I thought it was spinach type also but I know that the bags of mixed bird seed bought in shops can strike if these seeds fall on the ground, so that could be one reason you have this plant growing?
They have all sorts of seeds in them - you'll never guess what we had growing under the bird feeder at the Social Services offices where I work!
Ah that would make sense, we do feed the birds on the patio in the winter and the area the spinich is growing is right next to this.
Posts
I thought it was spinach type also but I know that the bags of mixed bird seed bought in shops can strike if these seeds fall on the ground, so that could be one reason you have this plant growing?
They have all sorts of seeds in them - you'll never guess what we had growing under the bird feeder at the Social Services offices where I work!

Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
Ah that would make sense, we do feed the birds on the patio in the winter and the area the spinich is growing is right next to this.