Because this is the 'down' time of year when few pics of all the gorgeous dahlias you grow are posted, I thought I'd offer a reminder to those who are new to dahlias. I am hardly the dahlia expert but I have made enough mistakes to qualify as an expert at screwing up.
I do not dig up tubers and replant in spring. I rarely even dig up tubers and divide them every year - though I do get around to them every 2-3 years or so. What I DO do is amend the soil and this is why I write.
Last year I did not amend a large section with 9 dahlias in it and though very acceptable as a crop, it was NOT as good as the previous year when I dug up the soil and mixed in new soil with fertilizer in it. After amending the soil, this particular section was a work-horse and I know it was the soil because I had one plant that was transplanted from another area (exiled in fact for doing poorly) and it grew like a weed!
I did however amend the soil in a nearby spot where I had only one 5 yr old dahlia growing and I amended the soil because for the prior 2 years the blooms had been shrinking and were not very prolific and the plant was looking very weak and tired.
But last year AFTER addressing the soil, this dahlia looked brand new. It sent out 8 stalks, had large buds and bright blooms and it bloomed from July until October. It got to over 6 ft tall and if I told you what it was planted over and how I created the section, you would never believe me that it could grow anything. That taught me that simply addressing the soil with new soil mixed with a fertilizer (last year I used chicken manure but very sparingly!) made all the difference.
So I now amend the soils in every dahlia plot I have AND in the containers because the difference between tired soil and fresh soil with manure is unbelievable.
All the dahlia veterans know this and preach the same tune but I can confirm it works and is well worth the effort. And I know how hard it may be to get out into the yard and in the rain (not that we are getting any of THAT) especially when you have 100 dahlias but at the least, just spread some fresh potting or garden soil containing MG or any fertz in your beds and that will work wonders as well. Note: If the soil you bought already HAS fertilizer in it, there is less (if any) need for extra manure. I do it because I use manure sparingly with new soil and need to get rid of it!