Dahlias grow the very best when the soil temp. is constantly 60*-- no 2 ways about it! If they are already growing & you put them out they might just sit there stunted until the temp. gets up to where they need it to be to grow- or the tuber will rot & not have enough feeder roots growing yet to sustain the plant- then you've got a dead plant. Also- dahlias & tomatoes are related-- so there you go- what's good for one is good for the other!
Your dahlias grow all summer long & as the temp gradually gets cooler, they keep going until frost- yes, it's colder than 60* but they've had all summer to put on semi-hardy stems & the fall to get used to the colder temps. The stems in spring are tender new growth & way more susceptible to frost than the woody thick stems that have grown all summer long.
Why do Home Depot & all those nurseries have tender plants out now?? When they frost & die you have to go back to the store & buy MORE, of course! Stands to reason- if they can sell you the same plants a few times over they're going to! Most savvy gardeners don't fall for their tactics & don't purchase tender plants until they can stand up to the local weather.

Perhaps the first question to be asked is how developed are the plants? If you have just small growth I see no problem with your putting the plants outdoors now--but that all depends on how you plant. I plant my dahlias deep--6" to the top of the tubers, and then I only backfill as the plant grows. If there is a small amount of growth you can plant, leave some of the growth showing, and in case of an anticipated frost you can backfill and cover up the new growth. I live in zone 5/6. Five because that's where I live, and 6 because I am surrounded on three sides by salt water, so I am relatively warm. If you have tall growth, plant now but be ready to cover the plants if a hard freeze is forecast. I have had dahlias that have gone thru the winter in the ground, and we had snow, freezing weather, etc. The bulbs will take some cold.
If you want to have me give you some answers, tell me your zone, circumstances, etc. and put the word "GardenWebber" in the subject area to keep your e-mail to me out of the junk pile. I'll do my best for you.

Here is a link that might be useful: Exchange List Link

Thanks for the replies.

It seems everyone feels strongly about their dahlias! Does anyone have any scientific articles on the temperature subject?

Since sending my first post, I have called the local garden centers, all of whom say that they get their dahlias about May 1st, right between the average last hard frost date and the last frost with 10% certainty date. I have planted these for years at this time and they have all done well.

Thanks for the advice. More follow up later.

Have you tried contacting your local extension service? They should have detail info about your area. If I may suggest - (stepping on my tip toes here) no one has experience in your garden's micro-climate other than you. I wonder why you are seeking information from others when you have had great success in the past! Go for it and enjoy.

The local extension knows about local climate and soil conditions, not about dahlias in particular. I am asking these questions because there seems to be a lot of folk wisdom out there about dahlias that is not based in actual science about the dahlia plant itself. There is also a lot of advice out there that was developed to grow dahlias for show instead of as landscape plants.

You say:
- last frost date with 50% certainty: May 15th
- last frost date with 90% certainty: May 29th
I would plant them half way between these two dates or May 22nd. Dahlias in the ground with tubers for roots are pretty hardy and a mild frost would only nip them back. Odds are with you that no frost will occur and almost no likelihood of a killing frost. I am always surprised how everyone wants to plant early rather than a bit late. We plant here beginning about May 1st and end about June 10th. All the dahlias planted between these dates do fine. The later planted dahlias look better in September than the ones planted earlier. However, they do start to bloom later than the ones planted in the early season.

"Also- dahlias & tomatoes are related-- so there you go- what's good for one is good for the other!

No relation at all...period! Tomatoes belong to the Solanacea family and Dahlia to the Asteraceae family. Just wanted to clear that up.

Vera

Please see this page from Corralitos Gardens. It goes against conventional wisdom regarding dahlia planting dates, but has, in my experience, worked. Please also not the more technical note about the temperature of the soil.

If you really do your research, the commonly quoted 60 degrees for first planting is just not workable in much of the country.

http://cgdahlias.com/dahlia_growing_tips.html

Here is a link that might be useful: planting advice

I start all new dahlia tubers in gallon pots starting in March, with them in a warm room in my garage with grow lights. I aim to have them about 18" to 2" tall when I set them out so that the slugs will not eat off every shoot. It has been working fine for years. The gallon pots give them enough room for root development. They transplant well for me at that stage. I try to get the tuber down 6" from the soil line when I transplant them. I pull off any leaves that are below the soil line. The last frost is usually not later then the 3rd week of April. And yes, our soil sometimes does not get up to 60 degrees until late summer. By that time I often have buds on my dahlias...first bloom usually in late June and they will bloom until frost around 31 October. I am in a very temperate maritime climate. I grow about 160 plants a year. Last year for the first time in 2o years 3/4 of my tubers froze in the ground. So you never know, there will always be the exception to the rule. But this works for me. I can't remember when I had the last rotten tuber that was not from being frozen over the winter.