In the work of many women writers who have never had a full separate biography written about them taken from archival sources, and in whose work few have taken serious scholarly interest, misattributions begin to appear.
At least three works regularly attributed to Isabelle de Montolieu in library, online and bookseller's catalogues are not by her. The Enchanted Plants and The Festival of the Rose are the work of Maria Henrietta Montolieu. The Fables of Flora are not by Isabelle de Montolieu either.
Montolieu, Maria Henrietta (b. 1765)
The enchanted plants. Fables in verse. Inscribed to Miss Montolieu, and Miss Julia Montolieu. Second edition.
London: printed by Thomas Bensley, 1801
[viii], 95,  p., engr. frontis.; 16.6 cm. (8º)
Anonymous. By Maria Henrietta Montolieu. First published in 1800. The second paragraph of the author's advertisement on p. [v] reads: 'The few notes she wrote for her children, [i.e. the Misses Montolieu] and which may be of use to young readers, will be found at the end of the book'. Maria Henrietta Montolieu was the wife of Louis Montolieu, FSA (1761-1817). In the ‘Biographical Dictionary of the Living Authors’, 1816, she is also recorded as the author of The Festival of the Rose, 1802 and her collected works in 1812. Her daughter Julia married firstly William Wilbraham, RN (died 1824) and then Lieutenant-General Sir Henry Edward Bouverie (1783-1854), Governor of Malta (see: Huguenot Library. Henry Wagner Pedigrees). Engraved frontispiece by L. Schiavonetti after William Hamilton. A series of moral verses on subjects such as gambling, scandal, vulgarity etc., drawing on the inspirational qualities of plants.
The problem seems to be that there were two women with the same last name writing around the same time, one in England and the other in Switzerland. Both shared the same tastes, and Maria Henrietta Montolieu may have translated too. Accordingly, I would also be suspicious of any books in verse never cited in French and attributed to Isabelle de Montolieu where the name of the author appears as "Mrs. Montolieu" or "Madame Montolieu." For example, "The Gardens: A Poem," is recorded in the National Library of France catalogue as "a translation from the French of the Abbé de Lille" by "Mrs Montolieu." However, the publication information only shows it published in London: "London : printed by T. Bensley, 1798." This is the same place and publisher for Enchanted Plants, Fables of Flora, and Festival of the Rose, and other poems.
Two further English texts regularly cited as translations from French texts by Isabelle de Montolieu may well find their source in her work, but the first rouses suspicion, and the second needs more confirmation. An entry from the British Library reads: "The avalanche; or, The old man of the Alps, a tale translated from the French. Joint author/editor: Montolieu, Isabelle de. 1751-1832. Clapham. printed and published by H. N. Batten; and sold by Simpkin and Marshall, Darton and Harvey, and Hailes, London. 1829." However, another entry in the same catalogue, which reports that the text is "an account of the avalanche of Bergemoletto in 1755," also says that this text may be by "anonymous." This item does not appear in French in any catalogue I have seen. Anther similarly titled tale is The Old Cobler of the Village said to be translated from "Madame Montolieu" by "Mrs Sherwood" (Martha Sherwood) in 1835. Sometimes The Old Cobler is found with another text whose title is The Idler. There is a "Marcel; or, the Cobbler of the Cottage" listed as one of the translations from Montolieu's short pieces by Mrs Plunkett (published 1811, see Working Bibliography), and Sherwood's may be another translation of the same tale. The source could equally be other late 18th century writers of romantic/sentimental "contes" set in Switzerland (e.g., Marmontel).
For further information see my Working Bibliography.