It was 1989.
Philip Sturrock bought Cassell in 1986 and started acquiring other
publishers, including Ward Lock.
A business article in the Sunday Times says, "In 1989 Sturrock
targeted Ward Lock, a general publisher best known for Mrs Beeton's
cookery book . . ."
This list of gardening books by Roddy Llewellyn helps confirm the date:
1985 'Beautiful Backyards', 1987 'Water Gardens: The Connoisseur's
Choice' (both Ward Lock);
1989 'Elegance & Eccentricity', (Ward Lock/Cassell);
The Sunday Times article, "Cassell writes exciting chapter on book
sales" by Claire Oldfield (5 January, 1997), also says that in 1987
Cassell acquired "United News and Media's book publishing arm", in
1988 it bought Mowbray, and in 1992 Gollancz.
I hope you now have all the information you need, but please let me
know if you would like me to clarify anything about this answer.
Best Wishes - Leli
Finding general background information about Sturrock and Cassell
helped with this research. Since general Google searches didn't answer
the question, I turned to newspaper archives.
In case you want to read the whole article in the Sunday Times, please
note that I found Ward Lock mis-spelt as Wardlock, making searching
News International Archives (Times and Sunday Times)