This is a topic of great interest to me, and surprisingly has not been clearly studied or documented. Anecdotal accounts of graft success are out there, often with little follow up or details of the varieties, types of grafts, climate zone, winter protection, graft aftercare and support, yield, longevity. Why do it? The potential to use these wild rootstocks for cultivated varieties of fruit should not be underestimated - where they grow they are prolific and extremely cold hardy.
Location
Our aspen forest where we live and have the nursery is in southern Manitoba (USDA zone 3b) on 52 acres of native forest, full of Aspen (Populus tremuloides) and Bur Oak (Quercus macrocarpa) with understory that's nothing but wild Choke Cherry (Prunus virginiana), Saskatoon/Service Berry (Amelanchier alnifolia) and Beaked Hazelnut (Corylus cornuta) - in about equal proportions. On a good year you can wander in any direction and pick a 5 gallon pail of Choke Cherries, and on a great year with a bit more time just as many Saskatoons. While our area on the Canadian prairies has mostly been cultivated into farmland, if you travel to the north there are vast expanses of forest with these species. Last winter our coldest day was -41C, and our first frost was mid September. We had a decent amount of snow, while not record breaking there was 6-12" of snow cover through all of the coldest weather.
Orchard History
Our orchard coming into it's third year and as it's expanded into about an acre area it's been carefully cut into the forest in an area with a sparse upper canopy so that plenty of sunlight will reach the trees. Each group of trees is nestled between Choke Cherry and Saskatoon, they've been thinned back minimally to allow for walking paths and room for the trees and diverse list of other perennials to grow. So while grafting this spring and looking at a bag of left over pear and plum scions, I found myself looking around in the orchard at all of the Prunus and Amelanchier and wondering if this could work. One can only eat so many Choke Cherries, although the wild variety we have here is quite large (8-10mm) and in the fall when the berries turn black they're sweet enough to eat by the handful. Besides the whole point of the orchard to explore what's possible for biodiversity in a zone 3 food forest, everything is an experiment - all of the trees are multi grafted this year with as much variety as can be fit on a frame while upholding good form and aesthetics.
Experimental Setup
In several areas of the orchard in varying conditions groups of pears were grafted onto Saskatoon bushes, followed by plums onto Choke Cherry. 6-8 areas were selected for varying amount of shelter and sun exposure. 50-75 grafts were performed with scions that were fully dormant and grafted out between May 15-22 - and here that's about 2-3 weeks after the trees came out of dormancy. The Saskatoons were in flower and just beginning to flush new growth, while the Choke Cherries were just prior to flowering. The weather has been cool ranging from 20C to 2C, with a chance of mild frost the last week of May and one or two nights forecast at -1C - 1C. All grafts were whip and tongue, with care taken to match scion and rootstock diameters and create 1-1.5" sloping cuts, and only use viable scion wood with green cambium. Grafts were performed with a sharp grafting knife and sanitized between trees, and wrapped with Parafilm. Care was taken to wrap the grafts tightly and seal the scion tips, often wrapping the scions completely in Parafilm. Plastic tree tags were used with a UV resistant garden marker to note variety, the rootstock variety, grafting date, the source of the scion wood, and where ambiguous to note the position of the graft. There's some debate over the longevity of the plastic tags, and so after the grafts have taken and produced some growth in the fall or following spring they may be replaced with more permanent metal tags, like aluminum with embossed lettering and and a wire to hang it.
Sun Exposure and Wind Shelter
Some grafts are in full sun, some in partial sun, some at the edge of a bluff which will be in full sun from the east and some from the west. Some are nestled in the bush and will receive dappled sun through most of the day with wind protection, some were planted on a slope.
Rootstock Selection and Graft Locations
Healthy and small trees were selected, and preference given to any branches or growth that appeared most vigorous by considering the length of the previous season's growth, identified by a the different colored and smoother bark. In most cases 12" of new growth was about the maximum found on many trees. The lowest grafts were done 12-18" from the ground, where the top of a small tree was replaced while keeping all the foliage below intact. The highest grafts were 4-6' up. Most grafts were done at apical growing tips, where the strongest growth would be expected.
Varieties trialed
In total about 10 varieties each of plums and pears were grafted out. As a control each of these varieties were also grafted around the orchard to pear and plum trees, where the goal this season was to framework single varieties into multi grafted trees, many 6' tall and going into their second leaf. Most of the varieties tested were considered hardy to USDA zone 2/3. A mix of European and Asian plums, cherry plums, cold hardy Canadian pears as well as Russian pears were used. A full list of the varieties will be posted along with follow up data detailing graft success and growth. Whether or not we have any success this time around, more varieties will surely be trialed next year.
Winter Protection
This is where we could try some different things out. For the most part, these are established trees that are fully hardy - 100% survival of the rootstock is all but guaranteed. If the grafts fail, these trees will be absolutely fine and recover without missing a beat. Where there is some consideration is the cold hardiness of the graft union, where in anecdotal reports of graft success with a flush or growth there are some reports of grafts failing to survive the winter. One explanation could be that the less than perfect compatibility of the graft union is more tender and could require some protection over the winter. If so, once the graft has grown for several seasons and developed a better union and thicker back it may not be necessary to provide as much protection. We see this concept with winter sun scald, for young trees 2-3 years old their bark is thinner and they are much more susceptible to sun scald. Older trees with thicker back have more resilience. Indeed we have 17 year old Malus baccata - which use as one seed source for our rootstocks - at our old farm property nearby that have never received any winter protection or tree guards and are completely healthy.
