Well, nobody has come up with any answers, so I'll just say that, since they prefer well drained soil in full sun, I would not think that conditions which favour Vinca would be good for them. And Creeping juniper can grow a few feet tall which would possiby hide the Alliums, even if they forced their way up through it.

Drumstick alliums are pretty but OMG I regret planting them! I’ve spent too much time the past two springs ripping out seedlings!

Not as decorative as one might think looking at promotional photos, with the purple color not registering that much visually and the heads starting to brown while still in season.

Have a look at agapanthus, some varieties have the same lovely blue colored flower heads, and they have an attractive, scrappy foliage. Clustered in a swath, they’d make a nice statement while blooming and provide a nice, green backdrop for other plants blooming earlier or later.

I have found vinca to be sort of a bully, swallowing up azaleas and threatening to smother slow moving grandparents…

I believe I meant “strappy”

There are perfectly winter hardy agapanthus that I have been growing in the ground here in zone 5 for the last 30 + years. They flower very well for me every summer and they come back strongly in the springtime - There are two types of agapanthus, evergreen and deciduous, the deciduous varieties are outside in the ground winter hardy in temperatures below freezing. The hardy agapanthus would probably grow and return just fine in zone 4 if it was planted in well drained soil full sun and fertilized!! (these must be fertilized regularly while growing to do well) The evergreen varieties are best grown in pots. I have had years of experience growing both.

Hardy agapanthus do not grow as large as the tender varieties from my experience. Hope this info helps anyone confused about agapanthus! No one asked, but these are much showier than the drumstick allium - which is nice too, in the right spot.

Here is the hardy one outside and tender in garage.