Grafted tree hardiness

[WNC Bonsai]

While reading back over some discussions of trees that are commonly grafted onto other rootstock the question struck me as to how this affects the overall tree hardiness. I know that often some trees such as white pine are grafted onto more vigorus root stock but then is above ground hardiness determined by the rootstock or the tree? For example blue atlas cedar are hardy to zone 6 but are commonly grafted on Cedrus deodara which is only hardy to zone 7. So in this case is the above ground still a zone 6 tree growing on zone 7 roots? Fortunatley I am now in zone 7b so a blue atlas cedar should be fine on C. deodara roots as long as I get it in protected for winter.

 

[Wires_Guy_wires]

It depends on how much exchange there is, mostly dependant on how closely related the root stock and graft are.
Sometimes a good root stock can raise the hardiness (in the sense that its more hardy) and sometimes it makes no difference.

 

[BrightsideB]

I volunteered at Cox arboretum that had trees that typically don’t grow well here due to the overall climate. Tom told me it’s because they had the right root stock grafted. He grew a lot of tree’s I hadn’t seen before in North Georgia. There was an area they planted grafted material as experiments to see how well the specie acclimated with different types of root stock.

 

[crab apple]

I think almost all orange trees in florida are grafted onto sour orange type root stock, I always believed it was to make them more hardy.

 

[Shibui]

'Hardy' can mean a lot of things, especially for fruiting trees. Hardy to disease, hardy to wet soils, hardy to drought, insect pests or to cold/heat.
Grafting is often used to influence rate of growth and ultimate size of the tree. Many apple root stocks have been bred for size of the tree.

In this case the question seems to be about cold hardiness but I believe that cold hardiness can be affected according to the rootstock used. Citrus here are grafted onto Poncirus trifoliata because that seems to make them better for cool areas with wet winter soils.
Pretty sure that is not about the hardiness circulating through the entire tree. It is more about the grafted roots surviving in cold soil where the normal citrus roots won't.

I have not seen any research into whether less cold hardy rootstock would make a tree less cold hardy overall. Maybe that would depend on whether the conditions are affecting the tree above ground or the roots.

 

[Paradox]

From my understanding, generally the hardiness is more dependent on the rootstock. The reason why JWP is grafted onto JBP is so they can tolerate a milder climate than JWP on their own roots can.

 

[Bonsai Nut]

In California, citrus trees come with two tags - one for the scion, and one for the root stock. In the case of California, they are interested in roots that do well in alkaline soils, and have resistance to certain pathogens.

Many people don't know this, but all European grape vines are grown on American grape vine root stock. Back in the 1800's, a tiny subterranean aphid from America was accidentally introduced into Europe and almost wiped out the European wine industry. American grape vines had resistance to this pest, but European grape vines did not. Once French winemakers started grafting their grapes onto American grape rootstock, the wine industry was saved.

 

[BrightsideB]

This has become an interesting thread!

 

[Leo in N E Illinois]

For "bonsai purposes" where die back of the scion, even partial die back of the scion would be bad, essentially a failure as bonsai, grafting does not reliably affect hardiness. The reason all those JWP are grafted is that from seed, only one in a thousand have that lovely blue color we love. Most are a more sickly greenish yellow. They graft the few blue ones found just to make the "good color" available. JBP is used as rootstock in Japan to make JWP more tolerant of heat in Tokyo and low elevations of Japan. If you have a JWP on zone 7 JBP leaving it outside in a zone 5 winter will kill the roots.

So in general the hardiness will be no better than the least hardy of the pair, if one is zone 5 and the other is zone 7, your best bet is treat the tree as zone 7.

Additional factoid, graft unions do not heal instantly. It may take up to 5 or more years for a "full fusion", during this healing time a harsh winter can have ice form in the graft union, causing damage and partial separation. Sometimes winter freeze thaw cycling can completely break a new graft union. For the first 3 winters after grafting, it is best to winter the tree at above freezing temperatures. Protect longer if there has not been a lot of growth, as in some cork bark pines. Protect shorter if there has bee good vigorous growth. as in some Malus.

 

[WNC Bonsai]

I was intrigued even more by this discussion so went to the Eshraghi Nursery website to check on the Blue Atlas tree I bought. They list it as cold hardy to zone 6 so it must not be grafted onto zone 7 C. deodara rootstock. Maybe they have been growing the green atlas cedar variant to use as rootstock to create a completely zone 6 hardy tree. I have read that one reason C. deodara is used is because the green atlas cedar is just not grown any more. I’ll send them an email and see if they will provide that info.