Lemongrass is not considered toxic to dogs. However, a dog can become sick if they consume large amounts of lemongrass. If your pet does consume a large amount of lemongrass or you suspect the cause of any vomiting or stomach issues is related to them eating lemongrass then consult your vet as soon as possible. Small amounts of lemongrass or other essential oils are used in certain dog collars and sprays with very little potential for problems. With cats, if they nibble small amounts of the lemongrass plant in your garden then don't be alarmed. But as above if they exhibit any symptoms after eating lemongrass or you are concerned, consult your local vet as soon as possible.