Hi Terry, thanks for the question.

Black Plastic and Clear plastic can both boost the soil temperature.  It can also keep the weeds down.

I hope this is helpful.

Hi again,

Yes, cardboard will work.  If you remove it, it can be messy once wet.  If you plant through it, it may obstruct gas exchange to the soil and inhibit the absorption of water and nutrients until it breaks down. 

Hi Terry,

Sorry if I was unclear about this.  There is a lot of buzz about using cardboard in the garden this year to preparing garden space and planting with the cardboard in place.  You can use plastic and plant through it, but it requires way to deliver water under the plastic for water and delivering nutrients.  Drip irrigation is often used in this method.  Black plastic is normally used only for warm weather crops (e.g. sweet potatoes, tomatoes).  Plastic is also used for solarization, occultation, and for warming the soil.  In your case, warming the soil was the goal. Clear plastic will stimulate weed growth and then fry the weeds, whereas black will just heat it up and withhold light.  Short term use of plastic should boost soil temperatures.  Put it down warm the soil and pull it up. Cardboard has an advantage of staying put if wetted down.  A thermometer (dial type) can be used to measure soil temp, usually measured at 2 inches down. Here are some relevant links:

Using the sun to kill weeds and prepare garden plots | UMN Extension

gardeningwithmulch.pdf (oregonstate.edu)