All varieties of apple and cherry trees are toxic to dogs, particularly the leaves. Hawthorn trees, which are closely related to apple trees, are also dangerous for dogs prone to chewing on plants and foliage.
Red maple trees are also toxic to dogs. When dogs eat the fallen leaves, it can affect the way their red blood cells function. The signs of this type of poisoning are subtle and include lethargy, abdominal pain and sore paws. Eucalyptus oil is also toxic to dogs, and some may eat it directly from the tree. Dogs suffering from eucalyptus poisoning may become depressed, drool excessively and have diarrhea.
In most cases, an entire plant is not dangerous, but rather certain areas of the plant may contain substances that are toxic to dogs. Apple seeds, leaves and stems contain cyanide, for example. The levels are usually safe in small doses, but they can build up over time. In another example, black walnut trees cause poisoning in dogs even though the trees themselves are not toxic. Instead, the fallen walnuts attract a type of mold that can sicken dogs.
Owners whose pets may have ingested a toxic substance should call their veterinarian or a pet poison control hotline immediately.