These are the seed heads that form after flowering. Seed heads can be harvested.
Cut the seed heads as they start to break open.
Bring them indoors and let them dry out.
Gently tap out the seeds and store them in labelled paper envelopes.
This article will help.
https://www.gardeningknowhow.com/ornamental/bulbs/allium/allium-post-bloom-care.htm
I think I may have cut the flowering stem back to the base too soon. They’re planted amongst lavender and campanula - is this ok? Many thanks.
It is OK to deadhead the blooms, but leave the foliage. When the foliage yellows and dies down you can cut back the whole plant. It may not have been anything you did that caused them to disappear. It could have been weather related or the bulb itself. You just never know for sure. I have lost lots of plants after one year.
This article should help:
https://www.gardeningknowhow.com/ornamental/bulbs/allium/allium-post-bloom-care.htm
This plant is four feet tall, and looks like an onion on the top, it doesn’t appear to be flowering. We have Alliums that have flowered, this is not the same.
Did you mean to attach a photo? I didn't get one. I looked up some onion lookalikes in England and the invasive plant Three Cornered Leek came up as well as Few Flowered Garlic and some lilies.
If it flowers, you will have a better idea. All of the above will flower.
My sister took a interest in Allium flowers after I share some fabulous photos from NYC's Central Park Conservatory Garden and other locations around the Central Park. She has a fabulous sunny Walk thru garden in her back yard and I'd like to set her up with what she needs for her birthday in July. I know it's post season for planting. But would like to put in an order for quality, reputable bulbs to receive in or around Sept when they need to be planted. It's pure ornamental. Would like large and medium sized in 2 or 3 color choices. Thank you in advance. Any book recommendations specific to Alliums,too?
We don't make vendor recommendations but there are plenty of reputable online nurseries that sell bulbs. Check out their customer feedback and also see if your local extension service will make some recommendations. Possibly friends or neighbors have purchased bulbs.
https://www.gardeningknowhow.com/ornamental/bulbs/allium/growing-alliums.htm
When deadheading an allium plant, should I just cut below the purple or white "globe" flower OR should I cut all the way to the bottom of the long stem?
Either way will not hurt. Some plants are sensitive to removing flowering hormones, but this will not be the case here.
What I do, is cut the bloom first. Then, once the stem has died and the plant has pulled back the nutrients and hormones, I will cut the rest of the stem.
This collection of articles will help you to grow Alliums:
https://www.gardeningknowhow.com/ornamental/bulbs/allium/page/2
I am in the 5b-6a plant hardiness zone. Our plants are outdoors (in a big planter box that has at least 12” of soil) in a sunny location without a shelter overhead. Should I deadhead them all now, and after a week or so, cut them down to just above the soil and put 3 or more inches of mulch on them. Do you think they would survive the winter? Thank you!
They are hardy to zone 4 so you should be OK if they are mulched well. Cut them to the ground when the stems have yellowed and withered, which allows them to gather energy for next year's blooms. Then cover them with at least 3 inches of mulch. Be sure to pull the mulch back in spring. If you are very worried about them, you can dig them up and store them in the garage over winter.
https://www.gardeningknowhow.com/ornamental/bulbs/allium/allium-post-bloom-care.htm
https://ask.extension.org/questions/438718
https://www.gardeningknowhow.com/ornamental/bulbs/bgen/bulbs-dont-need-chilling.htm
This article should have your answers:
https://www.gardeningknowhow.com/ornamental/bulbs/bgen/preparing-bulbs-for-winter-how-to-store-bulbs-for-winter.htm