Detailed Explanation of Key Techniques for Olive Top-Grafting to Change Varieties

    According to surveys of flowering and fruiting conditions in several contiguous olive orchards, among plants grown from olive seedlings, about 10% are male non-fruiting trees and a small number are low-yielding trees with sparse fruit. 

Olive as a fruit tree crop has two types: the olive (also called white olive, mainly produced in Fujian) and the black olive (mainly produced in Guangdong). Because progeny from olive seedling propagation are prone to variation and tend to produce smaller fruit, rare and superior olive varieties all require grafting (or top-grafting to change varieties) to maintain their genetic stability. 

Olive xylem tissue is relatively loose, and the sap contains a high level of tannins. When wounds are exposed to air, the tannins in the sap readily oxidize and coagulate, affecting graft healing. Therefore, the survival rate of olive grafting (or top-grafting) is relatively low. Over several years, through several different renovation approaches such as grafting seedlings before field planting, top-grafting young trees (2-3 years old), and top-grafting to change varieties on large trees over 5 years old, and based on observation and analysis of the results, we believe that top-grafting to change varieties on large trees is relatively feasible. 

　　1. Rootstock Selection and Treatment For top-grafting to change varieties, plants are mostly selected from inferior, low-yielding, non-fruiting trees, generally over 5 years old, with a trunk diameter of about 0.1 m or more at 2 m above ground, and with relatively well-developed root systems. When top-grafting, first saw off the upright trunk or main branch of the rootstock at 1.5-2 m above ground or at a height of about 0.5 m above the branching point, and retain at least 1-2 auxiliary nurturing branches to ensure a balanced supply of nutrients and water between the underground and aboveground parts of the tree. 

2. Top-Grafting Period It is best to choose sunny days from mid-April to mid-May for top-grafting. 
 
　　3. Top-Grafting Method Use the inlay grafting method. 

(1) Scion Selection and Treatment Select vigorous 2-year-old or older branches from the middle and upper part of the canopy, about the thickness of an index finger to a middle finger (diameter about 2-3 cm). Remove the leaves and the tender upper shoot portion, and immediately wrap with a clean damp cloth to retain moisture. The scions are best used immediately after collection or on the same day. 

When cutting the scion, hold the upper part of the scion branch firmly with one hand, and on both sides of the base of the bud-bearing branch section, cut horseshoe-shaped slanted faces about 5-8 cm long, so that the cuts on both sides form a wedge or slanted triangular prism shape. Note that the cuts should match the rootstock cuts as closely as possible. The upper end of the scion cut should be at least 1 cm from the retained bud, then cut off the scion 2 cm above the uppermost retained bud point, and smooth the cut with a sharp knife. The scion is about 13-18 cm long, with 2-3 buds retained. 

(2) Rootstock Cut Treatment For the rootstock after sawing the trunk, the saw surface should be level or slightly inclined outward. First, using a hand saw, saw 3-4 wedge-shaped or slanted triangular prism-shaped cuts at a distance smaller than the thickness of the two side cuts of the scion; then smooth the cuts with a sharp knife, and the prepared scion can be quickly inserted, so that the outer cambium layers align and fit tightly. Then apply grafting wax and tie with rope to secure. 

When cutting the scion and rootstock cuts, the knife must be sharp, and the movements must be quick and accurate. The cuts must not be contaminated with dirt (such as finger sweat, dust, etc.), and prolonged exposure to air should be avoided, so as to prevent oxidation of the tannins in the sap from affecting healing. 

(3) Fixation and Protection After Top-Grafting After top-grafting, grafting wax should be applied to the rootstock-scion union and the top cut surface of the scion to retain moisture, taking care not to disturb or displace them. Use damp straw rope to wrap from below the rootstock cut up to the saw surface to protect the scion. Then put on a cylindrical plastic film (slightly larger in diameter than the rootstock), tie the lower end tightly below the straw rope, fill it with clean fine red core soil of moderate moisture, cover the scion completely, leave about 20 cm of space at the upper end of the film, and tie the top tightly to retain moisture. Finally, wrap a layer of straw or reeds above and below the graft union to shade light and lower temperature. Note that the straw-wrapped part of the trunk should be at least 1 m above ground, so as to avoid 0.2 cm, to reduce the sap flow at the rootstock-scion union.  

    4. Management After Top-Grafting　

    Management after olive top-grafting is crucial to the success or failure of top-grafting. Management work mainly focuses on moisture retention and shading, timely film breaking, and rational use of auxiliary nurturing branches, etc. 

(1) Moisture Retention and Shading Because shortly after top-grafting the hot summer and autumn seasons begin, in order to prevent dehydration and drying of the scion caused by high temperatures, strong sunlight, and excessive ventilation, attention must be paid in the early stage to shading with the outer grass covering and sealing with film to retain moisture, promoting scion healing and sprouting. 

(2) Timely Film Breaking About 30-50 days after top-grafting, some healed scions may sprout. At this time, do not rush to break the film; let them grow inside the film. When the shoot is about 10 cm long, poke a small hole at the top of the film, just enough for a slight air flow. After that, let the new shoots grow and even bend inside the film until the new shoots are nearly mature, then poke open the upper end of the film and let them resume upward growth. The outer grass covering still needs to be wrapped around for protection. During overwintering, the new shoots sprouted from the scion must be tightly covered with straw or reeds to prevent low-temperature frost damage.