I'm tempted to try them this year but would love to hear your experiences with them.

I'd guess even the cages would require staking to keep them steady but is one stake okay or would the cage require two stakes?

Would the dahlia height be limited to those growing to 3 - 3.5'?

I've used them for my dahlias and think they do a good job. Some of the dahlias tended to bend over at the top of the cage from the weight of the blooms, but the majority of the plants stood upright. I only had two that started tipping from the weight of the plant, but mine probably aren't as full as yours... I even used small bungee cords for the "escapees" that got too tall to tuck back into the cage! As cheap as they are it's worth a try!

Thank you, Rose. Now that I think about it I did have one that had a dahlia and it did tip over. I didn't even recall trying it! Bungee cords always come to the rescue here with roses and tomatoes.

I've resorted to using T-posts for tall plants here. I just get some garden twine and tie them up where needed.

Last year I tied Bahama Mama to an 8' tall Limelight hydrangea but it sagged when I wasn't watching and I couldn't get it back in place.

I cut out sections of 2" squares green garden fencing into 2' heights and appropriate diameter. Mine don't grow as tall as yours outside (lower 48") so they work fine. Only had one or two that needed to be staked to hold the fencing upright. I would say that the maximum height I have ever had with a dahlia was around 40", mostly more like 30". Lots of flowers though.