planted them. You probably also know if one is larger or has grown more rapidly than the other.

You could have a detailed soil analysis done on samples from the rootball area of each, and I guess that's what I'd do. That, and start some cuttings of the one that flowers, in case you need a replacement for the 'bad twin' - if you can rule out all the possible variables you'll be down to genetics. I would not assume that that;s the problem until I looked at all the other possibilities, though.

diggingthedirt-

I also have a Doublefile Viburnum and would love to root some cuttings. How would I go about it?

Thanks,
suebot in CT

Hi Suebot -

Dirr says: "I have taken cuttings throughout the growing season and achieved 100% success with 1000 ppm IBA, peat:perlite, mist; one of the easiest viburnums to root."

and

"I have grown well-branched, 3 to 4' high specimens in 2 growing seasons from cuttings without special care."

I think I've had this book, the "Manual of Woody Landscape Plants" for over a year, and have yet to figure out what IBA is, but I'd guess it's either a rooting hormone or some kind of anti-fungal compound that's mixed into the planting medium; you could check via google.

IBA is rooting hormone - it stands for indole butyric acid.

(Sorry but I don't have any suggestions on the original poster's nonflowering issue.)