How do dwarfing interstems or rootstocks work?

Having started grafting a few conifer cultivars onto species rootstocks for the first time recently--a practice in which no influence of the rootstock is wanted or expected--I started wondering why and how carefully selected roots or interstem grafts so dramatically influence the vigor, the growth rate, and ultimate size of fruit trees? Anybody help me out on this one?

Scott

Some trees are more vigorous even within a species and that will influence factors such as form/shape and growth rate, or inversely some are less vigorous. These examples are few, far, and inbetween however. They're as much of a freak of nature as a witches' broom for example as would be a seedling/witches' broom from a fruit tree having "dwarfing" properties/characteristics. Why conifers don't have dwarfing rootstocks in general is because most all cannot be propagated via cuttings but only through grafting. Sure some can, (Thuja's, Junipers, Metasequoia's, etc.) but still I've never heard of anyone using them/anything like this for conifers as it just probably hasn't been found necessary yet. It would be however interesting to take a dwarf that grows say three inches per year when grown on a typical vigorous seedling rootstock and then turn around and graft it onto a rooted cutting having "dwarfing" characteristics and see how it influences the growth rate and shape of the plant.

A friend of mine once told me that he wished more propagation, if any regarding the matter we were speaking about, was attempted at/for propagating reversions on witches' brooms. Some reversions too grow slow, and are not "complete reversions" so that area still has a lot of interest for me. Near me, there's a Picea glauca 'Conica' with a reversion (reversions are very typical for this plant) on it that I've been watching for at least four years now and it seems to grow about 3" per year. It's a perfect globe unlike most reversions you'll typically see on 'Conica' so I continue to watch it to see if it manages to maintain it's current form. The tough part about reversions on this plant however especially is that it is one of the rare few conifers especially of the Genus Picea that will actually root as a cutting. So now I wonder and you too are probably wondering what will happen when a reversion such as this is grafted to a quote-unquote more vigorous understock. If I were to take a guess, I would say that it wouldn't have much influence. To support my last statement it's been said to me that a grafted Chamaecyparis obtusa on a Thuja occidentalis rootstock when compared to a Chamaecyparis obtusa (cultivar) on its own roots will have the same shape when planted side by side in exact conditions with the only difference being that the grafted one grows much quicker initially and then slows down to the original growth rate of that of the rooted cutting. One more note about this grafting technique (obtusa onto occidentalis) is that when planted in clay soil, the Chamaecyparis on it's own roots will not tolerate the clay and will die, however the grafted obtusa onto the occidentalis will thrive.

For those of you who may not know what a reversion is, it's when the plant characteristicly produces foliage/needles that are larger/longer and have a growth rate that's always quicker than it's 'host'. Inversely, a witches' broom is a "congested" form of growth always growing slower than the parent/host.

So, while I don't know jack about understock selection for fruit trees, I'd have to assume that the understocks/rootstocks being produced are propagated asexually, as they would have to be, which involves either propagation as a cutting, or propagation via through grafting which wouldn't apply to this case nor would it apply to any situation where a dwarf rootstock from any species whether it be conifer, evergreen, or deciduous was used as rootstock that is, unless it was propagated as cutting-grown which of course leads me right back to that thought about how the scionwood from a Chamaecyparis obtusa "acts" when grafted. So really I have no basis nor understanding for your question regarding the behavior of fruit tree dwarfing rootstocks. Maybe this will fuel some food for thought for further discussion, I hope.

Dax

Having dwarf fruit tree rootstock makes for super-easy harvesting. No ladders, trucks, etc. Pick from the ground-level. Think of the cost-savings if your workers don't have to climb up and down all day. I know Poncirus trifoliata (Hardy Orange) is used for citrus for its cold-hardiness and durability. Some superior fruiting plants have poor root systems so why not get a dwarf stock that's been cleaned up for viruses, etc.

Scott,
The dwarfing rootstocks used for fruit trees are usually dwarfing BECAUSE they are genetically less vigorous than seedlings of their species. Most, like the EMLA series for apples, have been cleared of viruses/viroids. Many of these dwarfing rootstocks have 'weak' root systems - which necessitates staking or trellising, as they don't anchor well enough to support the top, with a full load of fruit. Additionally, some of these, because of their minimal root system, can't withstand drought conditions - if things get dry, they die. And finally, some dwarfing rootstocks negatively impact fruit size/quality.

However, performance is variable, to some degree, based upon the area of the country and environmental factors. A rootstock that works well for, say, pears, in northern IL may not function well in Houston, TX.

All the dwarfing/semidwarfing rootstocks I'm aware of are propagated vegetatively, either from cuttings, or more frequently, by stooling.

Scott,
The dwarfing rootstocks used for fruit trees are usually dwarfing BECAUSE they are genetically less vigorous than seedlings of their species. Most, like the EMLA series for apples, have been cleared of viruses/viroids. Many of these dwarfing rootstocks have 'weak' root systems - which necessitates staking or trellising, as they don't anchor well enough to support the top, with a full load of fruit. Additionally, some of these, because of their minimal root system, can't withstand drought conditions - if things get dry, they die. And finally, some dwarfing rootstocks negatively impact fruit size/quality.

