It seems to me the average dahlia runs 4 ft tall. Some shorter, some taller. I've had some "4 footers" go on to attain 6-7 ft: "big honkers" as you say.
If they're subject to wind as it sounds from your post, I'd say the bare minimum you'd want for a stake is 5 ft long with about a foot sunk into the ground to anchor the plant. I used tall bean pole stakes last year for some 6+ footers and was glad I had them.

I can't tell from your post if the compost-y material is too loose to anchor the plant, but I don't know that I'd go to all the trouble to sink an even longer stake into the 'firm' soil below it. If I have a troublesome extra tall plant, I usually end up adding a second stake. Sometimes I anchor one tippy dahlia to another that has a better grasp on life in the garden.

Really like the idea of spray painting the rebar- great idea. Let us know how the paint holds up. And best of luck in your wonderful sounding dahlia patch.

The ones I made last year were given a rust resistant spray coat first, then John Deere green which I had on hand. Why my International hubby had green on hand I don't know. Looking at them this spring they look like new after one year on the job. And they really hide on the job!! I'm hoping for 5 years before a new paint job is necessary.

My new plant stakes 4 from a 20' length of rebar for 5 bucks giving me 4 stakes at $1.25 Got enough for 20 more stakes and got over half of them with a wire brush. Now to wipe clean and prime and then JD green. Figure I've got about $1.50 in each stake after painting twice.
Figure I've got 10 inches to go in the ground leaving me a 4' stake for my dahlias.

Sounds good Jamlover. You're going to have the prettiest bargain priced stakes in all of Iowa. And lots of exercise managing them all!

I am going to try rebar when my wood stakes break. I lose a couple a year so may as well convert to something permanent.

Excellent idea - good thing i didn't give away all that dark green paint in the basement. (I have some light purple Security Spray Paint too. Kind of tempted to try that)

Instead of buying the rebar -
I'm going to list a *Wanted* in Craigslist Free or Barter section - and on Freecycle (see Yahoo Groups for one in your area) - and keep an eye out for materials at my Spring Neighborhood Clean-up.

I know that lots of Contractors are happy to drop off left-overs to avoid landfill and storage costs.

How did you cut the Rebar?
(I have a Sawzall - but I'm afraid of it...
one has to be pretty strong to keep it
from jumping all over creation when it's running!)

This time the man at the lumber yard did it for me. Last time my son who has an auto shop cut them for me. 20' long was to long for the back of the pickup.

The husband cuts rebar with a chop saw- too dangerous w/ sawzall & there's always a hacksaw-- slower but builds up those arm muscles to get you in shape for digging holes!!

Last year I went to the neighborhood hardware store & they had scrap pieces of 3/4"PVC anywhere from 4 to 5 ft. long. They worked out just great. Didn't paint them till this year. Used other scrap pieces to stake up some floppy Mums.

Somewhere I read that you can use tomato cages for dahlias. Some people spray them green. Has anyone used them? It sounds like a good idea, that way all those heavy flowers are supported, like tomatoes are.

They work very well- esp. if you're growing in pots. I think the Colorado Dahlia Society site has a bit about tomato cages on it- they turn them up-side down but when I used them in pots I just stuck them in the normal way & they held the plant up fine. The only problem might be if you have really tall dahlias they might flop over the top.

I have a few tomato cages, bought several as well as those 4' square cages. Last weekend, a neighbor cleaned up his garage and threw out five 4' metal plant stands.... They are sturdy and when turned up side down, they stand just like tomato cages shown on the Colorado Dahlia site, except taller. I just used one for my brand new 'Camano Pet'. I might have to add a taller stake later. I use bricks to anchor the stand for now. I'll have to see if the bricks will be enough.