Introduction

With around 20,000 hectares of cultivation, white asparagus is the most important vegetable crop in Germany. Asparagus (Asparagus officinalis) is a perennial plant that is harvested for 7 to a maximum of 12 years due to its ageing [1]. After sowing or planting in spring, the perennials grow in the first year and form strong storage roots through assimilation, which increase in mass from year to year. From the second year after planting, the crop can be harvested for a few weeks, with the yield increasing up to the 5th year.

The storage roots are the reserve store for quickly mobilizable carbohydrates, which are used up during the 2 to 3 months harvest to form new shoots. In the following assimilation period (herb growth period) from June to September, which lasts only 3 to 4 months, this store must be replenished or increased for the next year's harvest.

To produce white asparagus, the spears must be grown in the dark. Exposure to light results in undesirable red discoloration due to anthocyanin formation or greening. For this reason, 40 cm high ridges are raised above the rows of plants in autumn or early spring using ridges ploughs or ridges tillers.

Early varieties start sprouting from a temperature of approx. 10 or 12 °C. The length growth of the spears depends on spear length and temperature at the spear tip. For example, a 10 cm long spear grows about 4.6 cm per day at 15 °C [1]. A good water supply and soil temperatures of around 20 °C in 20 cm depth promote the formation of new buds, sprouting and growth.

Depending on the climate and premature measures, harvesting begins around mid-April and ends on 24 June to give the plants enough time to form herb and store assimilates in the storage rhizome for the following year's harvest.

In the past, without the use of films, white asparagus was harvested twice a day before piercing the soil to prevent the spears from discoloring due to light penetration. By covering the ridges with opaque black/white film, discoloration can be prevented and less frequent harvesting, in some cases, only every three days, is possible (Figure 1). In addition, the ridge temperature and thus the growth rate can be influenced within limits. If the black side of the film is facing upwards and exposed to sunlight, the soil warms up more quickly; if the film is white, it cools down to a certain extent. If a more rapid warming in spring is desired, additional transparent films, double films and even film tunnels are used. In extreme cases, if cheap energy is available, for example from waste heat, the areas can be heated underground using ribbed tubes filled with water.

Figure 1: Asparagus ridges covered with white film

Bild 1: Mit weißer Folie bedeckte Spargeldämme

The yield of white asparagus is between 4 and 12 t per hectare of marketable produce [1], depending on site conditions, growth performance and year of growth. Assuming an average yield of 8 t, around 1.6 kg of asparagus or, depending on diameter, around 30 spears can be harvested per meter per season.

Harvest

Cutting white asparagus by hand is very strenuous and time-consuming and therefore expensive. First, the film has to be lifted or removed, then, the spear must be slightly exposed by hand and cut to a length of 25 to 30 cm with the long asparagus knife. Then, the hole in the ridge must be closed again. The worker places the harvested spear in a basket or crate, closes the film and moves on. With a row spacing of 2 meters, the ridge length is 5,000 metres per hectare.

The reliable cutting of asparagus without damaging asparagus spears growing in the neighborhood requires training and continuous monitoring of the workers. Depending on the cutting method - either "blind" (without free digging) or with "free" digging - and the soil condition, it takes between 4 and 16 seconds (pure piercing time) to harvest one asparagus spear (Figure 2) [2].

The increasing shortage of labor and rising wage costs in Europe are forcing asparagus growers to think intensively about concepts to reduce the costs of harvesting and processing.

Figure 2: Time without off-times to hand harvest one single spear by different digging and cutting principles [2].

Bild 2: Arbeitszeit ohne Nebenzeiten, um eine Stange Spargel zu ernten [2].

Manual harvest with basket or carriage

The traditional manual harvest with a basket is organized as follows. The workers walk along the ridge and cover the film. As they walk back, they harvest and collect up to 8 kg of asparagus spears in the basket. If the basket is full before the end of the row, the workers walk to the edge of the field and empty the basket into a transport crate. This means extra travelling. Once the row has been completed, the workers walk back along the ridge and close the film. In total, the workers walk a distance of 15 km/ha using this method, which requires almost 4 hours of labor solely for walking. For this harvesting method, an average of approximately 4 workers per ha is required [2].

When hand harvesting with a push carriage, the worker carries a small, one-wheeled carriage with space for up to two 20 kg transport crates (Figure 3). This means that even longer field lengths can be handled, without having to empty the crates in between. The handling of plastic films has also evolved. Around 6 - 8 meters of film are opened in one go and closed again after cutting. Going forwards and backwards and pushing reduces the ridge length to be covered to around 10 km per ha. The amount of labor required is reduced by around 1.5 h per ha per harvesting pass [2].

