I leave all of my Allium seedheads untill they break away by themselves. I love the look of them in the garden and I want to make the most out of the pricy bulbs, especially 'Globemaster'. And I absolutely love christophii at every stage. Last year, the seedheads of christophii rolled around in the garden. That was funny.

People keep saying the flowers will be smaller the following year if left like that. I read that they can be invasiv as well. So far, I have not had many seedlings of any Allium YET.

As to the decreasing size, for me, it is true on some Alliums but not others. My 'Globemaster' does not set seeds. If well fed, 'Globemaster' does not get smaller. It even multiplies. I keep adding other Alliums every fall, so I don't care if they get smaller the following years.

Leave the leaves till they start turning yellow, which is a lot faster than the tulips. The last 2 years I have left on the seed heads. Now I have many babies starting up all over the garden, and I have no idea which allium they are, or if they are hybrids. I am leaving some, most I have pulled.

This year I am leaving only one seed head for each type of allium so I can offer the seeds to others.

Interesting that pitis Globemaster has not set seeds. Neither has mine. I find I don't like the bare pods, so down they come.

I also like 'christophii'. So far it is my absolute favorite of all of them.

I do think there may be a slight difference in the size of the flowers if you leave the seed develop. I think my Mount Everest flower is smaller this year, and no increase in the one bulb I have. :o(

Hope that helps