It depends on what you are calling "poplar." There it yellow poplar (AKA tulip poplar, Liriodendron tulpifera). It is not rot resistant.
Cottonwood is a species of true poplar which is commonly used for heavy equipment trailers. If treated with used engine oil it will prevent rotting, and that wood is very durable.
You must have a different species of Cottonwood there. The stuff up here (we call it Cottonweed) isn't even good for firewood. It's brittle and branches break off just from snow accumulating on them. There's no way I'd try and make bed wood out of it. The fuzz covered seeds blow everywhere and the pods dropping on a car will ruin the paint.