However, performance is variable, based upon the area of the country and environmental factors. A rootstock that works well for, say, pears, in Oregon may not function well in Houston, TX. As they say, your mileage may vary.

All the dwarfing/semidwarfing rootstocks I'm aware of are propagated vegetatively, either from cuttings, or more frequently, by stooling.

Well said Lucky_P! Very concise and correct.

Dax

Interesting. I think I might have had one misconception, or maybe not. I thought that on some fruit trees, vigorous root stocks are grafted onto a dwarfing inter-stem onto which then the fruiting cultivar is grafted, thus producing a tree of all the finest characteristics. Is that done, or even possible?

Scott

That concoction would be termed Malus x 'Suckering Sensation'.

VV is right.
I've got a number of apples in my orchard on a combo M111/M9 rootstock. On its own, M111 should produce a tree 60-75% standard size, but M9 should cut that to 30-50% of standard.
In my orchard, it's not a good combo. Evidently, M111 does NOT anchor well in my soil - a good clay soil with high water table, and it's in an exposed site, so winds are fairly strong and constant - so...I've got several trees on the combo rootstock than have leaned over at 45 degrees or more(one's lying flat on the ground now, but I'm pulling it out this weekend and replacing with something else), necessitating staking/propping.
Additionally, the M111 understock suckers like crazy.

So, what your saying is: the dwarfing rootstock effects the size that the graft can become by limiting nutrient uptake, but the fruit will retain normal charactoristics, including size?
This is very interesting.
Somehow, this process seems remotely simular to Bonsai. Except in your example, the root system is replaced with a compatable but functionally inferior one. And in Bonsai, the root system is periodically reduced through pruning to decrease nutrient up take.
In your case, the size of the graft is effected, but not the general charactoristics.
In Bonsai, the whole plant is miniturized.
How come the grafted plant doesn't develope smaller fruit? Or does it...

Patrick, if what you say is true, then why have I seen photos of bonsai fruit trees with normal size fruit? http://community.travelchinaguide.com/photo/5101/51013013322533-s.jpg
I am thinking that bonsai so drastically reduces the root system's capacity to function, not ability. Thus, my take on this photo link is that that particular bonsai was extremely well care for, while others that produce smaller fruit than normal simply mirror the capacity of the root system.

Here is another: http://nakahara.ac/e-bonsai/english-page2.htm Malus cerasifera has cherry size fruit normally.

Also, isn't the production of smaller leaves one of the by-products of constant tip pruning in bonsai?

And to be clear about the Suckering Sensation phenomenom: am I right in saying this will appear whenever a more vigorous rootstock is used with a less vigorous top or interstem? If this is correct, than I would expect Lucky's M111 rootstock not to sucker as badly had there not been an M9 interstem.

Rick

Rick, Thank's for posting your photos and link. To be frank, I am not a Bonsai grower, even though I do admire that level of artistic acheivement some people are able to acheive with Bonsai, I just can't bring myself to mutilate the subject so radically and repeatedly. About 10 years ago , I had the idea I would try to Bonsai native trees from my area so I could own trees that I didn't have room to grow on my then tiny property. The process didn't stick with me and I wound up planting all the surviving seedlings I had collected out in different properties. I have seen many old Bonsai with miniture flowers and fruits. I am aware of the normal size ones as well. I am just curious why the fruit does not appear effected with grafted plants. Am I to assume that by using dwarfing rootstock, although fully functional, it is unable to supply sufficiant nutrients to the graft, preventing it from achieving its genetic potential. That would cause me to wonder why the fruit is not effected adversly...

I'm not a real bonsai grower either. I guess you could say I have pseudobonsai.

But I am sure what is happening in bonsai and using dwarfing rootstock are completely independent methods.

Bonsai changes the capacity of the same root system by confininement, and forces less topgrowth.

Dwarfing rootstocks/interstems may have somewhat of the same effect, but not so severe. While I adhere to my explanation above on why some bonsai have miniature fruit and some normal, grafting is certainly different. And I would only be speculating on the actual mechanisms involved. I guess I do have my theory, but am finding it difficult to put in words. Besides, it's just theory, and we should be hearing from someone who knows.

Who knows?

My guess was that the bonsai's with the dwarf fruits were dwarf-fruiting to begin with = "oddities of nature."

I too have seen "Orange Tree Bonsai's" with fruits the size of a monetary quarter or dime...

I don't know, but I would like to know.

Dax

There are a few species of citrus that naturally produce small fruit. Indeed I should have included genetic dwarfism in my discussion. However, I did not.

Patrick,
There is some concern, with some dwarfing rootstocks for apples/pears, regarding fruit size and quality. Fruit size/quality is diminished with *some* dwarfing stocks, while others mainly limit the vigor and rate of growth of the tree itself.

Thanks for the discussion everybody, it was very interesting!

Dax