Summary - After some initial successes and lessons learned, I started with new methods for the 2022 grafting season in May and tried about a dozen pear varieties and twice as many Prunus, all cold hardy to zone 3 cultivars. At sites around our property trees were grafted over to several varieties, healthy trees were selected with 2-3' of new growth last year and in sunny locations. By the end of season we found just one or two Prunus that did outstanding and grew 3-4' of branched growth when grafted onto wild Choke Cherry trees and survived until dormancy. Pears and apples had better overall survival at 90% across every variety grafted onto Saskatoon, and most pears grew 2-3', and apples about 50% as much. We found our success depended more on the methods used, timing and followup than it did the specific compatibility across cultivars. This was a very interesting result and we have ideas for new methods to try next year. Winter survival is going to be the real test, and after two or three seasons will we have these grafted wild trees hanging with new fruit.
Field Grafting Technique - Whip and tongue or modified cleft grafts were used on small branches, and bark grafts on larger ones. Bark grafts are the fastest and easiest. Modified cleft grafts did well, which is a cleft graft done very off center with just a flap of bark behind the scion. They healed better than bark grafts. All grafts were initially secured with Parafilm in one layer over the union and then lightly over the entire scion to the tip, and then tied with white 1/2" budding tape, which is a slightly stretchable and non-sticky - similar to flagging tape. One benefit budding tape is you can leave a long tail to label the variety, and come back later with a more permanent tag. No other waxes or paints were used, no tin foil or bags were used to shade the scions. Painting the length of the scion would be faster than Parafilm, it's a consideration for next season.
Pear and Apple on Saskatoon - In mid May I started with pear on Saskatoon just as they were starting to leaf out. Saskatoon and Choke Cherry trees look very similar when they're dormant, it takes some practice to distinguish them. We had about 10 varieties of pears to test. In same groups apples were as grafted to compare, if a tree had a good wye at waist height I would use pear and apple side by side.
July Graft Maintenance - Through May and June a constant need to remove growth below the graft was obvious. The larger Choke Cherry trees that were top worked wanted to regrow below the graft vigorously, sometimes I wouldn’t catch new shoots until they were a foot long. Epicormic growth was intense - growth from hidden buds all along the trunk would sprout dozens of new little branches. If they were left to grow, the graft grow was noticeable affected. This presents one reason to avoid grafting at shoulder or head height, when you do it’s less reasonable to remove all the lower growth. The best growing grafts were done at waist height at the beginning of the season and all of the undergrowth was removed on a regular basis.
Labeling and support - The budding tape labels fade within a couple months even with UV resistant garden marker. After experimenting with a few types of labels, I ended up using a thermal transfer printer and vinyl tree tags that come on a roll. For one off labels, I used a Sharpie paint pen. Both of these leave a layer on the vinyl label that doesn't fade. These permanent labels were added about six weeks after the initial graft. By then we were also see some girdling under the budding tape on the Choke Cherry - the plum grafts grew faster than anything else and the unions would tend to swell very quickly. I found the grafts needed to be unwrapped at 5-6 weeks, and then rewrapped for support. By July many had grown 3' with secondary branching and were like a sail in the wind. They require support in the form of a stake, a splint, or by tying on to nearby branches to avoid breakage on windy days. Some plum grafts actually broke above the union during a storm. Apple and pear didn't grow this fast, and the wood is harder. None of the pear or apple grafts needed support and overall grew 12-36" this season.
Plum Delayed Graft Failure - of all the Prunus varieties we tested, including many plums (but also a few other related species like Nanking Cherry and Chums) everything initially took and grew. The take rate was over 90% in June, kind of incredible. By July at the onset of the girdling and support issues after this flush of healthy growth, there would be a sudden onset of wilting followed by decline over the week. This presented like sudden mechanical damage to the union, but not in every case. This period of graft rejection occurred within a 2-3 week window. It's possible that more preemptive attention to girdling and support would have saved many of them. My expectation going in was incompatible grafts would either completely reject and show no growth from the outset, just like a poor graft that didn't take, and not for vigorous healthy growth for two months followed by sudden death. This makes me want to try it again with new methods. For example, we had a deer nibble back most of the new growth on a few of the grafts in June, and they survived this summery pruning and regrew - and that group also survived while nearby unpruned grafts of the same varieties failed . My hypothesis is that most of this failure was from small cracks - unseen mechanical breakage at the new union during flexations in high wind. Imagine a windy day and the sail of a branch flexes thousands of times. Another possibility is it was caused by heat and water stress and the union when it was unable to support the large mass of new growth with high moisture demands. This period of rejection coincided with the onset of hot dry weather and the beginning of the wood to transition from soft to semi hard. Maybe this summer pruning trick is worth trialing, and reducing the new growth alleviates both of these stresses in the first season. Across all of the varieties tested, a small percentage of every one did made it through until fall. It's completely possible that genetically distinct Choke Cherry are more or less compatible with some varieties and themselves could serve as an interstem.
