1) First year tubers bloom a little later.
2) That having been said, some dahlia are simply later than others--all my Sun Lady exploded early. All the Color Spectacle took their time. The Blue Boy and Sky Angel were in the middle.

Your care (off the cuff) sounds fine. Sunny, warm, well-drained soils with moderate feeding are perfect for dahlia. If the pots were large enough for them, that wasn't a problem. Plenty of people raise dahlia in pots all season long.

Transplanting into the garden might shock them a little bit, but I do that every year with my smaller, seed-grown dahla (Figaro and Harlequin). They barely seem to notice.

My oldest dahlia is about six or seven now. My newest was this year. Those and everything in the middle performed extremely well and I can't say I noticed any significant performance difference between them. The oldest tubers did tend to bloom a little earlier, but that's it.

My leaves have also darkened, and we had our first frost last night. I expect that either took them out or damaged them, so I may be digging tubers this weekend or early next week.

Some of my additions this year were very, very late (June 15th or so), and I shallow-planted the tubers to avoid waiting for sprout. Plus I wasn't sure how much energy they had left and didn't want to make them work for it!

Dahlia have pretty wide, shallow root systems that are capable of holding the plant up (wind may be a problem even with deeply planted tubers, however). All of mine, deep and shallow, featured strong, upright growth, strong stems, and good upright blooms. All survived multiple strong thunderstorms with no or extremely minimal damage.

One consideration would be winterizing in a marginal environment (which you probably don't have in Zone 6). There, deeply planted tubers are preferable. For those of us who lift (garden) or store (pot) dahlia, it doesn't seem to be that important.

They rot if left in the ground in zones where it is too cold or winter soils too wet. And if dug and stored properly, they keep very well over winter. Like morpheus, I have planted the same tubers multiple years and they just keep getting bigger and better.

You asked about topping techniques to get the plants to bloom earlier... That works to kickstart the plant to grow lateral branches quicker, but that actually delays the first flush of blooms by a couple weeks. But you will have a more constant supply of blooms when they DO start.

Kousa's method of piling soil on the sprouted tuber might indeed help stimulate growth earlier in the season, when soil temperatures under 60 degrees slows activity. Swan Island does exactly that, using a tractor to hill the tubers like potatoes. But SI also tops all their plants in July to keep them from growing high, as they don't use any supports. I bury my pre-started tubers with 12-18 inch growth about four inches down, and notice a stalling out of growth for 2-3 weeks.

It sounds like there is something else going on, if they are only blooming now...

I am wondering how many plants you are talking about, and how much you fertilized/watered them. Unless you were just growing 10 inch or larger varieties, you should have had blooms early August after putting out pre-started plants mid-may.

My experience is very different then yours. There are few acceptably firm mother tubers in any of my clumps when I dig. Most are hollowed out or soft, some rock hard and very dark.

I was taught by my mentors to toss them immediately, or if you must keep them, separate from the other tubers to keep rot from spreading to the rest of the stock.

Since our soils warm up quite late in spring, I now grow all my dahlias in containers, which I store intact (minus the top growth, which I remove after frost) for winter and bring out again in early April to set out on concrete to warm up fast. Warmer soil prompts earlier growth and IME, early growth translates to earlier blooming. About every 2-3 years, I unpot the tubers, divide and repot with fresh soil.

I've heard that growth from clumps are quite vigorous, and folks in the
UK make all their cuttings in spring from last year's clump, throwing out the 'mother' clump after it is spent or they don't have need of more plants.

If that is what Morpheus was referring to, that makes more sense.

For an active trader/seller like me, I wouldn't dream of 'wasting' tubers by throwing them all in the ground for just one plant. However, that is exactly what I was suggesting to some older folk I was just helping earlier today. They only want one plant of each variety, and don't have the dexterity required to splice out a dozen individual tubers from each clump they have. I was showing them how to cut down the clump of any extra broken tubers and squishy mother tuber, so it would store better and they would be assured of spring growth.

>>If that is what Morpheus was referring to, that makes more sense.

Pretty much. I remove the mother tuber(s) from the clump at the end of the season. They're clearly spent and not in great condition, but...well...

OK, I'll admit it. With 2,000 square feet of garden, I'm too lazy to split everything that gets lifted every year. My Sky Angel had seven or eight mothers, and I don't have room for another seven or eight Sky Angel dahlia. They're nice, truly gorgeous, but would take space from the zinnia and salvia and marigold and Easter eggplant and...

So instead I have two absolutely enormous, jungle-like clumps of those, plus (at this point) eleven other enormous, jungle-like clumps of other dahlias.

When I do split, I immediately start looking for people to adopt them, but I rarely do more than divide the clump in two.

Again, time. My dexterity and vision are up to the task, the amount of time I have during the window of frost to freeze is not up to the task.

Older clumps sprout earlier (of course, some of those eyes are more active than others and sprout first), produce huge "bushes" of greenery, and bloom profusely on many branches at once...so of course some blooms appear earlier.

Care of larger clumps differs, including keeping a close eye out for fungus, and they do seem to like a heavier feeding than a smaller single-tuber dahlia would. That's fortunate as I don't isolate them from the rest of the (demanding) garden.

Large clump dahlia perform well--I've included a link to my garden images, which get updated about once a month or so at worst.

I like to use them as central showpiece items with smaller plants around them for accent and color contrast. We should point out that I never met a saturated flower color I don't like, nor do I approve of blank space in a garden!