Yes, oleanders are EXTREMELY poisonous! I don't know exactly how much
would kill a horse, but the phamphlet I am looking at right now says:
"THESE PLANTS ARE DEADLY IN VERY SMALL AMOUNTS AND CAN CAUSE DEATH!!!".
Then it lists several plants with include oleander. The leaves are the
deadly part, it doesn't say anything about flowers, but I wouldn't want
my horses anywhere near the things. Go give 'em h--- at the meeting!
Good luck.
Susan - WA