We have worked at CREW with other Solidago species from the US (e.g. S. ohioensis, S. shortii, S. odora), and cold moist stratification for 6 to 8 weeks worked pretty well. When we had low germination was due to empty seeds.
I haven’t worked with Solidago squarrosa, but my Solidago spectabilis and Solidago velutina ssp. californica seed collections germinated well (71% and 75%, respectively), on agar with no treatment.