They are most definately poisonous to cats. Here is the list of toxic plants.

Aloe Vera
Arrowhead Vine (all parts)
Asparagus Fern
Azalea
Bird of Paradise (fruit, seeds)
Boston Ivy (all parts)
Caladium (all parts)
Calla Lily
Christmas Rose
Chrysanthemum
Creeping Charlie (all parts)
Creeping Fig
Crown of Thorns
Daffodil
Dieffenbachia
Dumbcane (all parts)
Easter Lily
Elephant Ears
Emerald Duke (all parts)
English Holly
English and Glacier Ivy (leaves and berries)
Heartleaf (all parts)
Ivy (Hedera)
Jerusalem Cherry
Lily of the Valley (all parts)
Majesty (all parts)
Marble Queen (all parts)
Mistletoe
Nephthytis (all parts)
Parlor Ivy (all parts)
Philodendron (all parts)
Poinsettia (leaves and flowers)
Pothos (all parts)
Pot Mum
Red Princess (all parts)
Saddleleaf (all parts)
Schefflera
Spider Mum
Sprengeri Fern
Swiss Cheese Plant
Tulip (bulbs)
Umbrella Plant (all parts)
Weeping Fig
(Ficus)

You either have to put it where your cat can't get it, try spaying the cat with water or get rid of it. You don't need to deal with a very sick cat or at worse losing your cat over a plant.