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Dwarf Rootstock Mortality in Cold Climates

The following is good reading when considering dwarf rootstocks in the colder climates. This is one of the many studies done through the US SARE system. (Look for more SARE project posts on our website.)

Authored by John O’Meara of O’Meara Family Farm with assistance from Renae Moran (U of Maine) :

Summary

This project attempted to determine the usefulness of dwarf rootstocks in a zone 3 orchard. Conducted over the course of four years, this project compared the growth and survivability of three dwarfing rootstocks with a standard, cold-hard rootstock normally used for extreme climates. Ultimately, the dwarf rootstocks did not survive well and generally showed poor performance at this location. The results of this project point to Antonovka rootstock as the most viable rootstock in a zone 3 location, especially when the orchard is under organic management.

Introduction

This project aimed to test new dwarfing and semi-dwarfing rootstocks in a zone 3 climate. Rootstocks G. 41 (dwarfing) and G. 935 (semi-dwarfing), developed at the USDA/ Cornell University Apple Rootstock Breeding Program in Geneva, New York, could offer the hardiness required of a zone 3 climate combined with the benefits of a dwarfing tree. Dwarfing trees could allow the northern grower the advantages of fruit-bearing precocity, ease of harvest, and high orchard density. In addition, G. 41 and G.935 have been bred to be resistant to serious apple diseases, which may make them particularly valuable to growers in various climates. To test the hardiness of G. 41 and G. 935, this project compared the survivability and growth of the two new rootstocks to Bud. 9, a commonly used hardy, dwarfing rootstock, and Malus antonovka, a hardy standard-sized rootstock.

Objectives/Performance

Targets

In the spring of 2009, we grafted 200 trees for this project. Liberty scionwood was used for all 200 grafts. 50 grafts were done on G.41 rootstock, fifty on G.935, fifty on Bud. 9, and fifty on Antonovka. The G. 41 and G. 935 rootstocks were provided by Dr. Fazio. The Antonovka and Bud. 9 rootstocks were acquired from Lawyer Nursery in Montana, the usual source of our rootstock. Success rate of grafting was recorded in the beginning of the first growing season. In the fall of each year, the trees were protected from rodent damage, in preparation for winter. During four growing seasons (2009,2010,2011, and 2012), the trees were weeded. They were also monitored for vigor, survivability, growth, and the presence of disease.

Methods

Grafting success: The trees were grafted in April, 2009 and planted on May 4th, 2009. As of June 4th, 2009, the following numbers of grafts were successfully growing, by variety. G.935 16/50 G.41 30/50 Bud 9 13/50 Antonovka 35/50 The G. 41 and Antonovka had significantly better grafting success than the other two varieties. Although every effort was made to acquire rootstock of comparable size and quality, the G. 935 was of larger diameter than the other rootstocks, requiring that the grafting was done a little differently. In addition, the Bud 9 rootstock lacked vigor from the start and did not grow well in general. Although these initial problems may have affected grafting success, we proceeded with the project with the belief that the project would still provide important data about the survivability and overall growth of the rootstocks in question, over the long term. Fatalities: Some rootstock died during the course of this project. As stated above, the Bud 9 rootstock arrived from our supplier nursery in poor condition. An unusual percentage of those rootstocks were dead on arrival or perished the first season. Deaths among the roostocks: 2009– recorded September G.935 5/50 G. 41 2/50 Bud 9 12/50 Antonovka 0/50 2010—recorded August G.935 42/50 G.41 13/50 Bud 9 25/50 Antonovka 2/50 2011– recorded August G.935 2/50 G.41 20/50 Bud 9 11/50 Antonovka 0/50 Total fatalities– 2009-2010 G. 935 49/50 (98%) G.41 35/50 (70%) Bud 9 48/50 (96%) Antonovka 2/50 (4%) Total fatalities– 2012 G. 935 49/50 (98%) G.41 47/50 (94%) Bud 9 48/50 (96%) Antonovka 2/50 (4%)

