What dahlias do you have? Some just don't have very strong stems & always hang their heads- like Fidalgo Splash- a gorgeous flower but after the first couple of blooms the stems get weak & the blooms hang from then on- or if they've been OK in other years it might be a nutrition problem- phosphorus makes for strong stems & roots.

I am wondering if you couldn't use wood ashes as a top amendment (for the phosphorus). Anyone know?

Wood ashes are usually considered for potash content which help tubers to form. Check out the website below which discusses wood ashes.
You can use Mor-Bloom 0-10-10 which is an Alaska Corporation product at the rate of one quart a week per plant as a drench and this will stiffen your stems without giving the plants any nitrogen. The mixture rate is one tablespoon per gallon. You can also use it as a spray. You can find Mor-Bloom on the net or Lowe's and Home Depot now carry some Alaska products.

Here is a link that might be useful: UCONN IPM: Homegrounds: using wood ashes in the garden

Thanks for responding Huey and the website.

I couldn't stop myself and just poured a little ash in a semicircle around a number of my plants. We'll see. Meanwhile I will look for Mor-Bloom.

Patricia