Figure 3: Hand harvest with push carriage.

Bild 3: Handernte mit Schiebewagen.

Manual harvest supported by harvesting aids

Around the year 2000, different harvesting aids were developed. The goal was to handle the plastic film, transport the spears in crates, and, in some cases, the workers themselves. So the row distance of 5000 m had to be walked only one time. From single pulled up to motor driven five-row systems many harvesting aids were developed and tested to reduce harvesting costs. The harvesting itself was still carried out by hand.

The simplest versions were pulled or pushed four-wheel one-row harvesting aids without engine. The film was lifted automatically like an arch and the transport of the spears in crates was simplified. In the end, the workers preferred harvesting with push carriages [3; 4].

At the same time different manufacturers developed one-row combustion engine-driven harvesting aids with sitting working position. The workers sat bent-over on a small seat in front of the ridge, harvested the spears and put them in a crate nearby. To improve the lateral working space of the workers the seats were laterally movable. But these machines could not gain acceptance because of the unfavorable sitting position (it is exhausting to dig and cut out of the shoulder), the necessary frequent stopping and starting of the harvester, the motor noise nearby the worker`s ear and the high machine costs.

To reduce the high machine costs per row, two-, three- and up to five-rows harvesting aids were developed, sometimes in combination with sitting worker position in the middle row [5]. A sun-roof gave shelter on sunny days. In 2015 a 14 row harvesting aid from Strauss Verpackungsmaschinen, came on the market [6]. The film was lifted, 14 workers harvested and put the harvested spears on a conveyor belt, which transported the spears to the central truck. There, the asparagus spears were pre-cut, pre-washed and pre-sorted and stored at the back.

However, all multi-row systems had the disadvantage that individual workers had to wait a long time when yields were low, as there were large gaps between the individual spears. Because of the high machine costs and various logistical issues, more-row harvesters were not successful and were phased out of the market.

During this time electrically driven one-row harvesting aids were developed from Engels Machines (Figure 4) [7]. The worker walks a ridge length of 5 km, harvests and determines the speed, but the film handling and transport of the crates is automated. The film is lifted and forms an “arch” for one worker. It is necessary that the arch is long and high enough to give the worker sufficient space and time for hand harvesting. The velocity of the device can be adapted to the daily yield. A simple stop and go switch supports the process. Additionally, optical sensors are available to automate this process. The battery performance should last one day, not to interrupt the harvest. If required the film can be turned from black to white and vice versa automatically. Non-productive times only occur for emptying the crates, turning the machine at the end of the field and threading and fastening the film strips. With good work organization and trained workers, one worker can harvest up to 20 kg of asparagus per hour or around 0.8 ha per day. The performance of electrically driven single-row harvesting aids can be further improved by shift operation. Two workers used one harvester and each worker harvested twice for four hours per day. With this work arrangement, productivity could be doubled from 221 to 463 spears per day [8].

Figure 4: Electrical driven one row harvesting aid [7]

Bild 4: Elektrisch angetriebene einreihige Erntehilfe [7]

Today harvesting with push carriages and with electrically driven single-row harvesting aids are standard and between 1.2 and 1.5 workers per ha are needed in the harvesting period.

Mechanical harvest

Non selective harvest

In the late 1950s [9; 10], different developments started to harvest asparagus for cans. The whole ridge is cut, taken and sieved by a moving sieve chain. Up to four workers remove the asparagus spears from the sieve chain and place them in crates. The ridge is then automatically rebuilt and covered with film. Depending on the temperature in the ridge, it can be harvested again after approximately one week.

Around the year 2000, this harvesting principle was further improved and tested at ATB [11 - 14]. HMF Hermeler Maschinenbau [15] built the Wisent asparagus harvester, for which the company won a gold medal at the DLG exhibition in 2003. Many open questions remain to be answered regarding this harvesting method including plant reactions, harvest frequency based on ridge temperature, yield, sorting, losses, spear quality, economic viability, and other factors.

However, this development could not convince the growers at that time and was stopped. Ten years later, similar systems were introduced to the market by Ai-Solution (Figure 5) and Christiaens Agro Systems, and they remain available today [16; 17]. The machines are available as self-propelled or tractor-drawn models.

