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Contribution from Bill Bussell and Mark Bonin, School of Agriculture
The University of the South Pacific Alafua Campus
Dr Bussell's present address is Horticulture Department
UNITEC Institute of Technology
Private Bag 92025
Auckland, New Zealand.
Asparagus is traditionally grown as a temperate crop but this article and the preceding article clearly show that it has considerable potential for tropical areas. John Bonnardeaux, of Agriculture Western Australia describes the results of research conducted in the semi-arid tropics of the Ord River Irrigation Area in northern Western Australia. In contrast, Bill Bussell and Mark Bonin describe the results of research conducted at the University of the South Pacific in the humid tropics of Western Samoa. All three authors are enthusiastic about the potential of asparagus as a new vegetable crop for tropical areas.
Asparagus is an unusual crop in that it is a perennial green shoot vegetable. It is generally grown as a temperate crop and there are few reports of it being successfully grown in the tropics. However, it is grown in Peru at latitudes 8-12°S, which compares closely with the latitudes of 13-14°S for the four volcanic islands that comprise Western Samoa.
The decision to commence research on asparagus at Alafua in Western Samoa was at least partly prompted by the severe damage to crops caused by Cyclone Val in December 1991. During the cyclone, many of the perennial crops, such as coconuts, cocoa and bananas, suffered severe, and in many cases, fatal damage.
In considering potential new crops for the country it was felt that even if all the above-ground parts of an asparagus crop were lost during a cyclone, new spears would grow quickly from the crown once the cyclone had passed. In contrast, the traditional perennial crops of coconuts and cocoa need to be replanted and have a relatively long juvenile period before they become productive.
Asparagus trials were commenced in 1992 and have involved cultivar evaluation, growth analysis and harvesting strategy studies. The area where the trials were conducted has a rainfall in excess of 3500 mm/ year, much more than most of the world's asparagus growing areas. The mean daily temperature is 26-27°C, which is close to the temperature at which asparagus has its highest growth rate. The soil is a freely draining basaltic soil which dries quickly after rain.
Seventeen cultivars obtained from the USA, Taiwan and New Zealand were tested with the objective of identifying cultivars which produced spears of international quality. International standards are very strict, requiring spears to be 18-25 cm long, with a minimum diameter of 6-10 mm and with the spear tips tightly closed. Five cultivars; Tainan Nos 1 and 3 (Taiwan), UC 157 and Apollo (California, USA) and CAS 15 (New Zealand) gave promising yields of high quality spears.
This suggests that cultivars from warm temperate oceanic areas are likely to be most suitable for the tropics.
A feature of asparagus production in Western Samoa is the rapid growth rate of spears which can be up to 8 mm/hour. One consequence of this rapid growth rate is that spears need to be picked twice daily to avoid excessive length. Harvesting the spears this frequently did not increase the proportion of closed spears, but did reduce the proportion of very open spears.
Growth analysis involved assessments of shoot, crown and root growth each month for 18 months, and showed that growth is continuous throughout the year in Western Samoa. This offers the possibility of year-round spear production and the opportunity to harvest at times best suited to meet local or export market demands.
Harvesting strategy studies were conducted on crops established by transplanting eight-week old seedlings with their crowns 150 mm deep in rows 1.5 m apart and with plants 0.45 m apart (ie 14 800 plants/ha). The seedlings were grown in a situation which was protected from heavy rain until transplanting. This was necessary to ensure the best possible crop establishment in the very wet Western Samoan conditions.
The studies examined the influence of age of plants at the time of first harvest on spear yields and the length of the harvesting period in the first two harvesting cycles. Accumulated yields from four harvest cycles were significantly lower from plants first harvested at 7 or 9 months after transplanting, compared to plants first harvested at 11 months after transplanting.
Long harvest periods of 14 and 22 days in the first two harvesting cycles did not reduce yields in later harvest cycles, compared to shorter periods of 7 and 11 days in the first two harvest cycles. With harvests commencing 11 months after transplanting and with rest periods of 4 months, the yield would be about 2.5 t/ha in the first year and about 4.5 t/ha in the second and third years.
It was found that spear diameter was much less in the second half of a 30 day harvest period during harvest cycles 3 and 4. If export markets require thicker spears than those typically obtained under tropical conditions, then the lengths of the periods of harvest and rest would need to be reviewed.
Asparagus has good potential to supply the local Western Samoan market and niche export markets, but the size of these markets is uncertain at present. The continuing success of a Western Samoan asparagus industry would be dependent on ensuring that the country remains free of the diseases and pests which cause such serious problems for the crop in other countries.
Any claims made by authors in the Australian New Crops Newsletter are presented by the Editors in good faith. Readers would be wise to critically examine the circumstances associated with any claims to determine the applicability of such claims to their specific set of circumstances. This material can be reproduced, with the provision that the source and the author (or editors, if applicable) are acknowledged and the use is for information or educational purposes. Contact with the original author is probably wise since the material may require updating or amendment if used in other publications. Material sourced from the Australian New Crops Newsletter cannot be used out of context or for commercial purposes not related to its original purpose in the newsletter
Contact: Dr Rob Fletcher, School of Land and Food, The University of Queensland Gatton College, 4345; Telephone: 07 5460 1311 or 07 5460 1301; Facsimile: 07 5460 1112; International facsimile: 61 7 5460 1112; Email:email@example.com
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originally created by:GK; latest update 6 June 1999 by: RF