Well, since no one else is responding, I'll take a stab at it: I've always planted my alliums in the fall - around October, so I would not recommend waiting until after your dahlias die back. I've had dahlias around Thanksgiving but I don't know how long they persist in your zone. I'm guessing that you will want to dig up your dahlias for the winter. We can often get away with a heavy mulch here. You could pot up the alliums and give them time to establish roots until you remove your dahlias, then plant them out at that time. Maybe plant the bulbs fairly shallow - just a couple inches below the soil to give more room in the pot for root growth, then plant them deeper when you put them in the ground. That's what I would do if I had to lift the dahlias for the winter. Here, I would just slip the alliums in between the dahlias as gently as I could. Hope this helps!

I read during a search to keep them dry in a cool place. That rules out the frig and sounds like a paper sack or mesh bag in the basement would be the thing. I am surprised by the lack of information and responses on this forum to those of us who have needed to dig up bulbs and transplant and posted questions regarding that. I hope more folks post responses to your questions.

There are many variations - but I suspect that yours may be starting slowly from too much heat. Can you arrange some shade/filtered sun using other plants nearby? If you started from seed, Allium can look like tiny grass the first year when it sprouts - so be careful not to pull grass-like seedlings in that area! Yes, for several years new seed and small bulbs will grow larger and be more rewarding each season. And if you cut (mince and eat like chives) the leaves about the point where they flop, you may find that your Allium begins to feed bulbs and multiply from the base. The flowers will throw seed - or you can collect the seed when it begins to dry - and perhaps begin seedlings indoors, or in shade before planting out, to minimize heat-struggle. Flowering Stalks should be firm - so provide mulch &/or Am water if possible. Once flowering begins, it is normal for leaves to flop and die back to feed bulbs. We have opposite problems. I recently moved massive droopy peonies that were not blooming enough, and some over-vigorous clumps of ornamental Allium (medium size purple-globes) to the sunny front garden, and took care - so that the ground does not parch in peak sun...as that is the only spot where that is a risk for me. I added compost and to help hold moisture, and bulbs are aided by a top moist mulch, and the variety of other plants all around for filtered shade at the base. I intend to eat most of the spicy leaves as I don't care for more Sword-play in my garden by late spring - after masses of tulips, narcissus, and hycinths have quit and the lillies and iris are taking over...and Crocosmia ileaves are waiting to join the dance. I'm adding a Dahlia border to the sword beds...and am excited that a Bachelor's button has already pushed thru the mass of swords to bloom cutely. All are worth growing - in their display seasons. Growth and dormant seasons is balanced with other Perennials and annuals. Adding sheer Dahlia foliage mass to existing Roses and monster Rhodies - and by gosh, there's not much dang subtlety about who rules each season in this section of the urban rain forest. Whoo-ah...now to get back to that rain-or-shine deadheading - or the jungle-look will rule. Having so much Spring rain in the PAC NW - we wind up with too much foliage & dense shade unless we deadhead, prune, pull, divide and hack back plants regularly. Wonder if I'll get the courage to prune that lush Magnolia this year - for more light?

I have one in my yard that smells VERY garlicky. It's called Society Garlic. It gets a purple flower on it. Every time I'm even working near it I can smell it.