Balloon plant (Asclepias physocarpa) is an invasive weed, introduced to Hawaii as a fiber crop. A member of the milkweed family, it is considered to have medicinal properties, but parts of the plant are also poisonous.
I saw several of these plants in a pasture occupied by livestock and, like the cow in the photo, they were avoiding the plants. Consequently, the pasture consisted of close-cropped grass and a generous sprinkling of these scrubby plants, 2- to 3-feet high, though they can grow to 6-feet tall.
The ‘balloons’ are actually the fruits of the plants. When ripe, they’ll burst and release a multitude of white silky-haired seeds.
Graham's Island 
It has naturalized all over South Africa too, though originally from tropical Africa. It is an interesting plant and in your photograph it looks great against the dark cow that is studiously avoiding it.
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The plant is dotted through the cow pasture but, according to what I read about it, the cows won’t eat it unless they’re starving. Fortunately, these cows aren’t likely to get anywhere near that hungry.
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Never seen this plant before. It is weird.
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I’ve probably seen it before, but not registered it until recently. It is a strange plant, that’s for sure.
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