YEAR-ROUND ASPARAGUS PRODUCTION

Mike Nichols
Massey University
Palmerston North, New Zealand

Introduction
There is a steadily increasing consumer interest in the availability of fresh asparagus in supermarkets on any day of the year. Historically asparagus has been grown in temperate climates, with the young spears being harvested in the spring; the summer being used to replenish the carbohydrates in the root system, which provide the “food” for the spear production the following spring; and the tops dying down and the plants then becoming dormant in the late autumn and winter. It is possible to harvest an autumn crop of asparagus in temperate climates, but this is generally lower yielding than a spring crop, and production of new spears ceases when temperatures are still high enough for spear production in the spring, because of some form of bud dormancy.

Because of the increasing demand for out-of-season asparagus throughout the developed world, it has been necessary to source product from other countries, where production can be obtained outside the normal harvest period. In northern hemisphere countries this fresh product is sourced either from the southern hemisphere (Australia, Chile, New Zealand, and South Africa) or from tropical countries such as Peru, Philippines, and Thailand. Because asparagus has a very short shelf life it is commonly air freighted (at great expense).

Although the product may have undergone a very efficient cool chain process in the producing country, once it is on the plane it is at risk from whatever storage conditions occur in the hold. Upon arrival it then has to go through quarantine (which may involve methyl bromide treatment) and the normal marketing chain process, so it already has a greatly reduced shelf life.

Some tropical countries are able to produce asparagus at any time of the year, for example, Thailand, Philippines, and other humid tropical countries. Here they use the mother fern system and, in most cases because of the climate quality, it is not as good as temperate asparagus because the high temperatures and potential fern disease problems make the spears thin, pale green, and “seedy.”

There are some exceptions, and in Mindanoa (Philippines) I have seen some excellent asparagus being grown at very high altitude. In Peru the absence of rain and the lower temperatures (due to the Humbolt current) provide an ideal climate for producing asparagus for harvesting year-round.

Of course, another important factor for less-developed countries is the low cost of labour ($2 U.S. per day), which makes it a very attractive crop for such countries because of its high labour demand for harvesting. Nobody has yet produced a satisfactory mechanized harvesting machine, an invention that would help to level the playing field for developed-country producers.

However, distance from markets and airfreight costs can rapidly reduce the advantages of a low-cost labour and year-round climate. The Philippines’ major advantage is proximity to the large Japanese market, which can be reached by a refrigerated sea freight container in three to four days.

There are very good reasons for not importing asparagus from tropical areas into temperate climates. Quarantine is one of the main ones, but quality of product is another, not only because of its appearance but also because of hygiene issues and pesticide residues. Air-freight costs are another factor and shelf life will also play a role in determining the value of the imported product.

Thus, there is a need for producing out-of-season product domestically, and this is the purpose of this article.

The Mother Fern System
Although the normal harvest period in a temperate climate for asparagus is only two to three months per year (normally in the spring), it is possible to harvest asparagus outdoors in a temperate climate for up to six months if use is made of the mother fern system. Whether this is, in fact, economical is a good question. The mother fern system for asparagus was developed in Taiwan in the 1950s as a means to produce asparagus in a sub-tropical humid environment in which the plants had no natural dormancy.

Normally, in a temperate climate asparagus goes through four different growth stages (Figs. 1 to 4) every year, namely

1. Dormant (winter),
2. Spear production (spring),
3. Fern production (summer),
4. Fern death (autumn).

In the humid tropics and sub-tropics there is no dormant period, and it was necessary to develop an alternative production strategy, which was called “the mother fern system.” Essentially, this involved growing the plants for some eight months (or until they had developed to a satisfactory size). Then, any new spears that developed were harvested and the old fern was removed as it died, until only four or five ferns remained per plant. As further fern dies and is removed, then it is replaced by allowing a spear to grow into a fern.

In temperate climates this method can be used to provide fresh asparagus throughout the summer months, but early production is reduced in the spring, because of the need to establish fern growth first and to support the fern with wires. The mother fern system is more demanding in terms of harvest because it is necessary look under the fern for the spears, whereas in the normal system there is no fern, so the spears are easily visible and, thus, much easier to harvest.

The mother fern system can be used for producing asparagus in a greenhouse throughout the winter months, but there is a cost involved in heating the house. In the field the yield potential of asparagus is dependent on the amount of carbohydrate stored in the swollen root system during the previous summer. The actual yield achieved in the spring depends on the weather during the spring — primarily temperature. Growing asparagus in the soil in a greenhouse provides the opportunity to ensure that a good level of carbohydrates is stored in the root system, but the problem occurs in the winter, productivity per week is very temperature dependent, and heating a greenhouse in the winter to 20ºC is an expensive operation!

Nevertheless, the mother fern system may well provide a means to producing a domestic out-of-season crop of asparagus in a greenhouse. It is very important to ensure that the top of the crown does not get too close to the soil surface, or spear diameter will suffer.

It is possible to produce either green or white asparagus in this manner. Green asparagus is produced when the developing spears are harvested at ground level when they are some 20–25 cm (8–10 in.) above the soil surface. White asparagus is harvested at crown level just as the spear starts to emerge, so it is grown in raised beds so that the blanched spears are some 20 cm long.

Massey’s New Method
We are developing at Massey University a new method of producing out-of-season asparagus. The plants are grown in large containers (pots) in a greenhouse during the summer to produce fern and, thus, plenty of stored carbohydrates in the root system. The plants are then made dormant by stopping irrigation. This is the process called drought dormancy, which is used in Peru to grow three crops a year in a totally arid but irrigated environment. It is also possible to make the plants dormant in the autumn simply by transferring them outside the greenhouse.

After dormancy the tops are removed, the pots transferred to a fully insulated building, and the temperature turned up to, say, 20ºC (68ºF). If white asparagus is required, the plants are grown in the dark, and if green asparagus is required, the plants are grown under a low-level tungsten light.

Selective harvesting is carried out when the spears reach the desired length. If the market demand is high, productivity can be increased by raising the temperature up to 30ºC (86ºF); if demand is low, the temperature can be reduced to, say, 10ºC (50ºF). Because the building is well insulated (unlike a greenhouse) the cost of heating is of much less importance.

Other Methods
There have been other methods developed for producing asparagus out of season in temperate climates.

In New Zealand Dr Peter Falloon promoted the use of plastic houses to produce out-of-season (in this case) early asparagus by growing a normal crop in the soil in the house and then making the plants dormant in the autumn by withholding irrigation. Applying water in the winter will result in spear production, but the rate of production will depend very much on temperature. Heating the house will enhance productivity but at a very real cost.

Similarly, near Christchurch (New Zealand), Klaus Prusas has developed a method to produce white asparagus out of season in a greenhouse by covering the dormant plants with black polyethylene film to ensure that the spears are blanched.

In the Netherlands a method of producing out-of-season white asparagus was developed by Dr Wagenwoort in which the crowns were dug in the autumn and transferred to a hydroponic system based on that used for producing endive /chicory). Unfortunately, the asparagus root system did not lend itself well to the system, which has not been developed further.

Asparagus Varieties
There are only small, but subtle differences, between the varieties used for white asparagus and for green asparagus. The chief difference is that white varieties normally have thicker spears, because the spears are normally peeled prior to cooking. Varieties normally used for green asparagus are selected to have tight heads when 20–25 cm (8–10 in.) Possible varieties worthy of consideration are

• For green asparagus: UC 157, Apollo, Atlas, Jersey Giant.
• For white asparagus: Cito, Lucullus, Gynlim.