USDA Zones and hardiness

[Kahless]

This may be a stupid question, but if I live in zone 4, and I would like a zone 5 or 6 tree, it's just a matter of not letting it get as cold as the other trees I have, correct? I have my cold hardy trees heeled into the ground and mulched. Would I need to put a zone 5 in a garage? Or, only when the temperature drops to a certain point, as long as the roots are protected once it's 15 degrees?

 

[BuckeyeOne]

Yes.
Just remember that a Zone 5-6 tree in a pot becomes a Zone 6-7. You lose a zone of cold tolerance when growing in a pot.
My Satsuki azeleas have to go in my unheated attached garage here in 6a as they are mostly Zone 7 tolerant.

 

[Bonsai Nut]

Each "zone" is 10 degrees of winter low temp. I wish it was more sophisticated... but it's not. There is a huge difference between a zone where it hits 10 degrees once or twice per winter... versus a zone where it consistently sits at 10 degrees. But they will both in the same zone - in the 7b range.

Now I'm here to tell you that I live in zone 7b and we have had two days of frost so far. Not 10 degrees... I'm talking night lows that hit 28... but days that are in the mid 40's. So you have to take the USDA zones with a grain of salt.

Bottom line... understand your zone and understand your trees, but realize that your garden can experience very different conditions, and a tree in a pot (particularly on a bench) doesn't have the protection of earth's natural insulation. Protect the roots (the most delicate part of your tree) and avoid major temperature swings. Trees go into and come out of winter dormancy slowly. A tree that is supposed to be zone 4 hardy can still die if it starts to awaken from dormancy and gets hit with a late zone 7 freeze.

 

[Paradox]

Yes.
Just remember that a Zone 5-6 tree in a pot becomes a Zone 6-7. You lose a zone of cold tolerance when growing in a pot.
My Satsuki azeleas have to go in my unheated attached garage here in 6a as they are mostly Zone 7 tolerant.

My Sastuki azealeas have to go in my unheated garage as well and I am in zone 7b because as you said, in a pot they are really zone 8 tolerant.

 

[Mikecheck123]

Yes, it's important to emphasize that zone is a very one dimensional aspect of climate. It's useful, but not the only consideration.

 

[Potawatomi13]

Big difference in tree hardiness vs root hardiness. USDA zones not about root hardiness. Michael Hagedorn explains very well in Bonsai Heresies.

 

[LittleDingus]

Being in Minnesota, do not discount the drying power of your cold winter winds! That can be a bigger hazard to your out of zone trees than pure cold. Drying over winter is the enemy!

Trees can dry out over winter for several reasons. The root ball can freeze. This prevents water uptake by the roots. Fryeezing solid happens more regularly to trees in pots. Also, cold winds are dry winds. They can pull a lot of moisture out of just about anything.

To more directly answer your question, I have zone 8/9 trees that I keep in zone 6. They move into the garage for winter. I leave them outside as long as I can to harden them up for winter but they move inside if it looks like we'll get to freezing. They will move back outside as soon as it is safecto do so. I keep them in the garage for the minimum time that I can to keep them safe...

 

[sorce]

Walking in Boston, there are no stupid questions!

Stupid concepts though, like zones and more specifically, how trees change zones inside pots.

Remember this is something that you will be doing probably forever. Every year that 5/6 tree is going to get a little better and your time into it greater. It dying will hurt more every next year too. So going about it, well, at all may be a bad idea, but half assed certainly is.

I'd ignore zones all together and see if that works. If not, decide wether to setup a legit area of moderate expense to ensure they never die. Otherwise you're setting yourself up for a sad day.

I'm convinced it's not the cold, but the more rapid onset of it that kills lesser zoned trees. They just don't have the time to prepare. Setting them on the ground to connect to the much network can help this IMO.

Next up for debunking....chill hours. Another stupid concept.

Sorce

 

[BuckeyeOne]

Michael Hagedorn explains very well in Bonsai Heresies.

Seems like Michael's book is gaining a big following!!
He explains the root tolerance issue very well. It is more important to those that grow sticks in pots than in the ground!
If I remember the table he shows was developed by Iseli Nurseries and Oregon State University. Great info!

 

[Leo in N E Illinois]

Soil is excellent insulation. 2 or 3 inches below the soil surface, the ground can be much warmer than the air. Heat rises. Heal your pots in ( bury to the rim) add a layer of mulch, then drape a piece of canvas, or a tarp over the trees. This will trap heat rising from the earth, and in zone 4 should be enough to get zone 5 and 6 trees through the winter. Remove tarp early in spring, lift trees 3 or so weeks later after deep freeze danger has past. Simple, works well for "almost hardy enough" trees.

 

[Kahless]

Soil is excellent insulation. 2 or 3 inches below the soil surface, the ground can be much warmer than the air. Heat rises. Heal your pots in ( bury to the rim) add a layer of mulch, then drape a piece of canvas, or a tarp over the trees. This will trap heat rising from the earth, and in zone 4 should be enough to get zone 5 and 6 trees through the winter. Remove tarp early in spring, lift trees 3 or so weeks later after deep freeze danger has past. Simple, works well for "almost hardy enough" trees.

Thanks for this reply. I am getting a trident maple which is hardy to zone 5 and I live in 4. I was thinking about growing it out for a few years in the ground in which case the roots would be protected but the tree would not be mobile. I wonder if I could do that as long as I kept an eye on the low temperatures so that I could protect the top of the tree when it gets colder than it’s zone. Or maybe a grow box is a better idea.

 

[leatherback]

Thanks for this reply. I am getting a trident maple which is hardy to zone 5 and I live in 4. I was thinking about growing it out for a few years in the ground in which case the roots would be protected but the tree would not be mobile. I wonder if I could do that as long as I kept an eye on the low temperatures so that I could protect the top of the tree when it gets colder than it’s zone. Or maybe a grow box is a better idea.

the hardiness zones are defined assuming ground growing. So you in zone 4 getting a zone 5 tree would be asking for winter damage the first time you get a "normal to cold" winter.

 

[Leo in N E Illinois]

If your tree in the ground is small enough to cover with a tarp during winter. That alone should allow zone 5 trees to survive in zone 4. The tarp holds some of the heat rising from the ground. Capturing this heat is part of what makes a cold frame effective. Digging a pit, putting the trees in a pit, then covering the pit with a piece of plywood or foam insulation board will also work. As long as you weigh down the board so it doesn't lift in a sub-zero windstorm the trees in the pit will be protected from the worst of Minnesota's cold.

Of course today, the cold has already arrived, the ground has begun freezing, digging is no longer possible. Where things are for winter now, is where they will have to stay. LOL

 

[Kahless]

If your tree in the ground is small enough to cover with a tarp during winter. That alone should allow zone 5 trees to survive in zone 4. The tarp holds some of the heat rising from the ground. Capturing this heat is part of what makes a cold frame effective. Digging a pit, putting the trees in a pit, then covering the pit with a piece of plywood or foam insulation board will also work. As long as you weigh down the board so it doesn't lift in a sub-zero windstorm the trees in the pit will be protected from the worst of Minnesota's cold.

Of course today, the cold has already arrived, the ground has begun freezing, digging is no longer possible. Where things are for winter now, is where they will have to stay. LOL

Right, if I tried this, it would be next year’s winter.

 

[Gustavo Martins]

It’s funny. People struggle to protect their trees in areas colder than their rated hardiness and I try to make sure mine get exposed to as much cold as possible by raising above ground and exposing to the winds.
Sorry for the rumble and side track.