By Jovita Turan, January 31, 2021
The kiwifruit (Actinidia) is native to north eastern China, specifically the Yangtze River valley, and the Zhejiang Province coast. Previously called Chinese gooseberry, it is now more commonly known by its marketing name of kiwi fruit. Several cultivars were selected by amateur horticulturalists in the 1930s and the New Zealand export industry was developed around fruit of the A. deliciosa cultivar ‘Hayward‘ in the 1960s. During the 1980s, breeding programs were established in New Zealand using additional germplasm imported from China.
About 15 Actinidia species produce edible fruit, but there are currently only three species of commercial importance: the fuzzy kiwifruit or A. deliciosa (the green kiwi) and A. chinensis (the yellow kiwi); both are widely grown commercially and the most important producers are Italy, New Zealand, China and Chile. In recent years, a growing attention is paid to a third species, A. arguta or kiwiberry.
Fuzzy kiwifruit and kiwiberry are usually eaten fresh or are mixed in desserts or salads. Some of the Actinidia species are grown for their berries, while others are used as an ornamental plant in gardens and parks.
BOTANY
All Actinidia species are perennial, climbing or scrambling plants, mostly deciduous, although a few are evergreen. The woody vines with young shoots can grow to a height of more than 9 m. Young shoots are covered with hairs. They have no specific organs enabling them to attach, but the young shoot tops curl around older woody shoots. Actinidia species produce long petiole leaves not equally shaped and colored.
Flowers appear approximately 60 days after bud burst in spring. The flowers of both the fuzzy kiwifruit and kiwi berry have white petals that become yellow as they age. There are some exceptions, but usually the plants bear only flowers of one sex, either male or female. Only female plants bear fruit, but only when pollinated by a male plant. Self-fertile cultivars are ideal where there is space for only one plant otherwise vines of both sexes are essential for fruit production, and they must flower at the same time to ensure pollination. Flowers of female vines have a well-developed style and produce non-viable pollen. In commercial plantations one male vine is required for eight female vines.
Botanically, the fruits of the various Actinidia species are berries. Since they are fleshy, they have many seeds embedded in the flesh, and they do not split open at maturity. The commercial interesting kiwifruits vary in size, shape, hairiness, and internal and external color. Some varieties have early-maturing fruits compared to others.
MOST IMPORTANT SPECIES
The fuzzy kiwifruit (A. deliciosa) is egg-shaped with a brownish skin covered with short stiff hairs has a fibrous, dull greenish-brown skin and bright green or golden flesh with rows of tiny black seeds. The fruit has a soft texture and a sweet but unique flavor.
Most of the yellow-fleshed fuzzy kiwifruit belong to the species A. chinensis and have a smooth- skinned and almost hairless fruit when ripe. The taste of this fruit is also sweeter than A. deliciosa and has a more aromatic flavor. Both Actinidia species are normally peeled before consumption because of the presence of surface hairs and a less tasteful skin.
In fruit gardens and orchard plantations the varieties of Actinidia arguta, also known as Mini kiwi or Hardy kiwi, are considered to have the biggest potential because of their frost hardiness as they may resist temperatures as low as - 35 °C. This rampant climbing plant can grow up to 30-50 m high in its homeland, while in a cooler climate, it reaches up to 4-8 m high.
The vines of A. arguta are vigorous, have shiny green leaves, with green or red petioles depending on the cultivar. At the base of the young green shoots, and on second-year wood, a lot of lenticels can be seen. Flowers are smaller than those of the fuzzy kiwifruit. Both, male and female flowers have a diameter of about 1 to 3 cm. Female flowers can be pollinated by male selections of A. arguta and A. deliciosa. As in all Actinidia species, only one year canes produce fruit bearing shoots.
An adult plant can yield between 10 to 20 kg of berries. The fruit weight of the A. arguta ranges from 2 to 25 grams. The berry can vary in color from green to red blushy, or Bordeaux red. The smooth edible skin of the fruit is one of the strong commercial.
CULTIVATION
Propagation
Kiwifruit can be propagated by seeds. The seed bed should be prepared in January – February. For preparing right seed bed, sand and forest top soil should be used at the ratio of 1 : 2 in composition. Seeds should be sown during the month of February either in poly-pit hotbed or in plastic trays and they need to be mixed with sand for line-sowing. As the seeds are so small, mixing is recommended so that it is not clustered too much in one space or line. Thin layer of fine sand should be applied for top dressing/covering of the seeds sown. Regular sprinkling of water is essential to keep the bed moist. As the seeds germinate, usually in March, and the plants are of 3 or 4 leaves, plants should be pricked out as soon as there is little chance of frost. The pricked-out seedlings should be transplanted in the nursery bed. The newly planted seedlings should be protected from wind, strong sun and hail-stone by providing shade. Regular watering of the nursery is required to keep it moist for healthy growth of the seedlings.
