Poisonous!

Asclepias engelmanniana
Narrow-leafed Milkweed

As a rule, all milkweeds should be considered poisonous, but the narrow-leafed, whorled species such as A. engelmanniana, contain glycosides that act on the nervous system and are considered to be the most dangerous.

Milkweeds are sought out by monarch butterflies as a nursery for their eggs so their hatchlings can incorporate the sap’s poisonous glycosides into their bodies as they feed on the foliage. The caterpillars become so bitter tasting that birds learn to avoid them. Even the adult butterflies have enough toxin remaining in them to give protection from predatory birds.

Poisonous!

Croton spp.
Croton, Skunkweed, Barbasco

Croton oil delivers a very strong and dangerous laxative effect, and just a drop will produce an extreme purgative response.

Crotonic acid is used in the manufacture of polyvinyl chloride (PVC) plastic, but because crotonic acid is known to migrate out of the plastic into food, both U.S. and European food laws restrict the levels that are permitted in food products.

Poisonous!

Senecio flaccidus (Senecio douglasii or Senecio longilobus)
Threadleaf Groundsel, Douglas Groundsel

Threadleaf groundsel may be found in overgrazed pastures, where it  increases because livestock avoid eating these plants until all the more suitable forage is gone. In badly managed pastures or in disturbed sites, livestock may eat them and suffer severe liver poisoning. Cattle and horses are very sensitive to these toxins, but sheep and goats must eat a great deal more before they are affected.

Poisonous!

Xanthium strumarium
Cocklebur, Cadillos

Seeds as well as seedlings contain a very toxic glycoside that inhibits germination and maintains dormancy in one of the pair of seeds. Its concentration is highest in the cotyledons (first seed leaves), and the level drops rapidly in the seedling. The toxin disappears after the second true leaves appear.

If animals or humans consume cocklebur seeds or seedlings, the toxic glycoside may cause death from hyperglycemia (excess blood sugar). Unfortunately, cocklebur seedlings are very palatable to cattle, sheep, and swine, and death is not uncommon among those animals that browse the young plants.

Poisonous!

Astragalus mollissimus
Purple Locoweed, Woolly Loco, Rattleweed

Many Astragalus plants contain a toxic alkaloid called swainsonine that permanently damages the nervous system of grazing animals and causes them to act loco, the Spanish word for “crazy.” Animals can even become addicted to locoweed, and the poison can be passed through the milk to nursing young. Affected animals will tremble and carry their heads low while staring vacantly. Locoweed is particularly dangerous to horses, for which only a small amount may be fatal. Horses that have recovered from the effects of the plant may be permanently affected and unsafe to ride.

Poisonous!

Tribulus terrestris
Goathead, Puncturevine

During periods of drought, goatheads accumulate nitrates that may lead to oxygen deprivation in grazing cattle, as the nitrates compete with oxygen in binding to red blood cells. They may also cause liver damage, blindness, peeling of the skin, and the loss of lips and ears.