S - St. John's Wort

T - Thyme

U - Umbrella plant

V - Verbena 

W - Wisteria

X - Xylosma

Y - Yew tree

Z - Zebra plant

Next Challenge: Comedians

A - Askey, Arthur

B - Bobby Ball 

and his comedy partner:

C  -  Cannon, Tommy

D - Jack Dee

E  -  Enfield, Harry

F - Dawn French

G - Gervais Ricky

H - Hill, Benny