Solar thermal clothes dryer feeds greenhouse/cold frames...

A big mass of soil.
In ground,above ground,made of  sand, whatever
Black,Corrugated,perforated pipes running through it.

Insulate ithe mass and put it under green house plastic, foil faced insulation,white plastic

A hot space.
Attic, solar hot air collector,the ridge of  a greenhouse.
Pipe  the heated air into the clothes dryer.
Dry the cloths.
Exhaust the moist, hot dryer air into the perforated pipes.
Moisture condenses in the pipes  taking with it, most of the heat.

Grow plants under  clear plastic or glass. Start cuttings under the white plastic or reflective insulation.

Solar+dryer+wet clothes=water vapor and dry clothing.
Water vapor +cool soil=condensation and warm soil.

My big concern would be mold.  From my experience with fiddling with dryer venting and using it to keep a greenhouse from freezing at night, the venting blows a lot of lint. When it was blown through longer runs of venting pipe, the lint starts to stick inside as well as collect at the opening.  And thats what tends to mold, it seems.

Im not sure how one could effectively remove the lint first. The regular dryer lint catch hasnt been good enough on the dryers i have owned.

I think mold in greenhouse airtubes is underrecognized. Besides being a problem for sensitive humans, it seems like it could create problems fot plants and starts, too.

I spent most of my gardening time in western Oregon though, where fungal diseases are a big problem.  And algae. I once worked in a commercial biodynamic and organic greenhouse that had a bad algae issue that the previous owner reported started when a worker left an aquarium in the greenhouse.  Ever after that event, every plant cell grew a thick layer of algae on it as it was watered. It became a race to see if the seed would start first, or the algae. Greenhouses can become a bit like petri dishes!

One thing im not sure is beneficial about wet air going through air tubes is that wouldnt the process of losing that heat in the ground negate the effectivness of the air tube? Wouldnt it be more effective to just run the hot air straight into the greenhouse? Thats what i did a couple winters, and that worked.

I do love the idea of recapturing waste hot air though, of course. Do you live somewhere that a dryer is needed? The attic heat idea - does your attic get hot enough in the plant starting season to make a big heating impact? How hot do you think that would need to be?

A couple thoughts:

The typical clothes dryer heats the air to 140-150F (60-65C). If it's cold enough out that a greenhouse needs supplemental heating, then a solar thermal collector is unlikely to produce enough heat for a clothes dryer.
I collect hot/warm air from under my roof for heating my house, but during the winter this air is typically only 20-30F warmer than the outside air.  So if the outside air is below 40F then the attic air is too cold to use for heating the house.
Now, during the summer time, that is a different story.  The air under my roof can easily hit 140-150F which makes it perfect for the clothes dryer.  Getting it from the attic to the intake on the clothes dryer requires a bit of work, but it could save you about 2kwh per load.

As Kim mentioned, lint is a major issue with using the output of clothes dryers.

Running moist air through underground pipes will often result in mold.

The color of the pipe is not important if it's buried.  Color mostly makes a difference in converting visible light to IR heat.