Figure 5: Non-selective harvester [16]

Bild 5: Nicht selektive gezogene Erntehilfe [16]

The harvesting time and the number of staff can be reduced in comparison to hand-harvest [18]. [19] measured a machine speed of 1.1 km h-1. Working 10 hours per day and 7 days per week, about 15.4 ha can be harvested with 3 to 5 workers by comparison to 18.5 workers for hand-harvest with one-row harvesting aids. For a machine speed of 1.8 km h-1 (statement of the manufacturer) the harvested area can be expanded to 28 ha per harvester. The high performance can be of great interest in periods of high yield. Additionally, loosening and sieving the ridge has a very positive effect for asparagus quality. Dead spears will be removed, sanitary problems will be reduced, the spears grow straight and following hand-harvest becomes easier. Additionally, yield peaks of the crop can be compensated. The disadvantages include poor sorting (e.g., many short spears), extended harvesting periods during bad weather, and high machine costs [20]. The reason for the poor sorting is that all spears, long and short, are cut about 5 cm above the root stock. After harvest, fully developed spears that are smaller than 5 cm will grow rapidly and reach the top of the ridge within a few days. The larger number of spears will need longer and will be cut in a too short state if only mechanical harvest is used. The amount of long spears can be improved, if mechanical harvest and hand-harvest are combined. The first spears after mechanical harvest are cut by hand until a larger amount can be expected. Unfortunately, an affordable optical method is lacking to inspect the ridge and determine the optimum mechanical harvesting date [21]. It may also be beneficial to change the cultivation method, e.g. by planting in double rows. In addition, a modified marketing concept for short asparagus is important.

Selective asparagus harvest

For several years, growers and scientists tried to develop selective mechanical white asparagus harvesters. Different detection, digging and cutting principles were tested but finally the capacity per hour as well as the precision and the high price could not convince the growers. In addition, the abrasion of the sandy soil led to rapid wear of the tools.

An interesting development in 2008 was the Asparagus Panther from Ai-Solution [22]. The three-row harvester lifted and removed the film, detected the spear tips optically, cut the spears with a specific device and placed them into a crate. Another development, the selective harvester “autospar” from a German consortium did not reach marketability. In 2018, a new device from Cerescon with asparagus detection by mechanical sensors and harvesting with a specific mechanism should come to the market. The three-row machine should have a performance of 40 ha and should cost between 500.000 and 600.000 € [23]. The development did not survive the so-called ‘valley of death’ phase, and has not been further developed.

Figure 6: Selective asparagus harvester [24]

Bild 6: Selektiver Spargelernter [24]

In 2023 the AVL Motion compact S9000 robot to selective harvest asparagus came to the market (Figure 6). With image recognition (recognizing the white tips) and patented harvesting modules, the harvester is able to identify the location of single spears, and software assigns one of the twelve harvest modules in the 'harvesting train'. All harvest modules are positioned next to each other on a conveyor travelling in a circle. When the ‘asparagus’ command is given, the rearmost harvesting module stops at the cutting location, moves at the same working speed against the direction of travel, dips in, cuts the asparagus spear, and places it aside on a conveyor. The module then runs through the circular process and lines up again at the front. This circular process, with its 12 tools, enables continuous harvesting with a high output. The robot can be operated by one person. Only a few tasks have to be performed: handling the self-steering robot with a few buttons, placing the asparagus spears manually in crates, replacing full crates with empty ones at the end of the field, turning from row to row, and placing the film 'into' the AVL robot. At a speed of 2 - 3 km/h, the maximum harvesting capacity in 2023 reached 280 kg of raw asparagus per hour. In two-shift operation, around 15 ha can therefore be harvested with one machine in the harvesting period [24].

Problems for a broad market launch of selective asparagus harvesters are the still very high price due to the low number of units built, the high development costs and the uncertainty as to whether the machine will fulfil its promises. This hinders further development and prevents asparagus growers from purchasing the machine in large quantities.​

Summary

Harvesting white asparagus is time-consuming, exhausting and expensive, and it is increasingly difficult to find labor for this hard work. Asparagus growers have therefore been looking for ways to solve this problem for a long time.

One of the most important aspects of hand-harvest is that the workers do a perfect job. This is advantageous for both parties. The workers earn more money for higher performance and the manager gets better quality and can reduce the number of staff. Good technical support is also essential. Using single-row harvesting aids with an electric drive in shift operation produces the best results. Film handling and transport of the spears are automated and only digging and cutting is made by hand. Between 1.2 and 1.5 workers per ha are needed during the harvesting period.

Non-selective harvest is still ongoing. The big problem is the unfavorable sorting with many short spears. It can be mitigated by combining mechanized harvest and hand-harvesting, and by changing cultivation process, i.e. planting in double rows. In addition, a modified marketing concept for short asparagus spears is required.

Selective mechanical harvesters are still under development. A new device entered the market in 2023. With image recognition and patented harvesting modules, the harvester can identify the location of single spears and harvests them fully automatically.

All in all, the development of white asparagus harvesting is not yet completed and most growers are still waiting for selective white asparagus harvesting robots which are inexpensive, fast, produce minimal losses, and are gentle with the produce.

