I used to on my long-stemmed cutting zinnia, but had it solved before I really started growing dahlia.

In my particular case--and that's not going to be universal by any means--it was a shortage of phosphorus and boron in the soil. Cells didn't develop properly, leading to weaker, easily broken growth.

Assuming it's not some strange disease, a soil test might help. While adding phosphorus in modest amounts is pretty harmless, you can't do that with boron.

I think at least two more factors are involved:
not familiar with your climate, not sure whether 1/2 day of sun is really enough.

and a rainy summer probably results in lanky plants, making them more fragile.

The other could be genetics, some cultivars have sturdier stems than others. Do all your plants have that fragile stem/ neck or only certain cultivars?

I grow my Dahlias on two plots, one with full sun, the other with some hours of shade. Some cultivars took the part shade not well, producing weak flower stems.

I planted those in full sun in the following season which helped.

About soil and fertilizer: K promotes healthy cells and probably good tissue. Proper soil testing service should include an explanation of the results and recommendation how to feed properly for your purpose, that is, Dahlias.

Good luck, bye, Lin

Hi;
The best way we have found to protect plants with heavy blooms and small stems is to stake them. Please see my info on how we stake them.

Hi again;
I failed to mention in my previous posting we have found that spending a little more and using yarn rather than twine, we use green and buy it in 2 pound skeins, there does not appear any harm to the stalks/stems at all when tied loosely.
Charlie

Thank you for the responses. I use 52" tomato cages for staking. I have only minor problems with broken stems. Just the blossoms.

I prep the bed in spring with 10-10-10. Then feed once a month with 15-30-15. Next spring I will prep with bone meal to boost the phosphorus. Thanks for the idea. I'm not sure about boosting the Potassium. I put a few left-over bulbs on the edge of my corn patch. Corn is K-intensive. Although I get adequate blooms, the plants are two feet shorter than the same bulbs in my main garden.

Yes it is only certain cultivars: Wheels and Poodle Skirt. Both are novelty blooms.