As the saying goes, “From little acorns mighty oaks can grow.” But did you know they could hold darker consequences as well?

Acorn ingestion can cause big problems for lots of animals. In a small dog, a big acorn can cause a blockage in the digestive tract. And for any dog, the tannins in acorns are toxic, causing signs from acute gastrointestinal problems—like vomiting and diarrhea—to full-blown kidney failure.

And it’s not just the acorns that cause problems. Oak leaves and buds are poisonous, too, particularly to grazing animals like horses, cattle and sheep. In fact, young green acorns, with their increased levels of tannins, are considered more toxic than the mature brown ones.

So, beware those little acorns. They might contain a world of hurt for your animals.