Grafting Avocado and Budding Citrus

1Key terms

• Rootstock (stock): the lower part that provides the roots, usually a seedling raised from seed.
• Scion: the shoot or bud of the desired variety, which becomes the whole top of the tree.
• Graft union: the point where scion and rootstock join.
• Cambium: the thin layer of dividing cells just under the bark. The cambium of scion and stock must touch, because new tissue grows from it to join the two and connect their water and food vessels.
• Budwood: a stick of the desired variety carrying several buds, used as a source of buds for budding.
• Mother tree: a healthy, productive tree of a known variety from which scions or budwood are taken.

2Conditions for a successful graft

1. Compatibility: scion and stock must be closely related (avocado on avocado, orange on a citrus stock).
2. Cambium contact: the cambium layers must meet and be pressed firmly together.
3. Clean, sharp tools: smooth cuts heal fast; clean tools do not carry disease.
4. Correct stage: the stock should be actively growing, and the scion healthy, pencil-thick and with firm buds.
5. Protection from drying: wrap the union tightly and keep the scion moist until it unites.
6. Aftercare: shade, watering, and removing shoots that grow from the rootstock below the union.

3Cleft (top-wedge) grafting of avocado

1. Raise rootstocks from fresh seeds of a vigorous local avocado, in polythene pots filled with a mixture of topsoil and well-rotted manure.
2. When the seedling stem is about pencil-thick, cut off the top with a clean cut, leaving about 20–30 cm of stem with some leaves below.
3. Split the top of the stem down the middle to form a cleft about 3–4 cm deep.
4. Prepare the scion: a shoot about the same thickness as the stock, from a mother tree of the chosen variety (for example Hass), with two or three buds. Remove its leaves and cut the base into a smooth wedge about 3–4 cm long.
5. Insert the wedge into the cleft so the cambium of the scion lines up with the cambium of the stock on at least one side.
6. Tie the union firmly with grafting tape from bottom to top, and cover the scion with a small clear plastic bag to keep it moist.
7. Care: keep in shade and water regularly; remove any shoots from the rootstock; remove the bag once the scion buds burst and grow (usually after a few weeks), and loosen the tape before it cuts into the stem.
8. Transplant when the scion has grown strongly, typically several months after grafting.

4T-budding (shield budding) of citrus

1. Raise rootstocks from seed of a hardy citrus such as rough lemon. Seed for rootstocks should come from healthy trees.
2. Collect budwood from a certified, disease-free mother tree of the chosen orange, mandarin or lemon variety.
3. When the rootstock is pencil-thick, make a T-shaped cut through the bark about 20–30 cm above the soil: a horizontal cut about 1 cm long, then a vertical cut about 2–3 cm long below it.
4. Slice out a bud with a shield-shaped piece of bark (and a thin sliver of wood) about 2–3 cm long.
5. Lift the bark flaps of the T and slide the shield in, bud pointing upwards, until it fits under the bark.
6. Wrap tightly with budding tape, leaving the bud itself uncovered or only lightly covered.
7. Check after about 2–3 weeks: a green, fresh bud shield has taken; a brown, shrivelled one has failed.
8. Force the bud: cut back or bend over the rootstock top above the bud so the bud grows. Remove all rootstock sprouts, then stake the new shoot.

Worked example: planning the nursery

Question: Uwase grafted 80 avocado seedlings and 68 took. (a) What is her success rate? (b) She has a contract to supply 204 grafted trees to plant one hectare. At the same success rate, how many grafts must she make?

(a) Success rate = successful grafts ÷ total grafts × 100 = 68 ÷ 80 × 100 = 85%.

(b) Grafts needed × 0.85 = 204, so grafts needed = 204 ÷ 0.85 = 240 grafts. She must raise at least 240 rootstocks.

Practice