Don't Rush Things
When you're planting dahlia tubers, wait for your frost date to pass, the soil to warm a little and check the forecast to avoid a long stretch of heavy rain at planting time. Spring showers are unavoidable in most areas and so long as you plant into well draining soil, they'll be just fine. If your garden gets really soggy during rainy periods, a slightly raised bed will benefit your dahlias... doesn't have to be a swanky wooden box number, just a raised mound of soil can work wonders.
You can sprout tubers early in pots, by a window, in a greenhouse and then transplant them into the garden and be the first in the neighborhood with dahlias, but make sure you transplant them after your last frost date and into soil that has warmed to around 60 degrees.
Dahlias can take their sweet time emerging and don't expect them all to pop up at once, that's just not their style. You might notice that some varieties emerge sooner than others, while others feel like they're taking forever. I've noticed that the darker varieties pop up earlier and suspect that the anthocyanins (the chemical that's responsible for red and purple colors) in those darker varieties offer the plants more protection from pests which allows them to get a head start during the Spring when pest pressures can be high. The lighter dahlia varieties tend to take a more cautious approach and emerge a little later when pest pressures are less intense.
August is the great equalizer and no matter how or when you started them, by the end of the month all the plants tend to be at the same stage, pumping out blooms and buzzing with happy bees.
