[A humble request: If you can, please donate to help me carry on tracing word histories. Thank you.]

 

Originally and chiefly British English, the expressions mother-in-law’s chair, mother-in-law’s cushion and mother-in-law’s seat are colloquial appellations for the globular spiny cactus Echinocactus grusonii, native to Mexico—also popularly known as the golden barrel cactus.

These are, in chronological order, the earliest occurrences of the expressions mother-in-law’s chair, mother-in-law’s cushion and mother-in-law’s seat that I have found:

1-: From an account of the Chelsea Flower Show, by H. H. Thomas, published in The Daily Telegraph and Morning Post (London, England) of Wednesday 23^rd May 1951 [Section B, page 6, column 5]:

All the exhibits were from the United Kingdom, except the group of cacti from Italy. This contained a particularly spiky cactus known as mother-in-law’s chair.

2-: From an account of a display organised by the Nottinghamshire branch of the National Cactus and Succulent Society, published in the Nottingham Evening Post (Nottingham, Nottinghamshire, England) of Thursday 14^th May 1953 [page 8, column 7]:

In the succulent section, there is some wonderful “mimicry” by the force of nature. The exhibits resemble the grey stony ground on which these plants are normally found, and as with the cacti classes many of their names are  almost unpronounceable to the average layman.
And no cacti exhibition would be complete without the pumpkin-shaped Echino cactus, or “mother-in-law’s cushion” as it is called, from Arizona.

3-: From the column News Diary, by John Carpenter, published in The Evening News (London, England) of Tuesday 21^st September 1954 [page 4, column 3]:

Her Mother’s Chair

Cacti are not at all my idea of autumn flowers. But they fascinated me more than any other display at to-day’s preparations in the National Hall, Olympia, for the great autumn flower show beginning there to-morrow and lasting until Saturday.
For one thing they have such odd names—“Mother-in-law’s chair,” a spiky cushion-shaped cactus; “Irish Mitten,” “Joseph’s coat,” “Angel’s wings” and “Lady Fingers.”
The tallest cactus plant was 9ft. high. Appropriately called “Old man’s beard” this Shropshire-grown cactus had caused something of a sensation by sporting a long overnight cob-web.

4-: From the Derby Evening Telegraph (Derby, Derbyshire, England) of Tuesday 21^st September 1954 [page unnumbered, column 5]:

Gardeners get ready for the big show

Men from the gardens of England were at Olympia, London, to-day, arranging their exhibits for the opening to-morrow of the Royal Horticultural Society’s Autumn Show.
It will be the biggest display since the society’s autumn show of 1937. Chrysanthemums, dahlias, begonias and gladioli provide the main masses of glowing colour and the scents of autumn, but a feature of this year’s show is the enormous variety of flowering shrubs.
The exhibits included an eight-foot six-inch 30-year-old cactus from Shropshire, and a hedgehog-like cactus named “Mother-in-law’s seat.”

5-: From the caption to the following photograph, from Autumn Glory in Olympia: Wonder Blooms at the Royal Horticultural Society’s Great Show, published in The Birmingham Mail (Birmingham, Warwickshire, England) of Thursday 23^rd September 1954 [page 1, column 5]:

“Mother-in-law’s Chair,” a strangely-named cactus.

6-: From the column Scunthorpe Day by Day, published in the Scunthorpe Evening Telegraph (Scunthorpe, Lincolnshire, England) of Monday 11^th July 1955 [page 4, column 4]:

The old-fashioned joke about a mother-in-law goes down well at the Simpson household at 158, Burringham-road, Scunthorpe.
Mrs. M. E. Simpson is a mother-in-law with a difference. She possesses a very prickly cactus called a “Mother-in-law’s Cushion”—just the type of cushion that many a chap, according to the music hall comedians, would wish his mother-in-law to sit on.
But this particular member of the cacti family is a rare one, and must be kept for a long, long time before it flowers.
Mrs. Simpson’s cactus, after braving the remarks passed about it and the urgings to its owner to “throw it out,” has confounded the experts.
Although only a three-year-old, it has burst forth into flower overnight—a magnificent bloom of variegated pink with a yellow centre.
Having seen what a remarkable flower the plant has produced, Mrs. Simpson has been trying to remember where she got it, but to no avail.
“It just seems to have turned up,” she says.

7-: From an account of the 106^th Mid-Somerset Show at Shepton Mallet, published in the Shepton Mallet Journal (Shepton Mallet, Somerset, England) of Friday 5^th September 1958 [page 1, column 2]:

The horticultural side of the show goes from strength to strength each year, and this progress was well maintained with an entry of 650. Breathtakingly beautiful, the banks of lovely blooms were a blaze of colour.
Gaining in popularity is the cultivation of cacti, a strangely fascinating plant regarded as a weed in its native parts, Mexico, the Mediterranean and Africa. There were 200 different varieties of cacti on a stand belonging to Mr. Gordon Nunnerley, of Redditch. Weirdly and grotesquely shaped, they had all sorts of picturesque names, such as: mother-in-law’s cushion, old man’s beard, Bishop’s cap and peanut cactus.