When it's perpetual apparently. We have grown varieties of spinach over the past few seasons and I really enjoyed some of the fresh young leaves.
Part of the reason for my misunderstanding lies in the way we grow things. Our main route for starting plants is to sow seeds in trays or pots and transplant them to the soil when the young plants can be handled, but that doesn't seem to work well for spinach (or, for that matter, carrots). We did have success with perpetual spinach and when we took a few leaves the plant just throws more out. It was easy to grow but I wasn't convinced about the flavour. The few spinach (non perpetual) plants that did grow didn't like having a few leaves taken, so they bolted, but those leaves tasted great.
The main part of my confusion is that a leaf beet is called perpetual spinach which I thought was a variety of spinach, well you would - wouldn't you? Even the gardening magazines list perpetual spinach as though it is a spinach. I finally realised my error because a seed catalogue pointed out the difference.
This all makes sense now. Perpetual spinach is not spinach, its a leaf beet. It can be sown in a pot and transplanted. It is tolerant of having leaves taken. It doesn't taste like spinach.
Real spinach doesn't like being transplanted. It is not tolerant of having some leaves taken so to keep a steady supply it needs to be sown at regular intervals, probably directly in the ground. It does taste like spinach.
I understand now, so I can buy some spinach seeds.