Rob Theweneti from Aquascapes and Landscaping explains why...
Another vet bill! This time I've found the cause, Hostas! Our dogs have chewed the life out of my plants, now I'm having to move them from the back garden.
Hostas are a garden favorite for shady areas; they make a lush display of greenery. But any part of the hosta plant leaves, flowers and roots can cause misery or worse to a dog if he eats or even chews on it.
What makes the hosta dangerous is a chemical substance it contains called a saponin. The name comes from the Latin "sapo," soap; you can actually make a soaplike cleansing substance from the soapweed yucca (Yucca glauca) by using the pounded root, and from common soapwort (Saponaria officinalis) by using the chopped leaves and stems.
Both plants contain enough saponin that, when agitated with water, it foams up. Hostas don't contain enough saponin to make soap, but they contain enough to make them harmful to dogs by this foaming action.
If a dog ingests saponin, whether from eating hostas or food that contains saponins, it stops the dog from vomiting, fills his stomach with foam and can even paralyze his intestinal tract so that for a while he can't get rid of his problem from either direction.
At the very least, he'll look miserable and show signs of a severe bellyache and eventually will vomit and have diarrhea. Some experts feel that in large dogs and giant breeds such as Great Danes and Irish wolfhounds, eating saponins can lead to bloating and even twisting of the stomach or intestines, which creates a surgical emergency.