The French name for the elm (Ulmus) is orme, but neither orme nor ormeau for a young elm are found anywhere in Van Gogh’s French correspondence. In his letters in Dutch, he only ever uses the word iep for elm and never the more traditional variant olm. There are no references at all, furthermore, to the yew (taxus).
In Belgium, by contrast, where Van Gogh lived on and off for a total of three years, the word iep (elm) was frequently confused with iebe (yew, also known as ijb or ijf) ‘due to the close similarity in sound’, as the Woordenboek der Nederlandsche Taal (Dictionary of the Dutch Language) puts it. The most recent edition of the Van Dale French-Dutch dictionary similarly notes that if can also mean iep in Belgian Dutch. What we can conclude from this is that Van Gogh really did mean ‘elm’ when he used the word if in French. He knew what tree it was and was evidently unfamiliar with taxus as the correct Dutch translation of if or with the French orme for ‘elm’.
The tree in the autumn garden is therefore a young elm and should rightfully have been called ‘un ormeau’ in his letter, while the sturdy tree trunk by the roadside is an old elm, correctly described in French as ‘un vieux orme’. There are no references to yew trees in his French correspondence any more than in his Dutch letters.