Asparagus Growing (Part I)
In the plant of the asparagus, we have to differentiate an underground part and an over-ground part.
1. Taxonomy and Origin
The first vestiges of asparagus appeared in paintings on Egyptian monuments (3,000 b.c.). The drawings showed bunches tied twice or three times, which were apparently offered up to the gods. This vegetable was highly regarded by the Greek, but it was the Romans who introduced this crop to Northern Europe. After the Barbarian invasions, the asparagus was only cultivated in Spain, but at the end of the Middle Ages, Northern and Central European regions restarted its growing. Current varieties seem to have their origin in the selection processes carried out in the Netherlands in the 18th century.
2. ECONOMIC IMPORTANCE AND GEOGRAPHICAL DISTRIBUTION
In the last few years, the world production of asparagus has increased significantly, especially in imports, as it is a preferential product in the international market which has enormous returns, due to an increase in its consumption and the variety of preparations.
The leading producer of asparagus is China, with around 85% of the world production, which is mostly dedicated to domestic consumption. Peru is in the second place, with a world production share of 5%. Its production is in steady growth, as weather conditions enable it to produce all year round.
In Europe, Spain is the second largest producer of asparagus, behind Germany.
The asparagus is a perennial, herbaceous plant that lasts quite a long time in the soil (around 8-10 years), if seen from a cost-effectiveness point of view.
Asparagus plant. Photo: Joi Ito
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In the plant of the asparagus, we have to differentiate an underground part (rhizome and radicular system) and an over-ground part (stems, branches, leaves, flowers and fruits).
- Stem: the main stem is only one, goes underground and is modified by a rhizome. In the soil, it develops horizontally as a base or platform from where other organs of the plant develop, according to their tropism.
- Roots: The main roots develop directly from the underground stem. They are cylindrical, thick and meaty and they can accumulate reserves, which serve as a basis for the next production of turions (asparagus). Rootlets or feeder roots, which absorb water and nutritional elements, develop from these main roots.
The life of the main roots is 2-3 years. When they die, they are substituted by new ones, which situate on the upper part of the old ones; consequently, the buds get higher. As years go by, the underground part reaches the surface of the soil.
- Buds: They are the organs from where the turions (edible and marketable part of the crop) sprout. When they grow, they turn into the future ramified stems of the plant.
- Flowers: They are small and bell-shaped and they have a 5-8mm (female) or 3-5mm (male) long yellowish green corolla. There is usually one single flower. Its cross-pollination has a high percentage of allogamy. Both male and female flowers have stigmas and stamen. In male flowers, the stigma is atrophied, whereas in female flowers, the stamens do not develop in full. The male flower has 6 stamens and the female flower has 3 pistils.
- Fruit: It is a 0.5cm diameter, rounded berry. At the beginning, it is green and when it ripens, it turns red. Each fruit has 1 or 2 seeds.
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Asparagus fruit. Photo: Rob Ireton
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- Seeds: They are dark brownish-grey or black and their shape is a middle point between round and polyhedral. They have a high germination power.
* The plant of asparagus is dioecious, i.e. that there are female plants that only produce female flowers and male plants that only produce male flowers.
Male plants produce many more turions than female plants; this is logical, as female plants use a great part of their reserves in the production of flowers, fruits and seeds, whereas male plants accumulate those reserves in their roots for the next turion production period. Moreover, male plants develop earlier and live longer than female plants.
When it comes to grow green asparagus, male plants are preferred to female plants, because, as they do not bear fruits, there is no possibility that the seeds produce new plants that could multiply the density of plantation. Logically, as years go by, the uncontrolled increase in the number of plants reduces the quality of the turions, which do not meet the minimum diameter requirements of the quality standards in force.
From the agronomic point of view, the asparagus plant has three different phases:
- Vegetative development.
- Production of turions.
- Dormant period.
4. EDAPHOCLIMATIC REQUIREMENTS
It is one of the most sensitive species to temperature changes, which is shown by the inertia of its vegetative movements.
The ideal average monthly atmospheric temperature for the development of turions is between 11 and 13ºC.
The optimum temperature for its vegetative development is between 18 and 25ºC. Its development stops if temperatures are under 15ºC during the day and under 10ºC during the night; on the other hand, it temperatures are over 40ºC the plant has difficulties for developing.
The ideal relative humidity for the production of turions is 60-70%.
If the asparagus is grown outdoors, the wind may have a special influence in the final period of development of the feathery foliage, as it may blow it down, although this effect has not been found pernicious for the crop. In areas where the wind usually comes from only one direction, the rows should run in said direction.
As this is the green asparagus, the colour is a quality factor, so it is important to guarantee that most of the turion (at least two thirds) gets this colour. That is to say that as we want to get the green colour, we should give the plant as much light as possible, so that the necessary chlorophyll could be synthesized to obtain said coloration.
