does hardiness change in a raised bed?

[Robert Pavlis]

The recommendation for growing things in containers is that they should be 2 zones more hardy than the zone you are growing in.

Does the same apply to larger raised beds? If I could grow a plant that is just hardy in my area (in the ground), will it survive in the raised bed.

Assume both locations have excellent drainage, and the same amount of winter moisture.

[Susan Band]

I have often wondered that. I presume in the middle of the bed it should be okay but maybe at the edges the frost could creep in. The heat will still come up from the ground into the bed. I grow a lot of things in polystyrene boxes and wonder if these will stop the warmth coming up from the ground?
Susan

[johnw]

I have often wondered that. I presume in the middle of the bed it should be okay but maybe at the edges the frost could creep in.

The heat will still come up from the ground into the bed. I grow a lot of things in polystyrene boxes and wonder if these will stop the warmth coming up from the ground?
Susan

Susan - I have a large raised bed in the yard and we have experienced some mighty cold winters without snow cover. I see no damage on the edges of the beds. The caveat, nothing in the bed is exceptionally tender - all zone 6 plants and we are in zone 6.   In the cold frames where the soil level in raised above grade there are zone 7b plants, all survive in the frame but the top is covered in December with white poly until late March, no damage even at the edges.

On the matter of styrene boxes,  I had a discussion with a nurseryman here just last week.  He had a new glasshouse with a cement floor and in floor heating (which by the way he does not turn on in the winter time).  I asked him if there was styrene under the cement and he said there was. I asked if he would not gain a bit of extra heat from the soil beneath. No, the contractor said better insulate and to protect from the cold beneath but that I presume was in spring when he turned the heat on.  The owner argued that there would be more heat in winter generated by the sun hitting the cement floor than radiated from heat gained below uninsulated cement.  I must say I left puzzled. He may be correct but there certainly is a great insulating factor from just having greenhouse walls below ground - ie the pithouse. Still not clear on the matter.

The temperatures you reported are shocking.  Would Glendoick have experienced the same lows?  Some mighty tender material there.

johnw  

[johnw]

The recommendation for growing things in containers is that they should be 2 zones more hardy than the zone you are growing in.

Robert - By this do you mean in containers, above ground without any winter protection?  

Our native hemlock  - Tsauga candensis - apparently is root hardy on to -7c so it might get knocked flat in such a scenario.   Also very hardy magnolias don't seem to survive much cold at all above ground unprotected. If either are heeled into a bark pile they come through fine.  More research needs to be done I guess.

I have seen great losses in white poly hoop houses locally.  Nurserymen just put their potted material in there, sometimes stacking them high. All sorts of material gets hammered.  I always recommend potted ericaceous material by plunged in wood chips or sawdust even in a hoop house, ditto mags and Japanese maples and countless others; few take the hint.

Ontario seems to have better luck than we do here with hoophouse survival.

johnw

[Susan Band]

John,
This is the lowest it has been for a while, the diesel in my pickup truck froze today. More worrying for plants is the fact it has been pretty cold in the west of Scotland which usually doesn't get any significant frost. Most of the great gardens are right next to the coast so perhaps they are okay. Glendoick is slightly nearer the river than here but that only really helps stave off light frosts, I presume it has been the same for them.
Some of the bulbs that are usually in the field I put in boxes as I never had time to plant them back this autumn so we will see in the spring if the field or box bulbs did better.
Susan

[cohan]

i dont have a lot of personal experience in this--except for losing some plants last winter that should be completely hardy here--in a big pot about 14" square, which had snow cover most but not all of the winter...
the following is more speculation than solid knowledge:

my feeling about raised beds improving hardiness is that it would more be an issue of altering conditions in the growing season than making winter any milder--
raised beds are considered good for veggies, for example, as they avoid some early/late frosts; for perennials, perhaps they would thaw sooner (not always a plus) and freeze later, giving a slightly longer growing season--this might depend on location, as well, i have less confidence that a shady spot would see much value--except the same avoidance of growing season frosts..

your area is not as cold as here, but i think once the deep cold sets in in mid-winter, a few inches up or down cant make much difference... but if you are able to increase the warmth/number of frost free days during growth, that might be an important factor for some plants--i am planning some beds to create that sort of micro climate for some hardy cacti-which i think can take my lows, but i want to make sure they get the hot summer they'd like...

[johnw]

Cohan - Raised beds here get things off to a roaring start in our very cool spring.

johnw

[Lesley Cox]

Robert, maybe it's one of those things you have to try, to find out. If you have enough material of whatever plant, try it in the raised bed and compare the results with those in the ground.

[Rodger Whitlock]

I have a raised bed that is about 20' long, 4' wide, and 1' high, running east-west and about 8' out from the south wall of my house the bed is parallel to. This bed was originally surrounded by old railway ties, but when the ties turned out to be harboring carpenter ants I got rid of the ties. These days I am left with an elongated mound of soil with sloping sides - my "Anglo-Saxon burial mound."

I have noticed this winter when we have had real frost, several nights in succession with temperatures down to about -6C, that the soil in the mound remained frozen when pretty much everything else has thawed out, even though this bed is in full sun.

Conclusion: my raised bed is both colder in winter and hotter in summer than soil in the open garden.

A point to keep in mind by those of you struggling with unusually cold weather: even a very little overhead insulation is a significant improvement over no insulation at all. Plants in coldframes will benefit from having a few layers of newspaper spread over them, just as in days gone by, tramps and hoboes would line their clothing with sheets of newspaper to stay warm.

If you have potted plants in pots in flats on the ground, nearly any kind of covering you have handy will give them significant protection. A few layers of newspapers topped by a sheet of plastic, the whole affair weighted down around the edge with bricks, will do more good than you might think.

Susan is quite right to suspect that styrofoam flats will block heat rising from the earth (an endless source of heat at 50F/10C), but nearly any kind of flat, because it lifts the pots above direct contact with the soil, poses something of the same issue.

[Lesley Cox]

A point to keep in mind by those of you struggling with unusually cold weather: even a very little overhead insulation is a significant improvement over no insulation at all.

Wasn't it Beverley Nichols who discovered and said in one of his books that even a spider's web affords some frost protection for the plants underneath it?