Serious Dicksonia Antartica Frost-related Question

[Tom2006]

I agree that the soil temps deep down don't fall low but in winter 2010 the soil was frozen for weeks up here, absolutely no heat was coming up through that.

But the soil froze as it was exposed to a radiation freeze, where the radiation rate was far faster than the conduction rate from below. If the soil was protected from radiation freeze it would not have frozen

Under one foot of snow the soil was not frozen here, but everything above the snow line was frozen.

Yes charlie, I do not wrap or cover for a whole winter, as in most, there will be mild spells and the plants are better off getting air through rather than incubating fungus. In general the SW thru NW is milder and wetter than here, so would be more important not to wrap for long periods.

I suppose, as always, it depends on where you are. The soil up here froze before the harsh radiation frosts hit and it was then a downward spiral and took weeks to thaw. No way I could have protected the ground.

[Dave Brown]

I suppose, as always, it depends on where you are. The soil up here froze before the harsh radiation frosts hit and it was then a downward spiral and took weeks to thaw. No way I could have protected the ground.

Ok I'll take your word for it, you must be a lot colder than Leeds and Hull then, and Brian Williams( Brian's Botanicals) in Kentucky, gets just as cold and for longer, but manages to get things 2 zones higher through winter by protecting the ground.

[mumfie]

cheers for all the replies, guys. always a bit of debate on this matter but plenty of valid points.

(nb not sure about the water pipe depth, dave. when i replaced my water service pipe i had to dig a 4ft deep trench and have it inspected by the board, not a 2ft one: maybe it's 4ft up north. however, still a valid point about lack of freezes beneath the surface - geothermal)

i might get a min max thermometer and plonk it in the crown before wrapping the fern. it would be interesting to see the outcome next spring.

[Tom2006]

I suppose, as always, it depends on where you are. The soil up here froze before the harsh radiation frosts hit and it was then a downward spiral and took weeks to thaw. No way I could have protected the ground.

Ok I'll take your word for it, you must be a lot colder than Leeds and Hull then, and Brian Williams( Brian's Botanicals) in Kentucky, gets just as cold and for longer, but manages to get things 2 zones higher through winter by protecting the ground.

We didn't get above freezing for two weeks, no idea what the weather was like elsewhere. How does Brian protect his ground?

Good luck Mumfie whatever you decide to do.

[dino]

Good luck to us all, it's horrible losing a decent size one

[Dave Brown]

How does Brian protect his ground?

Thick mulch gives him one zone more, and plastic sheeting gives another. Both stop the ground freezing

[Tom2006]

Is that a cloche type structure? I think I know the guy you mean, doesn't he use heating cables as well?...or am I thinking of a different person?

[simon]

I think Dave is right on this. The idea of mulch is that it effectively gives you a greater depth of protection for dormant tubers and rhizomes. If your soil is likely to freeze to a depth of 4 inches and you add 4 inches of mulch, the mulch freezes but not he soil (assuming for simplicity that the mulch has the same thermal properties as the soil. Hopefully better).

That is no help for Dicksonia above ground though, but a canopy like a duvet works in a very different way because there is also plenty of air trapped between the two. This makes it far more effective than mulch. With mulch, the radiation loss begins at the mulch surface and as heat is lost the freeze penetrates through the mulch and eventually directly into the soil itself. With a duvet, there is radiation loss is at the duvet surface but radiation from ground underneith it will come back from the underside of the duvet, resulting in the heat gain by conduction from lower in the ground being greater than the radation loss and therefore a net increase in heat. Coupled with that, the air trappen inside will be both insulated from the outside air by the duvet and warmed from below by conduction of heat from the ground surface. Even if the air freezes below the duvet and the ground eventually freezes below it too, this mechanism still continues. Remember that even frozen surfaces radiate.

[simon]

I forgot to give my answer to the original question. I think there is a considerable temperature difference between loss of fronds and death of the crown. I always loose my fronds and have lost young knuckles a couple of times but haven't had a total death. I think that that as Dave said fronds are killed around -6 and knuckles may be lost there or slightly lower or maybe after a more continued spell at that temp but depending upon thier level of esposure and stage of formation (i.e. more fully formed knuckles are more exposed to the elements and therefore more prone to frost damage). The growing point itself is much deeper down and therefore would require lower temps or considerably prolonged freezes before total death.