Decades ago, when Erythronium 'Pagoda' first began to appear on the commercial bulb lists at reasonable prices, I tried a few now and then. They never amounted to anything. The bulbs were probably near death if not actually dead when purchased - expensive mummies in fact. I never gave up on this one, and years later I obtained fifty bulbs in excellent condition from a reliable mail -order supplier. These were planted in a newly prepared bed full of Compro (composted sewage sludge). The following spring every bulb bloomed, and I thought I had arrived. I crossed this one off of my "difficult" list and assumed that this new feather in my cap was here to stay. In fact, the following year I waited in vain for them to reappear. I never saw even one of those fifty plants again: they were gone without a trace. Of the two usual culprits, rodents and "summer rot", there was insufficient evidence to pin the blame with certainty. 'Pagoda' went back onto my "difficult" list, and for years I avoided it.
In the meantime, my experience and insight into growing bulbs native to the West Coast of North America ('Pagoda' is a hybrid of two western species) began to progress. As is the case with most bulbs native to the West Coast of North America, they are unlikely to ever be reliable garden plants here on the East Coast for the sort of gardeners who plant them and walk away. But that does not mean that they cannot be grown here. What it does mean is that the gardener must step in and make sure certain things do not happen. The most important of these is to be sure the bulbs are never both wet and hot. Wet is fine during the winter, and hot (but dry) is fine during the summer. But never allow them to be both wet and hot. It's simply stated, but much harder to put into effect in the garden. That's why I don't think they will ever be successful garden plants for most of us.
The plants die down for the year in May here, and they should be dying down into drying soil. Check out the rain averages for May in this area and then consider the chances of dry soil in a typical year. It's not going to happen.
One solution is to dig the bulbs for the summer. Here's where a little bit of knowledge is not your friend. For those of us who garden on the East Coast, where we have native Erythronium in the woods (they are almost never seen in gardens), we know these plants as woodland plants, plants which grow in moist soil, soil high in organic matter. The same is true of the Eurasian Erythronium dens-canis: it's a woodland plant. Unless carefully handled, commercial stocks of Erythronium dens-canis are apt to be desiccated beyond help. Bulbs of these plants should not be dried out the way one would dry a tulip bulb for the summer. I've heard of at least one site where Erythronium dens-canis has naturalized here in the greater Washington, D.C. area.
But 'Pagoda' and many species and cultivars originating from West Coast species need dry summer conditions when grown on the East Coast. Here's what works for me: when the foliage dies down, I wait a week or two and then gently dig out the bulbs. By then the roots should be dead or dying. I pack the bulbs in improvised newspaper pouches and then store the pouches in a shady place outside for a few weeks. The idea is to avoid drying them out too quickly. Once the bulbs have dried they are moved to a place inside the house. The bulbs remain in the paper pouches until sometime in September, when the paper pouches are moistened, slipped into a zip lock plastic bag and then put into the refrigerator (not the freezer!). Within about two weeks the bases of the bulbs will show signs of swelling, and soon after that the roots will start to appear. Get them back into the soil at this point, and don't let them dry out once the roots start to appear.
The image above shows bulbs of 'Pagoda' which were stored dry for the summer in the way described above: as you can see, they are rooting vigorously and are ready to be replanted. Note that even the tiny little bulb is rooting well. Don't be afraid to keep these dry during the summer.