Asparagus is a perennial vegetable arising from a root system of fleshy rhizomes known as the “crown” with long feeder roots that can grow to a depth of 1.5 - 2 metres. Since asparagus is a deep-rooted crop, soils need to be well drained. Alluvial loams, peaty soils and sandy loams are the most suitable soils for growing asparagus.
In the Koo Wee Rup area of Australia, asparagus seed is sown in nursery beds in early September and produce young plants, which develop a “crown” under the ground. The needle-like leaves are modified branches that function as leaves in the axils of the scale, which are reduced leaves. The “crowns” are harvested with a potato digger and replanted in single row beds with plants about 25-30 cm apart between May and September
In spring the crown sends up shoots (spears), which are the edible portion of the plant. Spears continually emerge from the soil and can reach their marketable length of 25 cm in 24 hours if humid conditions prevail.
In the first season following transplanting of the “crowns”, spears are only harvested for three to four weeks, with the main objective being to strengthen the plant through maximum fern development and maximum crown growth.
The fern stage is very important to the plant as the “leaves” photosynthesise and produce energy, which is then transported back to the crown where it is stored for the following season’s growth. Prolonged harvesting or stress can impact on the following year’s yield potential if the plant is not allowed to progress to the fern stage.
The asparagus fern dies down between April and May as the plant undergoes a natural dormancy period over winter. The debris from the fern is usually slashed and mulched back into the soil, with care taken not to damage the crown. Other methods of removing the fern debris include using a flame burner.
Asparagus is dioecious, meaning the small yellow or pale green male and female flowers are normally borne on separate plants. The female plant produces small round berries that turn red when ripe (see pictured).
The sandy loams and warm climate in the Mildura and Swan Hill districts in Northern Victoria favour asparagus production between August and November, while the peaty loams and the cooler climate in the Koo Wee Rup area support production between September and December for the majority of growers with some growers continuing to harvest throughout summer extending the harvest season to the end of March.
Click here to download the Asparagus Growing Cycle diagram.
Asparagus spears emerge through the soil continuously throughout the season (although there are distinct flushes of emergence) and must be harvested before the tips begin to “fern out”. When the season begins spears may only need to be cut every third or fourth day, but as the season progresses and temperatures rise spears must be harvested every 24 hours. At the height of its growing season, in warm humid conditions, single spears of asparagus can grow over 2 cm in one hour!
Emerging spears that have reached the desired length of at least 25 cm of green colouring are cut with a long-handled asparagus knife at or below the soil surface. The cutter threads the cut spears between his fingers (see pictured) so the heads are not damaged and drops them in a bunch on the soil bed awaiting collection into crates and transferred to the packing shed.
Clydesdale horses were originally used to pull sleighs carrying the crates which meant bunches could be stacked directly into the crate. Today, growers use modified, self-steering pick up machines. These machines are unique to the asparagus industry and were manufactured and developed by grower Glen Roberts.
Maurie Cafra, an organic grower, has developed an electric harvesting aid that travels along with the cutters so bunches of asparagus can be placed directly into plastic crates rather than placed on the bed first. Reduced handling keeps the spears in better condition, preventing gravel rash damage and overheating from resting on warm soil during summer. Using the harvesting aid also reduces mud coming into the shed on wet days.
Asparagus spears are highly perishable once harvested. They also continue to develop after harvest, so cool chain management is essential. Wet hessian bags or weights are often placed on the top of asparagus-filled crates to prevent the horizontally laid spears from bending upwards in search of light before packing.
Full crates are sometimes passed through a pre-wash to loosen any dirt or debris and to provide some moisture so that the spears do not desiccate or lose their glossy appearance. In some cases the water is cooled to try and reduce field heat before packing.
Traditional practice was to sort and pack asparagus manually. However, most growers have now invested in computerised grading systems. Asparagus is brought in from the field and pre-washed in its crates to loosen any dirt and debris. The spears are then placed onto the header belt with all the spearheads touching the top edge so that they can be trimmed to a set length.
The spears are washed thoroughly with clean water, and are then straightened and loaded into individual cups of the grader. Any spears with obvious defects are removed at this stage.
Spears are trimmed to 24 or 25 cm. A photo is taken of each spear and the computer measures the length and diameter of the spear and the presence of white on the butt, then it designates a chute where that spear will be released. Spears that are too short, or still have a white butt, pass through the line and are trimmed at a later stage. Spears are collected from the chutes and packed according to their grade and market destination. The final stage in the packaging process is hydro-cooling the asparagus to remove field heat and prolong shelf life.
Regardless of whether the asparagus is manually or automatically graded, the principle is the same. The blemished, bent or insect-damaged spears are removed and then are cut to a common length and graded according to the spear diameter. Asparagus grades include M (9-12 mm), MM (12-15 mm), L (15-21 mm) and LL (21-27 mm).
Some markets demand that the spear be fully green with no white colouring around the base and so spears destined for these markets may need to be trimmed a second time to remove white ends.
Loose and bunched spears are packed upright into specially designed wooden boxes for export. These boxes are called “pyramid boxes” they have a flared base because the asparagus butt is thicker in diameter than the spearhead. Product destined for the domestic market may be loose or bunched and is usually packed directly into polystyrene rectangular boxes or recyclable plastic core flute “pyramid boxes”.
Once packed the asparagus is hydro-cooled to remove field heat and is placed immediately in cool storage. Maintenance of the cool chain is vital to prevent continued growth of spears and deterioration in quality and storage life. Pallets destined for export are sent by refrigerated transport to Sydney, Melbourne or Brisbane airports.