From early childhood Schweinitz was interested in botany, studying and collecting wherever possible. He accumulated a large herbarium, through his own collecting, through exchange and through purchase. His exchange correspondents numbered over one hundred as listed by himself. He is supposed to have bought only a single collection for his herbarium, namely the so-called Baldwin collection owned by Zaccheus Collins which was sold to settle the Collins estate. Schweinitz' herbarium was bequeathed to the Academy and contained, independent of the fungi and other cryptogamous specimens, 23,000 species of plants. When Charles Pickering, Chairman of the Botanical Committee, prepared to add the collection to the Academy's herbarium, he found such a great percentage were from Schweinitz that he decided to use that as a basis and interpolated the Academy's specimens in the newer collection. When finally arranged the Academy's numbered specimens had been increased to 28,000. It is still considered the basic herbarium of type specimens of Fungi. The Algae were not included in Schweinitz' gift and only years later when it was purchased through the Thomas B. Wilson fund did it come to the Academy.