Good luck to you growing it out of habitat and climate. None of my texts specify, some spp are to be sown with grasses. This particular plant is native to open areas and meadows in the montane ecosystem and in drier lodgepole sites. If you are so lucky to be able to grow these plants, you'll want to grow the paintbrush under eaves where they don't receive rainfall in a warm location, full sun. If you want to grow it in a large pot inside in a south window, I'd seed with some buckwheat and see what happens.

Dan

not necc. true the semi parasitic lore about castelleja ( Sherman 1996 et al ) pure stands are common ( i collect alpine and montagne seed commercially under special use permits). Often the close association with other species seems to be a moisture / shelter association rather than hemi parasitic.

I have germinated and grown several species of castelleja( always a variable genus) expect a germination rate of 40% at best!!!!!! more likely less.... as you are aware small seed has less stored energy so good seed storage is important to maintain what there is, castellleja falls into this category.

As to the host plant issue in practicalities, forget sagebrush as is often mis- reccomended any association with perennials in a mixed border or open shrubs will work ....it likes full sun and cool roots, at least at altitude... it doesnt like being moved once established
we have albeit ...anecdotly found that a well innoculated soil based growing medium really increases the root mass, development and consequent ability to retain moisture, nutrients etc....actually we do this for all natives we grow.
I use Roots organic but there are many options out there.

Many belated thanks to you two for the fascinating observations and advice. I'm afraid I dropped the ball, and haven't got around to sowing the castelleja. May still give it a try, but deterioration and loss of viability concern me now as well. Anyway, thanks so much for your thoughtful response. I hope to put your advice to use sooner than later.