Impacts

Clearly, the dwarfing rootstocks have displayed some serious problems during the course of this project. Although the Antonovka performed as it has for several years on this farm, the dwarfing rootstocks suffered severe problems of survivability. One cause of this may have been a weed problem that developed in the nursery during 2009. Quack grass became more prevalent in the nursery and possibly inhibited the vigor of the rootstocks in general. The more vigorous Antonovka was more able to compete with the weeds. On the other hand, the rootstocks that did survive performed well, indicating that weed pressure may not have been the major problem. Our advisor, Renae Moran, has expressed the opinion that weed pressure is probably not the primary cause of the fatalities among the dwarf rootstocks. Severe winter weather is a problem for fruit nurseries in our climate. This project was intended primarily as a test of the winter hardiness of these dwarfing rootstocks. Because the winter of 2009/2010 was one of the warmest winters in our location on record, as seen in the temperature log, and cold winter temperatures would not seem to be the cause of the high mortality of the dwarfing rootstocks. The winters of 2010-2011 and 2011-2012 have also been moderate in New Sweden. The problem may be tardiness in hardening off among the dwarfing rootstocks. Although the winters have not been very cold, G.935 and G. 41 may be slower to harden off in fall; the fall temperatures of Aroostook County may be causing a higher mortality among rootstocks not adapted to a zone 3 climate.

2009 Daily temperature log

2009 was an unusual year in northern Maine, weather-wise. April and May were unusually warm and almost incessantly windy. June and July were exceptionally rainy. Low and high daily temperatures were recorded daily 2010 also had some unusual fluctuations in weather. There were long dry spells during the summer but no exceptionally cold weather during winter months. 2011 was the wettest summer on record in Caribou, Maine. Although the exceptionally wet weather may have caused even more problems among the dwarfing rootstocks, the Antonovka were unaffected. During 2012, the dwarfing rootstocks used in this project continued to perform poorly. Only 1 G. 935, 3 G. 41, and 2 Bud 9 survived, as of October 2012. In contrast, only 2 of the 50 Antonovka rootstock have died since 2009.

Accomplishments

This established that though the rootstocks in question may have utility in other locations, they did not serve well or succeed in our northern, organic orchard. The Antonovka rootstock outperformed all of the dwarfing rootstock in this project. Although there are disadvantages to using standard rootstocks, the dwarfing rootstooks tested over the past four years did not survive well enough to be viable options in our orchard.

The results of this project point to Antonovka rootstock as the most viable rootstock in a zone 3 location, especially when the orchard is under organic management.

Publications/Outreach

An article on this project has been written and submitted to Farming magazine for Spring 2013 publication.

Northern Apple Rootstock Future Recommendations

Although the dwarfing rootstocks tested in this project did not do well in our orchard, our results point to the need for more testing in more locations and in different conditions. Perhaps a slightly more temperate location would allow for more survivability among the dwarfing rootstocks.

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Cold damage to Roots

It should be understood that plant roots and aerial portions of those same plants vary in their resistance to cold damage. Apple tree roots for instance, can be more than 60 degrees (F) more susceptible to damage or death from cold than their above ground counterparts. The reason for the difference is still under study, but some interesting information is available on the subject.

A Cornell study in 1921 shed some light.  Apple and pear trees were manipulated as to have their roots under different exposures to soil, oxygen, and light. By allowing the tree roots to be exposed (above the soil), a physiological transformation of the roots occurred. The excavated roots soon took on the appearance of trunk and other above ground parts. Roots that were uncovered many months before cold temperatures arrived showed no damage at temperatures of at least 3 degrees F. Roots uncovered directly before were killed at 13 degrees F. It was unclear to the researchers if roots could eventually attain as much resistance to cold as normal aerial parts (ie- trunk and branches).

The same aforementioned  study showed that trunk sections can undergo a reverse type of transformation. Fameuse trees planted were planted deep enough to have about 10 inches of scion covered with clay soil. They grew in this condition for a full season, were dug and then exposed to 3 degrees F. All tissue was killed below where the soil line existed. Control trees of same cultivar and grown with all scion tissue above soil showed no damage at the same temperatures. This shows a regression of cold hardiness resistance of the same type of tissue, when covered with soil for a certain period.  There is evidence also that the more (ie- deeper) the material is buried, the less cold resistant it becomes.

The type of soil does apparently have bearing. In a 1922 study, Jonathan apples planted a foot deep in sandy soil and later excavated showed no damage to the parts in question at 2 degrees F. In the 1921 study the soil type was much heavier (higher density clay material). A similar test showed no damage on parts as low as -15 degrees F.

Older roots generally are more resistant to cold than finer, younger specimens.

This can be of practical importance in that growers can prevent any root excavation in their practices when said practice is approaching cold weather. This can happen during cultivation, raking, or mulch redistribution. It also means that older roots that have been exposed to the surface for a long period are likely not to be easily damaged by cold temperatures.

