Propagation and Pollination of Kiwi Plants

Kiwifruit has gained significant popularity over the past few years, with consumers choosing it for its pleasant taste and high nutritional value and growers loving it for its hardiness, good yields, long self-life, and high prices. Successful kiwi production hinges on effective propagation techniques and optimal pollination practices. It is essential for growers to learn the best practices to optimize both.

Propagation of Kiwi Plants

Propagation is a critical phase in kiwi cultivation since it might affect the establishment of the crop on the field, adaptation, and the quick (or not) growth of the plants. Vegetative propagation (through cuttings and grafting) is used for commercial purposes; however, kiwis can also be propagated using seeds.

Seed Propagation – for fun and breeding

The genetic diversity enclosed in the kiwi seeds is beneficial for breeding programs but completely unwanted for the commercial cultivation of kiwi plants. If an amateur gardener wants to try out this propagation method, here is what he/she can do: You can extract the seeds from mature kiwifruits. After cleaning them, you can put them in the fridge to break dormancy. Then, you can sow them in small pots in a protected area (greenhouse or inside the house). The seedlings can be transplanted in the soil (garden) when robust enough.

Vegetative Propagation – for commercial cultivation and production

Hardwood Cuttings

Hardwood cuttings are preferred for commercial propagation due to their higher success rates and ease of handling. This practice involves taking mature, dormant cuttings from healthy parent plants during the winter months. The cuttings, typically 15-20 cm (6-7.9 in) long with several nodes, are treated with rooting hormones to enhance root development. They are then planted in well-drained, moist soil and kept under controlled conditions to ensure optimal rooting. This method produces uniform plants that inherit the desirable traits of the parent (true to the variety chosen), leading to consistent quality and productivity in commercial orchards.

Hardwood cuttings are less susceptible to stress and damage than softwood cuttings taken from actively growing plants. Additionally, they are stored and transported more easily and for longer periods without significant deterioration, unlike softwood cuttings that require immediate planting and careful handling. Finally, kiwi plants propagated from hardwood cuttings tend to be more vigorous and robust, as they start with a more mature tissue that is less prone to disease and environmental stress.

Softwood Cuttings

This type of cuttings is collected from the current year's growth in late spring or early summer. These require a more controlled environment with high humidity and bottom heat to ensure successful rooting. Some of the most important advantages of this propagation material are crop uniformity, fast establishment, and fruiting.

Propagating kiwis with grafting (whip grafting and cleft grafting) is also possible and, in some cases, necessary for specific cultivars that might be challenging to propagate through cuttings. However, it is not widely used in commercial kiwi production.

Pollination of Kiwi Plants

Pollination is a critical determinant of fruit set and quality in kiwifruit cultivation. Kiwi plants are dioecious plants, meaning there are male and female plants. As a result, cross pollination is vital for good fruit production and yields. It is essential that a grower knows the needs of the selected main variety and establish suitable pollinizers across the field.

Self-Fertile vs. Self-Nonfertile Varieties:

Self-Nonfertile Varieties: Most traditional kiwifruit varieties, such as Hayward, are self-nonfertile and require a male plant for pollination (pollinizer). Female plants produce the fruit, while male plants provide the necessary pollen. To facilitate that, it is also essential to boost the population of bees (and other insects) during the blooming period of the kiwi plants. While kiwi can be pollinated with the help of wind, insects like bees are the main helpers.
Self-Fertile Varieties: Some newer cultivars can produce fruit without a male plant, as they possess both male and female flowers. These are advantageous for small-scale growers or regions with pollination challenges.

Pollinizer Varieties

As we have already mentioned in the article about the selection of kiwi varieties, the compatibility between the main producing (female) variety and the pollinizer (male) variety is essential for successful pollination and fruit set. Male varieties like Tomuri and Matua are widely used as pollinizers for female varieties like Hayward (the most cultivated and popular kiwi variety – green fuzzy kiwis). The compatibility/suitability of the pollinizer is essential. In some cases, farmers may need to establish more than one pollinizer variety (one early flowering and one with more late flowering time) to cover the whole period of blooming of the main female variety. You must always consult your local agronomists and the companies selling the varieties for the suitable pollinizers per case.

