This is my second attempt to offer a reply; I must have pressed the wrong button last time.
There is a useful article about Maud Grieve, Dorothy Hewer and Margaret Brownlow, in the new-ish Oxford Dictionary of National Biography. I have also found out some more about their activities.
Maud Grieve helped Dorothy Hewer to establish the Seal Her Farm in 1926. Seal (where I live) is a small village two miles from Sevenoaks in Kent. When she closed her own nursery in Buckinghamshore, she gave some of her stock to Miss Hewer. Miss Hewer established a lavender of her own, 'Seal Lavender', which was widely marketed at the time, and sold it and otehr prodcue through a shop in North Audley Street, London, and by mail-order.
She in turn recruited Margaret Brownlow, who ran the herb farm through the war, expanding it considerably. She also published two books about herbs and about her farm, and lectured widely, helping to establish several other herb gardens (eg at Knole and at Lullingstone, near here, and others elsewhere in England). The farm closed after her death in 1968.
A big photoalbum has recently turned up containing many photos of the farm and of some fo the people who worked here. Our local history group is trying to discover more information, and we would welcome any contributions.