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Dwarf Apple Trees and Cold Hardiness

For those interested in planting a dwarf apple orchard in the cold climates of North America:

Walden Heights is zone 3 NE Vermont at 1700ft, and has seen -40 this decade. The lower snow cover and temperature swings lately will likely test the marginal species here.  Of the dwarfing stocks we have only tried in any real way the bud 9, since in literature it was touted as the hardiest commercially available dwarf stock. We did plant and are watching some bud 490, bud 146, bud 233 (and I think 491 is out there somewhere). 

The bud 9 trial (over 100 grafted trees in 2002) has been a near total failure. In terms of their susceptibility to the climatic conditions here they are third rate. This doesn’t necessarily point to cold temps  as we have not witnessed any dramatic tissue damage. What I am seeing is classic “failure to thrive” syndrome. When we trial things here, we treat them as the average homesteader will, and like we treat our standards. Not neglect, but not pampering either. The roots are horribly brittle even after years of growth, grow very slowly in our zone without heavy fertilizing and drip irrigation, and are highly attractive to borers. I have had 12 year old trees killed by a single borer. Burr knots have occurred on nearly all. Of the 100+ there are a couple of dozen left. Having seen better luck at orchards who use heavy applications of conventional fertilizers, or high nitrogen  and irrigation, folks who grow with a more heavy handed approach may be happy with them, and dwarf trees in general. To come full circle here, I believe the colder climates stress trees, delaying their growth or occasionally stunting them, in the same way that forest species are stunted at high elevations. Adding a paltry root system to the mix can be frustrating. The “bow echo” winds (90mph) that rolled through here on July 4 flattened another half dozen(broken at the base), but no standard apples were damaged.  I should also mention every one of our standards (same age and scion cultivar in this trial) offered fruit before the dwarf trees did. There was no more cold or other damage to the grafted portion of the dwarf trees, mortality and damage always occurred to the stock itself.

We have had extremely good luck with standard seedling here including: dolgo, ranetka, antonovka, prunifolia, Selkirk, bud 118(clonal); fairly good luck with robusta, and just began with baccata. Many of the Alaskan growers we ship to will only take ranetka and baccata and I know of none who will touch a size reduction clonal stock.

All the Malling and many of the other dwarfing stocks are rated zone 5 so we have not bothered with them in zone 3.

A caution- nearly all temperate trees will experience damage when the root system is subjected to 19 degrees ABOVE zero. Additionally, the root collar/lower trunk is particularly vulnerable, which is why we are experimenting with clonal stock as an interstem, and high grafting scions

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Cultivars for the Far North

This list is evolving, but here is a list of cultivars that have proved well in zone 3 (some will do well in colder zones as well) :

antonovka selections

duchess of oldenburg

red astrachan

yellow transparent

heyer selections

tetovski

centennial crab

chestnut crab

dolgo crab

haralson

baccata selections

Selkirk selections

beacon

mantet

norland

parkland

more to come….

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Rootstocks for the Coldest Areas

With regard to apples there is a decent selection for most locales. The coldest spots, however will find it necessary to seek out the toughest of the lot.  Among them are Dolgo crab, Ranetka crab and Siberian crab.

Siberian, malus baccata, is likely the most cold hardy. It has had some criticism in Alaska as being a bit slow in growth (too much so for some folks) but may be oart if the reason for its high survival rate. There are reported issues with compatibility in some variety combinations. There is anecdotal evidence that cultivars with crab in their ancestry are less likely have rejection. ..

..more to come..

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Rootstocks in the north.

Growers in northern climates should be aware of the considerations of rootstock hardiness.


Although most folks make good decisions regarding the variety, one needs to pay attention to the rootstock as well. The aerial portion of your apple tree for instance, will suffer more disease, dieback, and the like. If the rootstock fails due to being out of its zone for hardiness, the tree will just drop dead.

Even the most cold hardy of rootstocks are really only cold hardy down to around 19 degrees F. Since soil will insulate somewhat, even bare soil in winter will afford it some protection. Geothermal heat additionally will leak to the area for further aid. The real helping hand when temps get into subzero zones is through snow cover. This is why areas like the northern plains of the US and the Canadian prairies are the real test areas in most years. When wind strips bare the soil and temperatures plummet to -40F and beyond you had better hope you were smart enough to choose the toughest roots of the lot. Now, damage doesn’t always mean death, and plants can sometimes recover from assaults like this, but they often don’t. Furthermore, damage can be hard to assess since it is, well, underground. Often the only indication is that our trees aren’t doing so well, which can sometimes be even more frustrating than having them get on with it and die. So, choose wisely. we are in the process of giving as much info on this site so you can make a good choice for yourself, and when there is choice we always graft to the hardiest we can so there is less you have to think about.