But even if we had planted tubers, the soil would not have warmed up enough for dahlia tubers to quickly sprout. People point out that the dahlia tuber clumps that they left in the ground over the winter are sending up shoots long before our planting dates. These overwintered dahlias probably sent up the first shoots many weeks before one sees them poke above the soil line. But there are big differences between an over wintered dahlia clump and your lone tuber. The overwintered dahlia clump has probably 4 to 6 or even 10 tubers with eyes. Only one or two sprouts need to successfully grow to make a plant and if several are frozen or eaten by slugs and snails, there are very many sprouts to take their place. Our lone tuber may have only one eye and if it freezes or rots, or is eaten by a slug, may never recover. So the best course of action is to plant our tuber in warm soil, so that it will jump out of the ground and thrive.

There are at least two ways to get your dahlias blooming earlier and planting them in the ground too early is not one of them. One of the best ways is to plant your tubers in black gallon nursery pots. No special soil is needed as your garden soil works fine. The pots can be placed in a warm environment such as a greenhouse but the easiest way is to just leave them outside in the full sun. The black pots absorb the sun's rays and are much warmer than the garden's cool soil. Another way is to take cuttings from dahlia tubers or pot tubers and grow them in successively bigger pots. By the time the the dahlia is a foot tall or so in a gallon pot, it will really put on a growth spurt when planted in the garden. If you are a bit lazy, just leave the dahlia cutting in the small pot where you placed the cutting to root. Even a small pot dahlia will bloom two three weeks before tubers planted in the ground.

But the USDA has a a different system called the USDA Hardiness Zones. According to their charts we are in zone 8a.

But again there is another chart called the AHS Heat Zones that tracks how hot you get in the summer. They count days above 86 degrees F. We are in zone 4 for that one.

I still like the Sunset's Climate Zones because they take into account both cold and heat.

We typically have a few days over 100 degrees and about 20 days over 90 degrees. We seldom go lower than 10 degrees in the Winter and in a typical year the lowest temp is about 15.

Our last Spring frost date with about 95% probability is May 1st.

In the last 22 years at my location, the latest killing Spring frost has been April 22nd. In the Fall we typically have a killing frost about October 31st. This year we went well into November without any frost.

That is the long answer to your short question.