I have never heard of Alliums as being invasive or aggressive.  In my garden they have not spread at all beyond the bulbs I originally planted.

Susan, thank you for your response. It would help me to know how long have you had your alliums (it took a few years for my red trumpet vine to become invasive), and do all alliums spread at the same rate? I have three kinds of allium, but I am particularly concerned with the hairy allium. I cut the flowers off last week because I wasn't sure if they would drop seeds and self-sow. I planted bulbs to get the hairy allium, so one would assume that the hairy allium would multiply via a bulb; however, I believe allium are related to onion weed, and I get that popping up all over without having planted onion weed bulbs, so perhaps allium spreads by seed as well as by bulb. Maybe it depends on the kind of allium as to its invasive properties? For example, I had to pull out my Sedum Watch Chain Stonecrop because it became invasive, but I am still enjoying my Sedum Blue Spruce.  I don't want an invasive/weedy plant in my garden, but I definitely want to keep the plant if it is not invasive. 

Allium 'Beau Regard' is a drumstick allium and is in no way invasive.  That is what I have.  Hairy allium is also called hairy garlic (Allium subhirsutum), it is a bit more aggressive (I have that too, but didn't plant it).  It hasn't caused me any problems, but if you are concerned, remove that one but leave the purple alliums.

Susan, thank you so much for your help. I will take out the hairy garlic and keep the Beau Regard.  ~Donna

Thank you for using Ask an Expert!