I'm not a big fan of solarizing anyway. As Brandon has recently pointed out elsewhere, solarizing sterilizes the soil -- and that includes killing off all the GOOD bugs that are in there. Try smothering with some lasagna/mulch layers instead (thick newspaper plus mulch or something similar), to eliminate some of the weeds without killing off the good guys.

This is for an area with a serious Crown Gall problem and I do expect to have to re-established the good guys in this area, but A. tumefaciens is here and it was a long time tobacco plot.

In that case, why don't you irrigate the area (if possible) before putting down the plastic. If the area isn't around a water source, just wait until the next rain (or light sprinkle, as its been lately). That might make it more effective.