Best Plum Varieties - by mid July it was apparent that two varieties trialed were outgrowing the rest, and had suffered minimal graft failure. Toka (A hybrid of Prunus americana and Prunus salicina, American and Japanese plum) and an unnamed scion of Prunus americana. Toka also did well in our 2021 tests, and the few scions we grafted are still alive at 18 months. Toka had the best growth rate, by July many grafts were 3-4' tall with branching: they’re now as large as our 3-4 year old plums trees. If these survive the winter, Toka is our best candidate for an interstem. Second best was Prunus americana, which nearly grew as well as but had about 25% of the grafts fail - keep in mind this could be from mechanical breakage. Third best was Pitsin 5, where about 50% of the grafts survived through the first season. Since the P. americana had good success, it may be worth trying scions from more seedlings and if the is a range of compatibility, naming a selection that can be used as an interstem. On balance Toka stood out with the most growth, in many cases double the growth of P. americana, but we also grafted more of it on to the best trees.
Budwood from New Plum Grafts - one immediate benefit was the surplus of plum budwood available in July. We collected bud sticks from all of the grafted varieties growing out on Choke Cherry and used them to graft our cherry plum rootstocks next season.
Apples and Pears on Saskatoon - There's a couple of apple varieties reported to be compatible with pear include Winter Banana and Palmetta. It was reasonable to think that since pear is compatible with Saskatoon, and apples with some pears, that we should try apple on Saskatoon. Last year we tested a group of about a dozen late in the season with left over scion wood and got a high take rate, and happily 90% of those apple grafts survived the winter and put on healthy growth this next season. For 2022 we grafted two dozen varieties of hardy apples, again from left over scion wood - onto groups of Saskatoon, and the take rate was 90%. Some grafts grew 24". Nearby we also did Pear on Saskatoon grafts and generally they had the same take rate - but pear usually put on about 50% more. Maybe this is from better compatibility, and maybe we also have a cold hardy dwarfing rootstock for apples. It's worth noting that all of the dwarfing rootstocks available for apple have weak root systems and are not cold hardy to zone 3. Saskatoon also doesn't get girdled by rodents. There's some benefits that are worth pursuing.
2023 - Surviving the winter is a key benchmark, if the grafts survive then maybe we're onto something. So far we haven't found a pear or apple variety that doesn't grow on Saskatoon, but it will be interesting to follow which ones perform the best, how well they fruit, will they require support, and most important how is their long term health. For plums we have two varieties that did outstanding, so if they survive the winter we'll want to continue more testing including more varieties, and try intentional summer pruning and proactive support, rewrapping. Also up for testing are double worked grafts using Toka and P. americana as interstems. Another thing to trial is budding, so did some T-Bud grafts this fall and we'll be interested to see how they grow. We've also yet to see if any variables we adjusted play into winter survival. It's been suggested that intergeneric grafts may suffer from a cold sensitive union that may require some protection by wrapping with cotton or some other insulation. We'll try that on some in case it has a positive effect.
Recap - What's the Wild Rootstock Grafting Experiment and why does it matter to us? In 2021 I started a grafting experiment around our property to answer the question: could I grow all of the cultivars of pears, plums and apples on the wild trees around our nusery as rootstocks. Could I walk around with a scions and grafting knife in the spring and create a food forest. Garden centers want to sell you a 6' tree in a pot that was grown in a different climate half way across the continent with a dwarfing rootstock. The tree is usually grown in a sterile peat based potting mix, non-fungal dominated, pumped full of nitrogen after being field dug with a tractor that left half of it's root system in the ground. These potted trees have done poorly around our yard, it's the type of tree we planted when we started our orchard. What if instead we could grow our fruit on the trees around the forest that are already established, native to our area and grown from seed here. They have well developed root system and they thrive in our soil. They're bullet proof to our -40 winters. You never need to water it or mulch it. The voles will never eat the bark. Wild Choke Cherry and Saskatoon trees are common around our area, and they're both in the rose family (Rosacea) along with apples, pears, plums and roses. Top working or grafting over an existing established tree produces fast and precocious grow (early fruit bearing). The only question to be answered is one of graft compatibility. Maybe more importantly the question is: How healthy is the graft after a few years? It takes a few years for grafted branches to grow enough to produce some fruit. The common gospel is that you must only graft fruit in the same genus together, apples to apples. Apples to pears? Well obviously that doesn't work right? What's been fascinating to explore is that there may be far more graft compatibility in the Rosacea family than commonly assumed. Consider that graft compatibility - which I'll define as how perfect the union is between two joined pieces of plant material - is on a spectrum. If the presentation of some small incompatibility is comparable to the effect of grafting onto a dwarfing rootstock like a slower growth rate, smaller maximum size or a shorter lifespan then this is already a commonly accepted trade off. In fact there are studies which found that this could result in earlier and heavier bearing, and small precocious trees are in demand. Are there cultivars that are compatible enough for it to work, in any combination? If you found even one plum that grafted well onto Chokecherry, then it could be used as an interstem for other varieties. And the same for pears and apples onto Saskatoon.
