Nasturtiums taste similar to watercress and consist of somewhat the same nutrients. The flowers and leaves of Nasturtiums are ok for cats to consume if fed in small quantities. Like watercress, Nasturtiums also contain oxalates and, when fed in large portions to cats, can cause mild toxicity and adverse gastrointestinal effects. Moreover, high levels of oxalate bind with calcium and accumulate as kidney or bladder stones causing your cat to pass urine with difficulty.