Most cats nibble on grass, despite the fact they can't digest it. This is pretty much safe -- in fact, it may even be good for their health -- and common ornamental grasses aren't poisonous to cats. There are exceptions, though, so research the flora in your cat's prowling territory.
Ornamental Grass Toxicity
Ornamental grasses encompass a range of plants, most of which aren't poisonous to cats. Typically these include true grasses (poaceae), seges (cyperaceae), rushes (juncaceae), restios (restionaceae) and cat-tails (typhaceae). None of the plants from these families appear on multiple lists of plants poisonous to cats, including an extensive canon courtesy of the American Society for the Prevention of Cruelty to Animals.
Two varieties of poacae -- velvet grass and sorghum -- appear on a list compiled by Earth Clinic, a home remedy sales website, though.
Toxicity aside, cats who eat ornamental grass -- or any plants for that matter -- often throw up.
In the Weeds
Cats are obligate carnivores but it's not unusual to find them munching on grass until they throw up. Scientists offer multiple explanations of why this is can be beneficial: grass juice contains vitamins A and D and folic acid, the last of which spurs hemoglobin production; induced vomiting clears the digestive tract, which often contains inedible fur, bones and feathers, especially in outdoor cats; and retained grass acts as a laxative to clear the digestive tract.
Given the benefits, it's probably good to let your cat chow down on some ornamental grass.
What to Watch For
Although ornamental grass is an unlikely poisoning culprit, it helps to recognize poisoning symptoms. Throwing up, diarrhea, lethargy and tremors are all classic poisoning symptoms. These may escalate -- acute kidney or liver failure are usually in the mix -- as your cat enters a coma and eventually dies. Neurological symptoms are also common.
If your cat seems ill, call a veterinarian. Poisoning symptoms can take hours to manifest, so time is of the essence. Your cat may also excessively chew on non-food items -- that's called pica -- which is usually a sign something's missing from your cat's diet.
To reiterate, it's unlikely that the majority of ornamental grasses are toxic to your cat. Regardless, research the plants in and around your home to see if your cat has exposure to known poisonous plants.
A dozen plants are responsible for the majority of calls to the UC Davis' Veterinary Medical Teaching Hospital -- none of which are ornamental grasses. Common poisoners include lilies, Lily of the Valley, anemone, aloe vera, amaryllis, asparagus fern, daffodils, philodendrons, jade plants, chrysanthemums, cyclamen and cycads. If your cat eats a known poisonous plant, call a veterinarian immediately.
Chemical sprays -- including commercial and natural insecticides -- are toxic to many animals. If you must treat grass, limit your cat's exposure, and don't let him eat any of it.
You can easily grow a tray of untreated grass in a garage, laundry room or some other area with at least minimal sunlight. Offer your cat a handful of greenery every few days or, better yet, let him gnaw at the potted fresh crop.