I'm wondering if the fertilizer is really to blame. I grew dahlias among my roses last season as well, and they bloomed their fool heads off. I use high-nitrogen lawn fertilizer on my roses (like 30-5-8 or something like that), and everything did really well for the growing season. However, the tubers did not survive the winter in storage. I had expected that, because I had read that high nitrogen will make the tubers not keep. So this year, I'm planting the varieties that are readily available at the big box stores in among my roses, and saving another area of my yard for the more exotic dahlia tubers that I have on order.

Most Dahlias are hardy to USDA Zone 8 where they can be cut back and left in the ground to overwinter. In Zone 7 dahlias typically overwinter in the ground depending on soil drainage and severity of the winter.

Nitrogen or not I don't think you can expect a Dahlia to survive a zone 4 winter. The good news is that the tubers are like small potatoes and very easy to lift and store.

Select a sunny (minimum of 6 hours) location much like for roses. Dahlias grow best in a deep, fertile, well-drained soil with a pH of 6.0 to 6.5; they are not tolerant of water-logged soils.

Mine bloom well for me no matter what fertilizer I use....or never use. I have heard it recommended that you use a bit of 10-10-10 around them.

Why you didn't get any rebloom I cannot say. It is recommended that faded flowers should be removed to encourage continuous blooming.

HTH

Ronda

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I interplant roses and Dahlias all the time. They grow and bloom very well together. Some of dahlias come back and some not. I think certain dahlia varieties are more hardy then others, which is not surprising.
Olga

I used so-called miniature dahlias (around 2' tall max)in rose beds, I especially loved those with burgundy foliage. Some still returns.
The big Dahlias, however, get very tall and flop all over they just didn't look good among the roses, so I don't use them any more.

Just to clarify, my dahlias were in storage for the winter and did not keep. No, you most certainly cannot expect dahlias to survive the winter outside in the ground in my part of the world. SIGH. So many of the gorgeous flowers that I grow have to have special care for the winter.

I grow dahlias in amongst my roses. Thankfully, here in my climate I don't have to dig them up. They come back every yr, usually bigger than the yr before. They get so leggy, that if I don't stake them up, they can end up becoming groundcover! Which in some instances, is rather interesting.

Dahlias grow like weeds here-and do fine with any fertilizer. They need good drainage but lots of water. The tubers are edible. I suspect that voles and so forth are just as responsible for the disapperance of the tubers as freeze damage. If you lift them, wash off the soil and put the tubers in a cardboard box or paper bag with some slightly damp peatmoss or wood shavings and keep them cool and not too damp-they rot easily. It is hard for us these days to find a place to keep the tubers..you optimally need a root cellar. You could supply the world with tubers in just a couple of years.

patricia

My solution is to buy the really cheap ones at the big stores like Home Depot and just allow Nature to take its course. Some come back, some don't and they seem to bloom great with little care, unlike my roses!