"Deep" is measured from the bottom of the bulb to the soil surface. But you're not the only one that gets confused, Mary. When general gardeners give advice, take it with a grain of salt, because they might be using the "other method"
for depth, not knowing. (More often, they don't know the answer themselves, and are just repeating what they hear.) The smart ones that take the time and courtesy to explain a complete thought. They are the ones you want to befriend for advice.
Yes, in colder zones we plant deeper. But one can't plant "below the freeze-line in the soil". Mine, and Mary's , can be several feet deep! You are also confined to staying within the realm of good soil with ample drainage and aeration. The advantage of lower depths in cold zones is not as much temperature extremes, but temperature moderation, so seasonal
changes in temperature happen more gradually and plant tissues have a sufficient time to adapt.
