I thought I would add what I have learned. I have been grafting pines for four years. I have grafted though out the year and using a variety of techniques. First not all bonsai experts are expert grafters. I say that from experience. Grafting experts are usually in the propagation business.
For root stock two year old trees are often ideal. You should ideally match the Scion and rootstock diameter. If you don’t have a matching diameter you will line up the cambium on only one side. Usually the rootstock will be the same or wider than the scion. Generally for bonsai we want the graft union as low as possible. Preferably starting just above the first root and ending below the first root. To do this I usually taken the rootstock out of the pot and remove the top layer of soil to expose the first few roots. I usually clean the trunk using baby wipes soaked in alcohol or hydrogen peroxide. I usually prepare the scions first by removing some needles. I’ve had the best luck wrapping the scion loosely with stretched buddy tape, aka grafting tape or parafin wrap. It’s important to have the right stuff. Bonsai tonight sells it. It’s worth the price.
https://store.bonsaitonight.com/products/buddy-tape-for-grafting?variant=23297606275
Don’t wrap the Scion too tight. You want some air around the needles. It’s also important to start removing this wrapping as the Scion shows survival. The humidify which we depend on early to keep the Scion alive until it gets water from the rootstock can also kill the Scion from fungal growth.
When you make the cut try to end the cut in the rootstock in the center of the trunk. The cut in the rootstock should be slightly longer that the cut on the scion. You can secure the graft using half diameter buddy tape, wide cut rubber bands aka grafting rubbers or a combination of both. Grafting is best done during late dormancy. In California that is in February. It should be about a month before you see bud break on most plants or a bit earlier. Grafting later in the year using fresh candles can be done but I haven had much better success using the second growth from the previous year. The professional growers typically do not wrap the scion because they have a designated green house which is kept moist with controlled light. Some prefer to bring the rootstock into the growing season while keeping scion in dormancy if you have facilities to do that(greenhouses).
Get a good grafting knife if you don’t have one. You should also have a sharpening stone. The grafting knife needs to be really sharp. Personally I find most razor blades to be of poor quality unless you’re using Personna brand. If you don’t have a grafting knife, you can also use a straight edge razor, which you can buy used on eBay for cheap. Straight edge razors tend to be extremely sharp and work well for this kind of work.
Once you see evidence that the scion is surviving.
This will be evident if it’s still green at around six weeks. At this pint you want to slowly expose part of the scion to the elements. The newly grafted trees should be put in a sheltered position with about four hours of sunlight per day gradually increasing overtime.
If you have an early heat wave you need to protect those trees. As time goes on you want to expose more of the scion and slowly start taking foliage off the rootstock. Because the graft is placed lowest under the tree, auxins produced by growth above, will inhibit the growth of the scion, which is trying to stay alive. The goal is to slowly wean the scion off the rootstock tree by removing upper foliage and slowly unwrapping the buddy tape. I typically do this from six weeks and through the summer. Typically the buddy tape is removed when I have seen the scion push. Some rootstock foliage can be left on until the next year.
The reason for previous failures have IMO been due to fungal infection from leaving the scion covered too long in wet conditions, not protecting from heat and wind and probably due to technical error. It is not a static process that is done and forgotten. Some growers take this approach leaving the grafting tape on until the scion pushes through it. That may work in some conditions. I live in San Francisco where it’s often wet and windy during the summer. If it’s really dry where you live then leaving the Scion protected may be better. There are some subjective variables like how long the cut is and how hard you wrap the tape. It’s technically much easier to graft a little higher on the trunk but for bonsai the goal should be to have as little rootstock bark as possible on the final tree. I have also experimented with ways of managing the grafting union over time. Once the union is solid typically the following year I trim off as much of the remaining rootstock bark at the graft junction as possible. Exposed cambium will heal over, preferably with tissue coming from the scion wood. If the graft union starts swelling I typically wrap it with aluminum or copper wire. Even if the wire bites in to the tree it will look better than a bad graft union. The scar tissue and eventual new bark will look better than an obvious graft union. I can’t find my grafting pictures otherwise I would post them. Learning to graft takes experience. For that reason I graft many more threes than I need. I want to master the skill of tree grafting so I practice a lot. You said you will be grafting JBP onto ponderosa pine. I think most people would just grow or buy JBP and avoid the graft union. Like I said I’m a big fan of grafting for the sake of grafting simply to learn. I also graft fruit trees and whatever I can learn from. Typically I graft pine varieties that can not be grown from seed like ‘kotobuki’ or cork bark pines. I find very few of the commercial growers graft well for bonsai purposes. It’s much easier to graft higher on the trunk but this is typically acceptable for bonsai. I have bought well grafted tees from Western Evergreen nursery in Oregon. Stuff from Conifer kingdom has not been grafted well for bonsai. Once I have the genetics I can do quality bonsai grafting at or below the root level on my own. I currently have one pine that has five cultivars grafted on it. If I ever get access to a branch from an unusual pine I stick it on that tree to have for the future. Thus way we can store genetics and have access when we may need it in the future. The bonsai community seems pretty anti-grafting for many trees because the graft union looks unacceptable. To me it’s just a matter of doing quality grafting which I hope to some day master. Personally I like the many variegated pine trees but we rarely see these in the bonsai community because they require grafting. To graft juniper foliage or branches on a mature pine tree approach grafting is easier and generally has a better success rate. In regard to compatibility I think you can graft any pine to any pine, It is possible bit graft juniper to a Cypress but generally its best to graft to the same genus. If you do everything right you may still have graft failure. It’s good to repeat the process. If you have failure. Always ask yourself why it did not work. Sometimes we know sometimes we don’t.
Have fun!!!
Sorry about the length of this response
