I'd like to see 15-18" depth, but 12" is doable. 2 ft. is overkill, and put the liner all the way to the top.

Yep, the bog freezes solid sometimes, I have venus fly traps that freeze solid in the bog too! along with native orchids, Sarracenias, and Iris. All the plants you list will be fine. If they are hardy in your area they can take freezing.

Fredsbog, et al:

I appreciate the insight, especially since you live in a colder zone such as I do.

1. When some sites write: "2-2.5[/3] feet depth is preferable for mature root systems", do you think this applies for specific, very large varieties of marginals - i dont know any marginals that go that deep.

2. Re: Iris, Orchid, Pickerel Plant, etc., how deep is supposedly "ideal" for these; I suppose you mean that 1.5 feet is ideal.

3. Lastly, I did the following and the plants died: I placed two root-stocks of pickerel plant, one aquatic iris variety, and several arrowhead bulbs into a shallow tray of water and left is outside during the winter; all of these plants are from local, native water-sources. When they naturally thawed after a 6 week period in which they froze in solid ice, they died - the iris barely made it and mostly rotted. Yet, you indicate from experience that those plants you mentioned will survive freezing solid, as I myself seem to observe from examples in the wild - is this because they are in soil/water, instead of straight water; that, perhaps, soil limits the formation of some ice crystals?

Steve,
You may be right about the difference between water versus wet soil. Ice has a volume about 9% greater than water, so repeated freezing and thawing will crush the plants (I am going to name this phenomenon 'ice jaws'). I would guess that the volume taken up by the soil drastically reduces the expansion and the roots may even expand to the same degree as the wet soil, so there is no crushing action.
Here is an arrowhead anecdote: I had a plastic pot with some arrowhead plants in it that lay on top of the ground all winter and went through numerous freeze/thaw cycles. They came right back in the spring - much to my surprise, since I thought it was a pot of soil with nothing planted in it - otherwise I would not have left it sitting on the ground all winter.
Bob

Root systems of many plants are not as hardy as their tops (in the case of woody plants). Plants that are left in pots above ground are subject to the temperature of the air. Hence if it goes to -5F then the pot is going to get pretty darn close to that temperature. However, in the ground the roots are not subject to such extreme temperatures because of insulating factors such as snow cover, mulch, leaves, etc. Plus when there are extremes in temperature only the top layer of soil will get to the extreme. Plants have natural anti-freeze each species has it's limits to how cold it's cells can stand before they freeze.

I do not have Pickeral plant, but the iris and orchids I have have their rhizomes within the top 3-4 inches of the soil. I suggest 18" depth because it's a reasonable depth to dig, helps hold more moisture, and has worked very well for my bogs. I was unaware that your plants had been left above ground for the winter. I lost 24 seedling Ginkgo trees that I did the same with. Two that were planted in the ground survived and are growing fine, the ones in pots all dead, So there is a big difference between inand out of the ground.

Fred,
Love your bog garden, it looks excellent!

1. I live in New Jersey - my area is supposedly zone 6b; however, i know that these zones are sometimes misapplied. Would you say that you are in a colder zone then I am?

2. When we say that iris and orchid have their rhizomes in the upper 3 to 4" of earth, as definitely seems to be the case in the field, then how would greater depth then 10" help these plants thrive and to overwinter?
The only two advantages to extra depth that I can think of would be: greater water/moisture retention in the growing season and less water per soil ratio in winter so that the roots are not frozen in solid ice [mud].

3. I think my frostline might be 18" - i was thinking of making my bog 24", so that its well beyond the frost line. However, since I'm only going to have mostly the types of bog plants that you have [orchid, iris, hardy carnivorous plants, etc], is that a waste of liner - since the roots will not be going down that deep - and maybe wont even go deeper then 10"? [keep in mind that I wont be having any sort of cattails or very large plants that might have enormous root systems?

Thanks Fred,

Steve