Kiwi Pollinizer - How to select them and where to place them

Typically, one male plant can pollinate 6 to 9 female plants. However, this ratio can vary based on factors such as plant vigor, environmental conditions, and pollination efficiency. Usually, growers say that 10 to 12% of their plants in the field should be pollinizers.

To ensure adequate pollen transfer, pollinizer plants (males) should be evenly distributed throughout the orchard. A common practice is to plant one row of male plants for every six to eight rows of female plants or intersperse male plants within female rows at regular intervals. Some farmers say that one male plant is usually planted every 3-4 female plants in every other row. Regular pruning of male plants is essential to maintain good vigor and ensure a prolific supply of pollen.

Establishment and Protection of Bees: Bees are primary pollinators for kiwi plants. Ensuring a healthy bee population by providing adequate forage and minimizing pesticide use can boost pollination. It is essential not to spray with pesticides during the hours of the day when bees are active (spray only during the night). To be safe, growers should also inform their neighbors not to spray since pesticides can be transferred by wind drift. Some growers place beehives strategically within the orchard to maximize flower visitation. Usually, 4-5 beehives for every hectare are more than sufficient. Some growers and agronomists have experimented with using bumblebees for pollination since they are generally more hardy in extreme weather conditions and can fly even when the wind is stronger.
Wind Management: While wind is not a primary pollinator for kiwi, creating a windbreak can help stabilize the microenvironment, aiding in flower pollen retention.
Artificial Pollination: Pollen is collected from male flowers, processed, and then sprayed onto female flowers using specialized equipment. This method is becoming increasingly popular in commercial orchards to enhance pollination efficiency or in orchards that had problems with the initial selection of pollinizer varieties. However, this method has a high risk of spreading Pseudomonas, which is one of the most destructive bacteria for kiwi plants.

Frost or heavy rain during the flowering period can significantly negatively impact pollination and fruit set. Similarly, water stress can have similar effects. It is important not to stress our plants and pollinators during flowering.

How to enhance kiwi pollination and fruit set

Apart from the practices described above, some kiwi growers may also employ different substances to enhance flowering, pollination, and fruit set. Below, we present some common examples.

Hormonal Treatments:

Gibberellins: Gibberellins are plant hormones that can stimulate flowering and fruit development. Applying gibberellin-based products can enhance fruit set and size. You must always consult your local licensed agronomist.
Cytokinins: These are another group of plant hormones that can promote cell division in developing fruit, potentially leading to larger and more uniform fruits. These phytohormones (as CPPU) are usually applied three weeks after full bloom to enhance fruit weight. Based on research findings and field experience, spraying with CPPU can lead up to 22% heavier fruits. More than one application is usually performed without causing any negative effects on the safety and quality of the fruits.

Spaying with Nutrients and Applying Biostimulants:

Foliar application with boron (B), zinc (Zn), and calcium (Ca) is known to boost fruit sets during blooming. More specifically, boron can improve pollen tube growth and fertilization, enhancing fruit set. Similarly, calcium sprays can strengthen cell walls in flowers and develop fruits, reducing the risk of fruit drop and improving overall fruit quality. Calcium nitrate is usually applied after the fertilization of the flowers. And finally, zinc is important for synthesizing auxins, which are essential for proper flower and fruit development. Zinc applications can also address deficiencies and promote healthy growth.

Biostimulants are another group of products that are increasingly used in agriculture to enhance plant growth and resistance to biotic and abiotic stresses. One of the most popular products applied by farmers is seaweed extract. Rich in natural hormones, amino acids, and micronutrients, seaweed extracts can stimulate flowering, enhance pollination, and improve fruit set. Humic and fulvic acids can enhance nutrient uptake and stimulate plant growth, supporting overall health and reproductive success.

Some other products that can be used to boost plant health, vigor, and resilience and, as a result, positively affect flowering and fruit set are beneficial microbes such as mycorrhizal fungi (which improve nutrient and water uptake) and rhizobacteria (which can promote plant-root growth and health).

By strategically using these substances, growers can significantly enhance the flowering, pollination/flower fertilization, and fruit set of their kiwi plants, leading to higher yields and better fruit quality.