Dehumidifiers are essential up here in Ohio. I deal with foreclosed properties all the time; one of the first things we do is have a dehumidifier installed to keep the mold down.
They work in temperatures above about 55F. Anything colder and they just ice over. The hotter it is, the better they work.
On another note, look at your air conditioner. They perform two functions - cooling the air, and drying the air. If you have an over-powered air conditioner, it will cool the air a lot faster than an underpowered air conditioner, but it won't dry out the air effectively.
We work together every damn day. --Jon Stewart