NOTES ON SUMMER GRAFTING
The previous articles on grafting Japanese maples have covered operations carried out whilst understocks are dormant. The associated costs in providing warmth during the winter, if not already available in the form of a heated greenhouse etc, can be a disadvantage if you are grafting only a few plants. Summer grafting, by contrast, requires only that the resultant grafts be kept relatively cool.
The technique is identical to that illustrated in the slideshow as a side veneer graft is probably the easiest to produce. Collection of scion wood can begin, depending on your growing season, as early as the beginning of July and last until mid August. Diverging from Harold Johnston's technicue, all leaves are removed from the scions prior to grafting and no plastic bags are used to cover the graft. In addition, the understock is not cut back but left to grow on. The plant is then placed in a cool, shady position whilst the graft calluses to avoid drying out. A daytime temperature of about 65 deg. F is recommended. Once callusing of the graft is observed (typically within two to three weeks), plants can then be exposed to more light and natural heat.
The reason for the above is to make sure the scion does not come into growth before the plant goes dormant. The growing season for the scion is overwise going to be short and new shoots may not have time to fully ripen before going dormant. By not cutting the understock back the plant is allowed to continue building up strength in it's root system. Cutting back during growth can be a shock to the plant, particularly for the small sizes used in grafting. The success or failure of a graft is almost entirely dependant on the quality of the root system and where that cannot be guaranteed it's best to err on the side of caution.
The understock is eventually cut back as close as possible above the graft once the plant is fully dormant and the resultant growth the following spring will be more vigorous than if the understock was trimmed back whilst still in growth.
One aspect of grafting that is often mentioned in articles on techniques is the need to 'condition' the understock. This requires that the understock is partially dried out to prevent the union from being 'flooded' by excessive sap flow. The whole point of sap flow is to ensure the cells of the cambium layer remain alive so that they can callus and any excessive amount of sap will drip away. A problem is only going to occur if the graft is completely sealed and the pressure from a build-up of sap dislodges the mating surfaces. As can be seen from the tying in the slideshow you will be unable to produce a 100% seal and neither is it recommended.
There is one other additional method of summer grafting Japanese maples that can be employed and this is now illustrated by a slideshow. Although greater accuracy than that needed for side veneer grafting is required, it does have the advantage of groducing a far neater graft.