I have some information on tulips, daffodils and lilies I can share which might be useful for zones
2-6, maybe zone
7, not sure.
Daffs & Tulips -- foliage only, without flower
stems coming up: Temperatures down to 20° are fine. Between 17-20 is generally okay, but below that is bad news. If you get these low temperatures, be sure to watch for black foliage indicating fire and scortch. Remove leaves and discard. The leaves are full of spores, o get these removed before rains comeand spread those spores. The leaves will continue growing
albeit with ragged edges.
If they have flower
stems, which usually happens later in the season, the flowers
and stems can usually go down to about 25. You will see them laying down when you leave for work, but if you come home for lunch and the day is sunny, they will be back up. I have seen the same plants
do this maybe a total of 5 or 6 times. If you picked theflowers for a vase, you could tell they weren't intiptop shape, but as a landscape plant
you can't even tell!
Under 25° and the stem could collapse. If this happens, it looks like frozen lettuce from the fridge. Your flowers
are gone and done with until next year. Remove them before they fall prey to fungus-stuff.
If they are up 4" or less, it's easy to try to cover them with leaves or something.
LILIES are totally and 100% different. The flowers
for the entire year are at the terminal tip of the stem. If they get frozen or even frosted, they are done for the year. It is very important, therefore, to keep putting leaves, dry leaves over and on top of them. If they are over about 8-9", you'll have to snug the leaves around them for support, sort of making columns, then put more over the top so you don't bend them, and break them.
Trumpet lilies are the most susceptible to frost, and Martagons can maybe take a couple of degrees of frost. It is so hard to tell because most people don't have multiple outside thermometers to check exactly how cold it got. Martagons often grow in the woods, on the woods' edge and are more protected. The woodland soil, too, probably could hold a degree of heat more than the exposed soil in a sunny bed. Trumpets must have full sun, or nearly, so they are usually more exposed. Does this mean they are more frost susceptible? Or is it that they grow in generally more exposed conditions? Not sure, but Liles are only beautiful when they have blooms or buds, so it's essential to get them covered!