The soil should have a loam texture, tending to sand or lime loam texture. The plant also admits a clay loam texture, although clay soils are not convenient. To get the maximum commercial advantage of turions, the soil should not be rocky to prevent damage of the apical bud of the turion by contact with stones when it is developing underground.
No puddles should ever form on the soil, as the plant is highly sensitive to radicular asphyxia.
The ideal pH is between 7.5 and 8, although it can grow in soils with a pH of 6.5.
It has a high level of resistance to the salinity of the soil and the irrigation water; it is possible that this is the reason behind the reduced longevity of the asparagus plantation, although this plant is a garden vegetable that has one of the highest resistance to salinity and tolerance to high electrical conductivity.
5. VEGETAL MATERIAL
There are different criteria that could be applied to classify varieties, although the colour of the shoots is the most important. Some varieties have purple shoots that become dark green when exposed to the light, while others have violet and pink shoots that turn light green if exposed to light.
The varieties of green asparagus should have the following characteristics:
- Resistance to the ripening of the head.
- Resistance to illnesses, especially those caused by rust.
- Medium thickness turion.
The aptest varieties for the production of green asparagus are: Verde de California, Huetor, Plaverd, UC-157, Mary Washington, Jersey Giant, Eros, Grande, Mastric etc. We must take into account that there are varieties more adapted for the production of green asparagus and other varieties more adapted for the production of white asparagus.
Huetor: this is a native variety from Huetor-Tájar, province of Granada. It has a tendency towards “green”. There two types of green: dark green and purple green. It has an average earliness and its productivity falls below most hybrid varieties with a tendency towards "green".
Plaverd: it is a specific variety for the production of green asparagus. It does not bloom as early as the variety Huetor.
UC-157: it is a variety that was created in 1980 in the United States. It is a specific variety for producing green turions. Hybrids F1 and F2 are the ones which are commercialized. Hybrid F1 is more productive and of a better quality than hybrid F2. This is one of the earliest and most productive varieties of the market.
In some regions of the Americas, the following varieties are used:
- White or light green: Connovers Colosal, Mammmouth White
- Dark green: Martha and Mary Washington, Palmetto, Argentenil, UC157 and UC72
6. GROWING TECHNIQUES
6.1. Preparation of the land
The soil should be prepared with a subsoiler, to facilitate subsequent exploration and easy development of a strong radicular system. A cultivator and a milling machine should also work the land horizontally as well as vertically in order to favour the uniform distribution of organic matter and basic manures and to obtain a soft structure for ploughing furrows, in whose bottom vegetal material will develop. The furrows should be up to 50cm deep. In this case, the bottom of the furrow will be 10-15cm deeper than the soil level. The required depth depends on the type of organ to be planted (crowns or seedlings). The difference will be around 5cm. The rhyzome of the crown is not higher than 5cm and it should grow upright to keep its tropism. As it is not necessary to whiten it, no ridges are needed.
The type of transplant depends on the kind of vegetal material to plant: crown or seedling.
- Seedling: it is obtained from hybrid seeds. At the moment of the transplant, the seedling should have a 10-12cm long feathery head and a second partially development feathery head in its base.
In case of planting root balls, turions can appear in two years provided they have been planted in their final soil location. In warm climates, turions may appear after only one year.
- Crowns: they are obtained in seedbeds. Their recommended weight is 60g and those under 20g should be discarded. In warm climates, one year after planting the crowns, the first harvest can be done. This is the best type of plantation for greenhouses.
6.3. Direct sowing
Two lines of seeds should be placed 25cm apart and the distance between seed-rows should be around one metre. Therefore, 3kg of seeds are needed for each hectare.
Weeds not only compete with the plants for water and fertilizer, but they also make it difficult to find and harvest green turions.
In the phase of formation and development of the feathery foliage, the part of the plant that is over the ground synthesizes the reserves accumulated in the crowns; during the harvest, there is a migration of said reserves to be used in the formation of the turions. Therefore, it is possible to use systemic weed-killers through the foliage during the harvest, as they only affect the turions and not just the underground part of the plant.
6.5. Density of plantation
The density of plantation for growing green asparagus should be between 25,000 and 30,000 plants per hectare. But if the asparagus is grown in a greenhouse, the correct density of plantation should be 33,000 plants/ha for a planting distance of 1x0.33m.
For 25,000 plants/ha, a planting distance of 1.25-1.50x0.30m is recommended.
After the end of the period of supply and restitution of the reserves taken from the plant during the harvest, the part of the plant that is over the ground starts to dry up.
The cutting should be done near the base of the stem. We recommend doing it mechanically, although it can also be done manually. In the market, there are motor-operated circular saws that can be used for this activity. The only disadvantage is that it leaves on the ground all cut vegetal material, which must be picked up.
A harrow can also be used, as it cuts the entire plant and removes the rests. However, to use it the soil must be levelled and the plants have to be very dry.
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