My understanding is that dahlias need lots of sunshine. I grew some last year from tubers on the south side of the house where they received full sun all day and they did wonderfully. I also read that they like deep waterings.

She's a beauty. I hope someone will have better feedback for you and she'll perk back up for you.

The tag which came with the plants says partial sun.

Do spider mites live in the soil? Or is spraying the leaves/stalk enough to get rid of them?

Once an infestation is noticeable and correctly diagnosed, the plant might be a gonner if immediate action is not taken. You have to realize that spider mites are extremely small, and the numbers have exploded to crazy amounts for you to see damage on the plant. They are hard to get rid of, and Sevin only increases their numbers. There are several chemical products out there that have been used unsuccessfully, even when the directions says it will work for spider mites. Soapy water or horticultural/Neem oil works most times.

For a plant that size, I'd cut all the flowers off with any damaged leaves, and rub every inch of remaining stem and leaf with sudsy water, saturating the ground around it with the solution. I'd repeat every few days, keeping every other plant well-away from the contaminated plant. You can also use horticultural oil, but I believe actively rubbing with soapy water is better. If you have lots of plants, or a big plant, horticultural oil or Neem oil eventually works, especially if you take the damaged parts off.

Damaged leaves/blooms will NOT recover, and only slow the new growth down.The roots are unaffected, and will grow new leaves/flowers as long as the mites are not allowed to demolish them.

i have to say that I don't know if the plant in question has mites, as Abid did not respond. It's hard to diagnose with just a picture.

Thanks i just read the post today, was little busy. I checked the undersurface of leaves and didn't anything you mentioned. will check again. again thanks a lot.

The don't like to be too wet IME. Is the area well-drained?

Yes area is well drained rather i feel the soil little dry when i dig around the plant with my finger. i don't understand what is IME.

IME = In my experience