It is true that oriental trumpets are generally hardier than straight oriental lilies. It's not surprising, as trumpet species usually have better, longer lasting constitutions.
The four best tips for orientals in very cold climates.
1. As long as you know the ground will freeze anyway, put on a heavy mulch before the ground freezes. (That's not what I normally recommend.) Here in zone 4 Minnesota, that's not the case. In fact this season, we had a heavy snowfall relatively early for a thick blanket. The frozen ground has already thawed in many places under the deep snow. If you are unsure, than apply a mulch after the ground freezes well.
2. Make sure you soil is acid based. A pH of 5.5-6 or 6.5 is best. Neutral soil will lessen the cold hardiness, and alkaline soil will affect overall health, which will in turn decrease cold hardiness.
3. Plant the bulb 2-4 inches deeper than recommended. But you must be sure that the soil way down there, and below the bulb where roots will establish, is adequate (i.e. rich, acid, still well draining).
4. Plant in spring, not fall.
Remember, if the stem has died back, there is no need to water in the fall, unless you are positive the soil is bone dry down where the bulb is. It is always better for bulbs to go into the winter slightly lacking water than the other way around.