One way to protect the grafts over the winter would be to bend the supple trees over and steak them so that the graft is below the snow line. A heavy application of wood chip mulch could bring up the ground level 6-8" to help. In an extreme case where the scion is a tender variety, the tree could be pegged down and cover with wood chips or burlap for the winter. This method of protecting trees is common with Russian training methods, such as the Melon or Arctic Stanza where the tree is grown very low (8-12" from the ground), allowed to flush new vertical growth during the warm summer months, then pegged down and protected for the winter. We have about a dozen trees undergoing this training method around the orchard, it's both fun, creates creative and aesthetic feature trees, and might allow us to zone push many varieties. Similar to step over espalier training, a tree can be trained to grow in unconventional shapes and kept pruned small to contour along a path or against a backdrop such as a fence or wall. A combination winter protection ranging on a spectrum from neglect (just leave it alone) to pinning the trees down (the small ones are so flexible you could easily do this without harm) and covering them in wood chip and burlap for the winter will be trialed. Another possibility at the extreme end of this spectrum would be to plant the tree in a trench or shallow hole, or to mount of the earth around it so that it could be held down - or trained to grow low - and then just covered by planks for the winter. The method has been successfully done in Russia to grow citrus in climate zones approaching as cold as ours, at an extreme the trees are grown 3-6' below ground in something akin to a Walipini greenhouse.
Graft Support and Follow up
One cause of graft failure reported is structural where a branch failed at the graft union. It's possible these unions will need some support, ranging from temporary to permanent staking or trellising. As the season progresses and once some growth has come one, some grafts showing vigorous growth will be tied to a supporting bamboo steak to support it through high winds and snow. If these fair better we'll know that support is needed, at least while the tree is young. Indeed a permanent support would be fine if any of the trees end up being trained in espalier fashion, whether that means some staking, trellis. Another consideration will be pruning, we like to keep our fruit within reach for picking and generally are training for delayed open center and modified central leader trees. New growth will likely be pruned back by half or more each season. This also provides a completely secondary reason to do this - an improved supply of scion wood. Left over scions at the end of the grafting season? Keep them for next year by grafting them out, then collect the new growth the following winter. For the home orchardist, this purpose alone could be of interest even if the resulting grafts never survive enough seasons to produce fruit.
Apples on Saskatoons?
Apples on pears. Your intuition might be that this is a newby mistake: conventional wisdom is that you can't graft these together. Yet for specific cultivars it's well documented that pears can in fact be grafted onto apple trees, and apples onto pears - and grafts can be long lived. Some reports were found on grafts surviving for over a decade. Yet here I've come across the same issue, the details might be a passing comment on message board, cultivars are usually unknown, and typical the example found on youtube is one poorly placed branch haphazardly grafted on an overgrown tree. How many cultivars does this work for? Why do it? The mostly likely explanation for the lack of rigorous study is the same: there just isn't a commercial application. In fact if you're bench grafting, it would only make sense to just get the right rootstock. But what if this compatibility means that apples could graft to Saskatoons?
Given accounts that Malus domestica (var Winter Banana) has graft compatibility to Pyrus communis, and Pyrus communis has graft compatibility with Saskatoons, is it reasonable to hypothesize that Winter Banana could also be directly compatible with our wild Saskatoons? In fact at minimum we strongly suspect that it could be done if pear were used as an interstem.
Winter banana (zone 4-5), an apple cultivar developed around 1890 is known to have this rare property of compatibility when grafted to pear. Is it the only one? Very likely there are a range of apples that have some amount of graft compatibility with some pears, but formal studies on the topic and their resulting charts with all the details seem to be missing from our fruit growing knowledge base. Palmetta (zone 2), an apple crab developed in Novosibisk, Siberia also has pear compatibility and is more suited to our climate. To round out the list, Northern Spy (zone 4) has some reports of pear compatibility.
This grafting season we happened to have a handful of Northern Spy scions left over, and so - just to see if this works as a long shot - these have also been grafted out onto Saskatoons around the orchard in a similar fashion to the pears. About a dozen grafts were done with large scions (6-8") in different locations onto medium and large rootstocks. If they take, since this variety is borderline hardy in zone 3b some additional winter care will be taken. Next year we could continue the experiment with Palmetta, and try it in combination with a pear interstem using the pear varieties that perform well on Saskatoon this year.
Results?
To be determined. As of this initial draft we're in late May and the orchard is coming into full bloom. The real value in this experiment is the follow up. Expectations are modest, we've heard that this both works but could suffer graft failure possibly after one or more growing season. The purpose of documenting some of the variable to are determine the cause if and when failures occur, and to find what works better and build on the results in an iterative fashion. If you have any results you would like to share with us, or if you just want to let us know what you think of this experiment please let us know. Feel free to send us an email: doug@oaksummitnursery.ca
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- Doug McGregor
(Owner) Oak Summit Nursery
More to come!
Russian pear on wild Saskatoon
Delecate Plum on wild Choke Cherry
