Seed Stalks/Umbels

Yes, the umbels are pretty, but if you are not going to use the seeds they produce, I suggest you cut off the seed stalks because they drain energy from the bulbs; essentially the bulbs grow very little, if at all, after sending up a seed stalk and you will have smaller bulbs. I cut my seed stalks close to the base without cutting any other leaves.

Forcing potato onions to go to seed.

Potato onions are biennial plants. When you plant a seed the first season, it will produce a bulb, and that bulb will usually not go to seed (I say "usually" because if the bulb is stressed in some fashion, it may bolt during the first year). It is during the bulb's second season that it will naturally send up a seed stalk.

Common thought is that stress causes potato onions to put up a seed stalks after its second year (i.e., third year and beyond). Having said that, it seems there is no guaranteed way to induce the bulb to go to seed.

One of the easiest ways to persuade a seed stalk to grow is to overwinter the bulbs; that is, planting them during fall, and allowing them to freeze and break dormancy naturally in spring. My fall-planted bulbs have flowered every subsequent season since I've been growing them. So far, the umbels have been male-sterile, but they do flower, and I am hopeful that one day they will produce viable seeds.