There is a great article in The Horse.d.o.t.c.o.m. on Plants that are toxic to horses.
here is the link:
For Florida.. pointing out these three paragraphs:
"Other plants outside pastures can be toxic to horses as well--ornamental plants used in landscaping, such as rhododendron, azalea, and yew, for example. The latter ornamental plant is an evergreen, so it and other evergreens remain toxic year-round because they never lose their leaves. Between 10 and 15 ounces of yew leaves are sufficient to kill a 1,000-pound horse.
Oleander is a leafy, flowering drought-resistant plant often used in ornamental landscaping in California and across the Southern states to Florida. It contains cardiac glycosides--naturally occurring compounds that can cause colic, loss of coordination, profuse sweating, difficulty breathing, abnormal heart function, muscle tremors, and potentially death from cardiac failure.
"A few mouthfuls of oleander leaves can be fatal to a horse," says Knight. "The toxin is similar to digitalis (a drug that comes from the foxglove plant and is prescribed to certain heart patients); it works on the heart, affecting normal heart rhythm."
Just things to keep in mind.