Climate and rootstock selection - on the Canadian prairies in southern Manitoba we experience a climate with some extremes. From June to August we often get days up to 35C (95F), while from December to February we will have as many night that reach -35C (-31F). Our frost free growing season is short at only 100 days or so - the seasons are intense here. The short gardening season requires some innovation, starting many vegetable indoors ahead of time, and our cold and frosty spring and fall makes you appreciate the hardy perennials that can take it - they help extend our gardening season. Your apples can stay on the tree and take a light frost, but don't try this with your tomatoes. It's incredibly important in this extreme climate to grow zone 2 and 3 hardy fruit trees, and in fact geographically this is a very large area of Canada including everything in the middle of the country. Garden centers in our area love to offer trees only rated for -25 or -30C. Cities are heat island which are typically a few degrees warming than the surrounding countryside, and one of these out of zone trees might survive there, but often they'll succumb to winter injury after a year and two. To have the best success here you need trees that can shrug off our winter extremes and have vigorous growth. This limits fruit tree rootstocks for us, and many of the most common options are less than ideal. Dwarfing rootstocks have weaker root systems, require ongoing staking and watering, and that root system reduces the tree's winter hardiness and ability to bounce back from injury. Dwarf trees are also shorter lived. They're great for commercial orchards, but for a yard tree here in very cold climates it's recommended to use the most hardy full sized rootstock you can find. In our nusery when we graft fruit trees we use Malus bacatta (Siberian crabapple) for apples, Pyrus ussuriensis (Ussurian or Harbin pear) for pears, Prunus americana or cherry plums seedlings for plums and apricots. These are the most cold hardy, most compatible rootstocks for fruit trees that we can grow. They're commonly used by many nurseries in our climate zone. So these options present a standard we can compare against when attempting to graft onto something new.
Lessons learned from 2021 trial grafts - with our initial grafts there was a tendency to be minimally destructive to the trees. If this wasn't going to work, I didn't want to indiscriminately murder every tree I experimented on. What I learned was that this conservative approach - grafting just one branch of a tree - doesn't work well, and given common instruction for top working or frame working maybe this should have been predicable. The grafts that performed the best done on vigorous growing trees in a sunny location and which were completely cut back and grafted over (topworked). The undergrowth needs to be completely removed for most of the growing season, and would benefit from debudding early on. Also grafts should be tied tight and with good knife technique. Bark grafts and modified clefts are more forgiving. In 2022 I switched to budding tape over top of Parafilm with improved results, and every tree was top worked.
Most studies are thinking about commercial industry - Most of the fruit production in the world is done in commercial orchards. When you find a rootstock study, the outcomes they care about are generally the goals of a commercial orchard, they're not for the hobbyist or home owner. For most of history a large commercial orchard grew full sized seedling trees spaced far apart. In recent history, trees are grown on carefully bred clonal rootstocks with dwarfing properties because it's more profitable to grow many small trees spaced close together at high density. When researching this topic it's surprising to find studies of Pear or Apple on Saskatoon that were done years ago and had good results, but for one reason or another - usually funding they didn't continue. If grafting onto wild trees in cool combination is possible, but not very easy to capitalize on, there's often no one funding the study. As a matter of curiosity, and because this seems of value to the hobbyist it seems to be up to the community to try new things out and share what works.
Combination thats might be possible - Pears are often grafted onto Quince trees. But pear is also somewhat compatible with Saskatoon, Cottoneaster, Mountain Ash, Ariona, some apples, and maybe Roses. Pear is known to be long lived on Cottoneaster, which is a rootstock combination popularized in Russia. Many pears are compatible enough they will survive for years on Saskatoon. How long lived is enough to make this worth while? Consider that peaches and nectarines often only live for 12 years, and apples on Bud 9 rootstocks may only live for 20 years. In addition to that lifespan consider renewal pruning, scaffolds may or live for the life of the tree (but often don't) and fruiting wood can be renewed after just a few years to maintain healthy fruit production. Given that, how old does a pear branch need to be? If a graft can survive and fruit for a few years, and a branch at some point needs some heavy pruning and renewal, is that good enough? The Rosaceae family (the Rose family) contains thousands of species in over ninety genera, and then thousands genetically distinct cultivars including hybrid species. There are enough variables to consider when grafting different Rosacea together, that in the set of all possible combinations - good data about what's could actually work doesn't seem to be available. The limits of what's possible when you start exploring this are fascinating. Sometimes scans of old publications and studies can have incredible details, and encapsulate a real bygone enthusiasm for discovery. An old Harvard study actually tested grafting trees together upside down - yes roots in the air.