Self-rooted vines perform better in the Northwest’s cold winters, where cold injury may occur. Softwood cuttings can be taken in the spring. However, using rootstocks in kiwifruit production could offer many advantages: vigor reduction, greater tolerance for adverse soil conditions such as water-logging, resistance to pests or diseases, greater physiological cold tolerance, more precocious and higher-yielding vines. For grafting, older than one year transplanted seedlings are used for root stock. The scion stick (male or female depending on the propagation desired) is taken from the father or the mother plant. The scion stick should be from branches older than 6 months and should have two nodes. Side grafting is easier and more successful in kiwi propagation. Regular watering of the grafted plant is essential. Technique of layering is highly efficient as well.
Environmental conditions
Kiwifruit has a huge climatic range and it require a sheltered sunny position, preferably a south- or west-facing wall, although they can be grown in the open in milder areas. Land gradient (slope) preferred is 10 – 14 degree. The site should be well drained because kiwi vines are very sensitive to standing water, especially after bud break in spring but adequate irrigation facility is essential. In heavy soils it is advisable to plant vines on raised beds, since it also may protect against phytophthora root rots. They grow best in a fertile, slightly acidic to neutral (pH 5.5-7.5), aerated and moisture-retentive soil which is rich in organic matter.
Temperature is a major limiting factor for cultivation. Fuzzy kiwis cannot stand temperatures below -15 °C, but in a dormant stage kiwi berry plants are resistant to temperatures below -30 °C. Kiwi fruit requires abundant water; during the dry season, the newly planted kiwi vines need deep watering once a week.
Plantation, training systems and management
Kiwi fruits grow on large vines that are similar to grapevines in their general growth and fruiting habits as well as their training and trellising requirements. Kiwifruit plants are trained to a permanent framework, either using hitching post trellises, T-Bar or Y-Bar like clotheslines with 4 to 5 parallel wires on the bar top.
On commercial plantations, a T-bar is used to support the solid canopy of foliage and fruit rather than Y-Bar. For easy harvest, the wires of the trellises should be at least 1.8 m above the ground. Cross-arms on the T-Bar and double wire systems should be 1.5-1.8 m long. Vines commonly are planted 4.5-5.5 m apart, while the row distance should be 5 meters at least to provide adequate aisle for care and management of the plants. With this spacing, a hectare of land can take in around 300 to 330 plants.
Alternatively, kiwi berries are best grown as an espalier. This method is used for training apples and pears as espaliers is a space-saving way of growing fruit on a wall or fence. They require little pruning once established and are attractive in blossom and fruit and architectural during winter.
The pit size should be 1 m by 1 m. The grafted seedling should be planted at the raised center of the pit. The grafted part should always be above the soil / ground.
The management factors after planting include weed control in young vines in case to reduce competition for moisture and fertilizer, pest control and protection of long shoots against wind damage on plantations in full production. Kiwifruit is a fast-growing plant, an adequate fertilizer management and water supply are indispensable for optimum productivity. In most cases, annual applications of a balanced compost should be sufficient. Compost or a light application of a prepared balanced organic fertilizer should be applyed around the trunk area from the spring (after risk of frost is past) into early July. To retain moisture, regulate soil temperature, control weeds, and encourage healthy root growth the application of 10 cm of wood chips or other mulch is required.
Intercropping (grass for fodder, MAPs, tea, etc.) is possible, which however should not directly compete with the nutrients applied for the main plant (kiwifruit).
Pruning
It takes 3 to 4 years to establish the plant, but once it's done, Actinidia will continue to grow and bear fruit even for 50 years. Vines need to be pruned both in summer and in winter to maintain a balance between kiwi plant growth and profitable fruit production. Kiwi pruning is done from December to February every year. During the first year one straight trunk should be developed by tying it loosely to the stake as it grows.
In the second year two warm (or Cordon) must be encouraged on opposite sides of the vine and drape one in each direction on top of the wire and tie them loosely. The lateral shoots must be trained perpendicular to the Cordon in the third year.
