Well, you're still starting with less total plant than you had before. It is true that there might be a suckerlike behavior to the regrowth for the first years after it sprouts from the roots. After two years you are probably seeing about as much of this effect as you are going to, unless you do something like topdress with compost, fertilize (sample and test soil first) or increase irrigation that gives the plant a boost.

For what it's worth, I wanted to share our recent experiences in transplanting a couple of very large conifers. Late September 2006 a friend gave us a very large, old Tsuga canadensis 'Pendula' or similar cultivar, that is probably 7-8' tall, 5-6" caliper, that had to be removed from a property he bought. We dug it out with a backhoe, hauled it about 10 miles on a flatbed trailer, and transplanted it in our yard. The rootball was very shallow, so we dug a very shallow but very large (5-6' wide) bowl-shaped hole to spread out and accomodate all the roots, hand-packed loose soil under, around and over the rootball, created a nice large berm around it, doused it heavily with root stimulator, mulched, and kept well-watered. The transplant was done in early September out of necessity, and the tree has done tremendously. We have been amazed, especially since Hemlock are very hard to get established in our area. About a month ago, we were at our local nursery when we overheard the owner tell several employees to "cut down" a huge Picea pungens 'Globosa', because it had outgrown its space next to their office. The owner told us he personally planted it over 30 years ago. My husband expressed his disbelief to the employees that they would cut down such a specimen, the employees were also hesitant to destroy it, so they hopped on their Bobcat with a forklift attachment, stuck the forklift into the rootball, lifted it out of the soil, and sat it in the backend of our pickup, nearly barerooted. This tree was planted in a foundation bed that had been heavily amended over the years, and the soil was extremely loose. Again, the roots were all very shallow, but widespreading. We brought it home and spent half a day planting it, similar to the Hemlock. It's too soon to tell how this blue spruce will do. By contrast, we have bought quite a few species conifers, all B&B and dug with a tree spade. Huge root balls, but all of the lateral roots were severely cut in the spading process. The bottom 1/2 of the root ball never contains roots. What we have observed in these experiences is that conifer roots tend to spread much more laterally and shallowly, and it would seem to me more important to dig at least to the drip line or a bit beyond, but not necessarily as deep, to make the size of the rootball more manageable and cut fewer roots. I've also come to believe the root stimulator is an important ingredient when transplanting a tree with cut or disturbed roots. As Dave stated, the conventional wisdom is that trees should grow their roots deeply, thus slow deep watering, but this is certainly not what we have been observing over the last several years in conifers. It would have been nearly impossible to move either of these trees without heavy equipment. If you're a strong do-it-yourselfer and have someone strong to help, I would think you could carefully hand-dig the trees, then rent a Bobcat to shallow out your new planting holes and move the trees, provided you have room to maneuver equipment on your property. I agree with Ken that early Fall would be the preferred timing to give the trees more time to establish before the heat and dryness of summer. Go for it!!

Was it sold as a planter? I would put it somewhere that perhaps it is shaded during the hottest part of the day. I would also plant it outside of a pot. For a really large plant like that, the pot you will need is huge!! I have a neighbour that grows large Dahlias in planters very successfully, but urns are HUGE!!! Must be nearly 10 gallons!! I have heard tomato fertilizer is good for them. My MIL uses MG with great success... I tend to forget and all mine grow fine. I have heard that high nitrogen can cause the tubers to shrivel up when they are recovered in the fall, I would suggest laying off it at the end of the growing season if you want to recover the tubers in the fall. I have also heard this is when they typically do most of their growing, in cooler weather. Dahlias like soil that is not wet. It sounds like it either does not have a good root system yet, probably due to the pot being too small, or the sun is just too overwhelming in the zone you are in.... I don't know much about zones and dahlias past zone 6 though. My dahlias like really dry soil. I have them close to the house and they get almost no moisture and lots of sun.... Your biggest challenge is probably that the pot is too small.

" I could kick myself because years ago, I passed up a nice sized specimen." You passed on a specimen whose carcass you might have been digging out of the ground this week, when the record rains of early summer killed it. As I'm always saying, at least hundreds, probably thousands, of (expensive, usually) monkey puzzles have been planted by gardeners along the east coast since the 60s and before, and yet we hardly know of many mature specimens. I can't remember if you are on LI or in NJ, but either place should have plenty of mature specimens - as they do of other desirable, sufficiently-hardy collectibles of the 20th century like Sequoiadendron. Just in my random existence, not even involving contact with collectors of rare plants, I know of one in the neighborhood in Columbia, MD, where I briefly lived in the mid 2000s that was killed off by a wet spell one summer. I never even spoke to the person who owned it, I just noticed it prominently planted in their yard. It was 8' tall and had probably been there a few years, looked established and healthy one year but was gone the next. I once found a primary source/gardening journal in google books from the 1920s attesting to their difficulty in CONUS non-maritime climates! Specifically on Long Island IIRC. It's pretty clear that in places with cool summers to their liking, winters are too cold, and where winters are sufficiently mild, summers are too hot. The one at Polly Hill Arboretum has supposedly died back a few times, to the trunk. (that's just what their website once said about it: seems a bit implausible to me but the take home message is: was probably planted decades ago, yet is not a mature specimen even in that somewhat benign climate. NB the Fairweather guys claimed to have found a huge Austrocendrus on Cape Cod or the islands, which would have been the only one of the east coast) The known US east coast ones over 25' are: The Barnes Arboretum (who also had the horticultural savvy to bring the northernmost Sequoia on the east coast to maturity: the key for both probably just leaving them alone and not "feeding" them), the one in Bethesda MD that survived <<0F in 1994, and one at an old estate in Wilmington, DE, that was cut down for "not looking English enough." (the best of all possible anecdotes concerning the overall idiocy of most people who practice so-called "landscape architecture" in this country)