Literature

[1]     Ziegler, J.: Spargelgewächse-Asparagaceae. in Laber H. und Lattauschke G. (Hrsg), Gemüsebau, 4. Auflage, Eugen Ulmer KG 2023. S. 511-529; ISBN 976-3-8186-1831-5.

[2]     Brückner, B., Geyer, M., and Ziegler, J.: Spargelanbau. Ulmer Stuttgart, 2008, ISBN 978-3-8001-4627-7, pp.84–95.

[3]     Geyer M., Tischer S. and Rohlfing H.-R.: Stechmethoden und Mechanisierung der Spargelernte. Gemüse 39 (5) 2003, 29-31.

[4]     Geyer, M., Jakob, M. and Rohlfing, H.-R.: Spargel-Erntehilfen unter der Lupe. Gemüse 37 (12) 2001, 29-34.

[5]     Fischer-Klüver, G.: Fünfreihige Erntehilfe für Spargel technisch verbessert. Gemüse 37 (10) 2001, 34-35.

[6]     Strauss Verpackungsmaschinen: https://www.strauss-pack.com/anlagen/spargelverarbeitung/erntehilfe/, Zugriff am 01.01.2025.

[7]     Engels Machines: https://www.engelsmachines.nl/de/spargelspinne-a1-2/, Zugriff am 31.12.2024.

[8]     Allofs, F.P.: Spargelhof Allofs. 8. Produkttag Spargel 29.01.2015, Ingolstadt. Bayrische Landesanstalt für Landwirtschaft.

[9]     Kepner, R. A., and O´Brian, M.: Mechanical harvesting and handling of white asparagus. Trans. ASAE, 1967, 145-149.

[10]   Mears, D.R, Moore, M.J., and Parashuram: The potential of mechanical asparagus harvesters. Trans. ASAE, 1969, 813-815, 821.

[11]   Geyer, M., Oberbarnscheidt, B.: Nichtselektive maschinelle Ernte von Bleichspargel, Landtechnik 56 (5) 2001, 326-327.

[12]   Geyer, M., Oberbarnscheidt, B., and Paschold, P.-J.: Non selective harvest of white asparagus. In Proceedings of the 6th International Symposium on Fruit, Nut and Vegetable Production Engineering, 2002, M. Zude, B. Herold, and M. Geyer, eds. (Potsdam, Germany: Institut für Agrartechnik Bornim e.V.), ISBN 3-00-008305-7, 227–232.

[13]   Paschold, P.-J. and Geyer, M.: Maschinelle Ernte von Bleichspargel. Gemüse 34, 2, 1998, 114.

[14]   Paschold, P-J.; Artelt, B.; Hermann, G.; Geyer, M.; Oberbarnscheidt, B.: Nichtselektive maschinell Spargel-ernte. Gemüse (8), 2002, 26-28.

[15]   Hermeler Maschinenbau GmbH: https://hmf-hermeler.de/. Zugriff am 01.01.2025.

[16]   Ai-solution: Kirpy. https://www.ai-solution.de/, Zugriff am 01.01.2025.

[17]   Christaens Agro Systems: https://www.christiaensagro.com/de/produkte/spargel, Zugriff am 01.01.2025.

[18]   Kress, O., Strobl, P. and Schaser, M.: Erfahrungen mit dem Spargel-Kirpy in Bayern. Gemüse (47) 2011, 37-39.

[19]   Strobl, P.: Vergleich von Hand- und Maschinenernte bei Spargel. 8. Produkttag Spargel 29.01.2015, Ingolstadt. Bayrische Landesanstalt für Landwirtschaft.

[20]   Strobl, P.: Spargelprojekt Kirpy in Bayern. Sonderdruck Spargel & Erdbeer Profi 5/ 2012, 1-4.

[21]   Seyfried, D. and Schoebel, J.: Ground penetrating radar for asparagus detection. Journal of Applied Geophysics. Vol 126, 2016, 191-197.

[22]   Ai-solution: Spargelpanther. https://www.youtube.com/watch?v=4JVQUgiry_M, Zugriff am 31.12.2024.

[23]   Cerescon: Cerescon stellt selektiven Ernteroboter vor. Spargel&Erdbeer Profi 2017,

         Rheinischer Landwirtschaftsverlag: https://www.spargel-erdbeerprofi.de/nachricht-spargelerdbeer/cerescon-stellt-selektiven-ernteroboter-vor, Zugriff am 31.12.2024.

[24]   AVL Motion: https://www.avlmotion.com/de/avl-compact-s9000/, Zugriff am 31.12.2024.