Later thereafter, new leaders forming around the base have to be cut off of each vine plant above the soil surface. Branches or tendrils wrapped around other branches or the wiring system should be pruned as well. Shoots that are less than a pencil width in diameter should be cut back, as well as the wood that fruited the previous year. Deadwood and overlapped branches should be removed. Each year there is a need to cut back about one-quarter to one-third of the oldest laterals to a bud around 5cm from the main stem. New growth will be produced from this stub in the growing season.
Excessive plant growth is removed during the growing season to keep the kiwi canopy open and to remove non-fruiting wood. Once the fruit has set, the side shoots can be pinched back leaving four or five leaves beyond the maturing fruit. When used as an ornamental plant, Actinidia can be left to grow in an unrestrained manner, with only occasional trimming of errant branches. However, such a plant will start yielding fruit later and it will be of poorer quality.
Harvesting
Actinidias usually start bearing fruit in the fourth year after planting. Harvesting is done manually and can begin from the end of August as kiwifruit reaches almost of full size at that time. However, it is not matured enough for harvest until late October or early November. When the fruit is ready for consumption, it should contain 12 – 15 percent sugar. When picked unripe, they will soften and ripen if kept together with fruit producing ethylene, e.g. with apples. It is best to put them in a cellophane bag and leave them for a few days at room temperature.
Fruits are harvested by snapping the stem at the abscission layer at the base of the fruit. Commercially, kiwifruits are harvested all at a time. However, in the home garden, the largest fruit can be removed first and the smaller fruits allowed to grow and mature in to bigger sizes. While the grated plants start fruiting at the nursery bed only, kiwi vines reach the full/optimal production level (commercial production) within 8 to 12 years.
Storage of fuzzy kiwifruit is between 3 (A. chinensis) and 6 months (A. deliciosa) while for fresh kiwi berries it is limited to 6 - 10 weeks.
HEALTH BENEFITS AND USES
Actinidia fruit is very healthy. Being almost totally devoid of fat, low in sodium and rich in vitamins (E and C), potassium and other microelements, including zinc, it is a perfect supplement to a wholesome diet. The fruit of Actinidia arguta contains about 400 mg% of vitamin C, which is up to 4 times more than the vitamin C content of a lemon or an orange.
The fruit helps to clear impurities and eradicate aging and stress, it helps to reduce cholesterol level and lowers arteries narrowing it maintain healthy skin tone and texture, maintains bone, tooth, vision, and skin health, reduces high blood pressure and hypertension. It improves iron absorption. Kiwifruit is a perfect fruit for pregnant women attributing to the abundance of natural folate. Kiwifruit has low glycemic index which makes it suitable for the individuals with diabetes. Kiwi contributes favorably to regulation of innate and adaptive immune system.
As berries are smooth-skinned, they can be eaten without peeling. They can be eaten raw, added to cakes, jellies, salads, and other dishes enriching summer and autumn meals, dried as grapes, frozen, and marinated. They are perfect for preparing wines, liqueurs, jams or marmalades.
REFERENCES
1. Atkinson, R. G. and Macrae, E. A, 2007, Kiwifruit, Biotechnology in Agriculture and Forestry, v. 60, p. 329-346, (editors Pua, E. C. and Davey, M. R.), Springer-Verlag Berlin Heidelberg.
2. Debersaques, F. and Mekers, O., 2010, Growth and production of kiwifruit and kiwiberry, Verheye, W. H. (editor), Soils, plant growth and crop production, v. 2, Oxford, UK: Eolss Publishers. Vancouver.
3. Hoover, E. E., Luby, J., Tepe, E. S. and Guthrie, B., 2015, Growing hardy kiwifruit (kiwiberries) in the home garden, Yard and Garden, http://www.extension.umn.edu/garden/yard-garden/fruit/growing-hardy-kiwifruit/
4. Sherpa, S., 2013, Kiwi fruit cultivation, Natural Resource Management Approaches and Technologies in Nepal, published by ICIMOD, http://lib.icimod.org/record/28277/files/Technology%20(5).pdf, (March, 2013).
5. Strick, B., 2000, Growing kiwifruit, http://content.libraries.wsu.edu/index.php/utils/getfile/collection/cahnrs-arch/id/499/filename/96136182432004_pnw507.pdf
6. http://www.bcci.org.bt/wp-content/uploads/2016/12/ODTP-2.pdf
7. http://www.clematis.com.pl/en/vines-in-garden/other-plants/588-actinidia#s5_scrolltotop
8. http://bioweb.uwlax.edu/bio203/s2012/cejka_laur/history.htm
9. https://www.healthbenefitstimes.com/kiwi-berry/
10. https://www.rhs.org.uk/advice/profile?pid=600
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