Why did my alliums die?
A few autumns ago, I planted two allium bulbs in my garden–a gladiator and schubertii. They’ve come up each year and seemed healthy. This year, both started to grow and then stopped, and all of the foliage shriveled up and died, including a flower bud that had already started on one. I know this has been a weird season for temps, but I was under the impression that allium are pretty hardy plants, and everything else around them looks great (save my weeping cherry, which skipped right over the flower phase and into sparse leaf). Other allium in the same garden and in another nearby are doing just fine, as well.
Is this just a result of one of the late freezes, or should I assume that the bulbs are kaput and replace them this fall?
There are several reasons why alliums can begin to grow and then die off. First, confirm that you are growing your alliums in optimal conditions, per the information in this NC State Extension fact page on alliums (i.e., full sun, rich sandy well draining soil, etc).
Since you’ve been growing them in this spot for several seasons without trouble, and since you have other alliums growing around them, I’m not immediately leaning toward soil issues or widespread infection. I can’t rule them out 100%, so if you haven’t had your soil tested in a while, you can always submit a sample to the UMaine Soil Analytical Laboratory. We had a generally mild winter and a pretty wet spring, so rather than a late freeze, I wonder if you are getting some rot in overly wet soil (which allium is prone to). I recommend partially exposing a bulb to see if it looks healthy, or soft and gooey–if the former, you can leave it in place to see if it recovers. If the bulb is rotten, it will need to be replaced and you may need to give some thought to amending the soil for better drainage.