i have two different solid plant extract ethanol and water to check antiepileptic activity which is the best saline to dissolve the solid plant extract to i.p in mouse.
As plant extracts composed by many different compounds, so PBS and normal are not good candidate for complete dissolving and DMSO is better choice for you.
i think the best way is to prepare suspension of the solid plant extracts in 0.05% or 0.1 % Carboxy methyl cellulose. I have followed this method for administration for extracts of powder herbs. But this is for oral administration.