Growing parsnips in Western Australia

Introduction

Parsnips (Pastanica sativa) belong to the Apiaceae family which also includes carrots, celery and parsley. Parsnips are biennial but are grown commercially as an annual. The edible portion is the enlarged fleshy tap root.

There is good demand for parsnips throughout the year, especially in cooler months. Production is mainly from Wanneroo, Carabooda, West Gingin and Myalup.

Soil, climate and rotations

Parsnips have an effective rooting depth of 35 to 45cm and grow well in deep, sandy soils. A soil pH of 6.0 to 7.0 is adequate for growth. Acid soils should be limed before planting and heavy soils and stony soils are not suitable.

The optimum temperature range for growing parsnips is 16–20^oC and the foliage may be scorched above 30^oC. Roots are not harmed by frost but cold weather may result in loss of roots due to flowering or ‘bolting’.

The optimum rotation is to include one crop of parsnips in the rotation every four years to avoid diseases such as Rhizoctonia and canker. Do not include too many crops of carrots, celery and parsley in the rotation as these are in the same family as parsnips.

Parsnips are often grown following a well fertilised leafy crop such as brassicas or lettuce.

Varieties

For many years, the major varieties in Western Australia have been the open-pollinated varieties Melbourne Whiteskin and Hollow Crown. Another open-pollinated variety, Dusk, is supposed to be easier to establish in summer when parsnip germination can be a problem.

Newer F1 hybrid varieties that promise better disease tolerance are now also available. Check with seed companies for current varieties.

Growing the crop

Parsnips can be seeded throughout the year in Perth and the South West but germination is better during cooler months. Seed remains viable for one year if kept cool and stored in a dry container.

Row spacing will depend upon the type of harvester to be used. Sow the seed in the field at 5 to 15mm deep in rows 30 to 40cm apart. This requires a total of 3.3 to 4.6kg/ha. They may also be planted in double rows with 40cm centres, with the two lines in the double rows 6cm apart and plants at 8 to 10cm within the rows.

If seeds are hand sown, thin seedlings to 75mm three weeks after germination. Roots may be deformed if the plants are crowded. Sowing seed with a vacuum or air seeder results in precise spacing and plants are usually not thinned.

At optimum temperatures, germination takes about 14 days but may take four weeks in cold conditions. Establishment is better in the cooler months. Germination percentage can be low in summer. The seed is more difficult to germinate than carrots and requires regular moisture for establishment.

Parsnips may be seeded with a nursery crop such as cereal rye which germinates more quickly and protects the seedlings from wind and sand-blast. The cereal rye is sprayed-out when the parsnips have three leaves.

Fertiliser

Applying compost at up to 30 cubic metres per hectare to other crops in the rotation will supply organic matter to the soil, add nutrients and help to retain moisture. Applying compost before sowing parsnips is not recommended because of the risk of increasing root forking with fresh organic matter in the soil.

Apply the following trace elements and magnesium per hectare before planting:

• 11kg borax to supply boron
• 18kg copper sulphate to supply copper
• 18kg ferrous sulphate to supply iron
• 25kg manganese sulphate to supply manganese
• 2kg sodium molybdate to supply molybdenum
• 16kg zinc sulphate to supply zinc
• 50kg magnesium sulphate to supply magnesium.

The program in Table 1 shows the main fertiliser requirements for a crop planted on the Swan Coastal Plain and which is harvested 21 weeks after planting.

Notes:

1. This program refers to Karrakatta sand with a Colwell soil phosphorus test of 50mg/kg. If the soil test is above 50mg/kg, reduce the rate of double superphosphate. Double superphosphate is preferred to ordinary superphosphate because of its lower cadmium content.
2. Reduce the nitrogen rates if analysis shows more than 10mg/L nitrogen in the irrigation water.
3. Fertilisers are best applied by fertigation, provided that sprinklers are applying water uniformly.

Analyse soil and irrigation water for nutrients before planting, plus one to two analyses of the youngest mature leaves after planting. This will provide information on nutrients that are deficient or toxic and allow you to adjust your fertiliser program. Some of the suggested nutrients may be deleted or reduced if they are sufficiently high in irrigation water or soil, including sources from compost and fertilisers from previous cropping.

Do not apply excess fertilisers because nitrogen, phosphorus and potassium are easily washed through sandy soils by rainfall and irrigation. This may lead to pollution of groundwater, rivers and estuaries.

Irrigation

Parsnips have a higher moisture requirements than many other vegetables. As a guide, apply 170% evaporation replacement on the Swan Coastal Plain once the crop has reached full ground cover. Use an efficient irrigation system with good uniformity.  In warmer months, apply half the water from 7 to 9am and half from 2 to 3pm. In cooler months, apply all of the water in early to mid-morning. To avoid root splitting, do not allow the roots to dry out.

Ideally, water used for irrigation should contain less than 500mg/L of total dissolved salts, or have an electrical conductivity (EC) reading of less than 90mS/m.

Table 2 shows the amount of irrigation water required in the Perth area in different months, based on average evaporation at Medina Research Station. The irrigation data have been adjusted to compensate for the average efficiency rating (85%) of butterfly sprinklers, spaced at 277/ha with an output of 15L/minute or 4.15kL/ha/minute. Adjust irrigation time if the sprinklers have a different output to the above and for marked changes in temperature, humidity, effective rainfall and wind speed.

Pests, diseases and weeds

Canker (Itersonila pastinaceae) can be identified by small spots on the leaves and may cause some loss of yield and quality in February and March-planted crops and to a lesser extent to crops planted in April and early May. The following control measures are recommended:

• Harvest the crop as soon as possible once market maturity is reached.
• No effective fungicides are registered for control of canker. Applications of a copper hydroxide fungicide used to control leafspot will slightly reduce the incidence.
• Adopt a rotation of four years between parsnip crops. If disease reduces marketable yields, discuss management options with your local adviser or consultant.
• Powdery mildew causes a powdery white appearance of the leaves. Check with your adviser or consultant regarding control options.

Pests

Root knot nematode and other nematodes may cause serious problems. Discuss management options with your local adviser or consultant.

Two-spotted mite is a major pest, especially from February to June. Infested foliage looks unhealthy and mottled. Mites are visible to the naked eye and recognised by a dark spot on either side of the back.

Aphids, leafhoppers, redlegged earth mite and thrips can sometimes cause damage. Weevils and black beetles may damage the roots.

Parsnips are often seriously affected by an orange (rusty) discolouration on the roots. The cause is not known.

Weed control

Parsnips compete poorly with weeds which must be controlled early in the life of the crop. Check with your adviser or consultant for registered herbicides that may be used on parsnips.

The registration and availability of chemicals for disease, pest and weed control change regularly. Consult a trained and experienced horticultural agronomist or the Australian Pesticides and Veterinary Medicines Authority (APVMA) website for chemicals which are currently registered or have a permit for use on this crop. The information on the label or permit for a chemical must be followed, including the directions for use, critical use comments, withholding period and maximum residue limit.

Quality assurance (QA) schemes for horticultural crop production require producers to have current information on chemical registrations and permits at their fingertips. The information to be found at this website allows this requirement to be met.

Harvesting and marketing

The time from sowing to harvest varies from four to six months. At harvest, roots should be 250 to 400mm long and 30 to 70mm in diameter.

The harvest period may extend for up to two months without damage to the crop in cool weather. In spring, harvest promptly as flowering (bolting) occurs and adversely affects root quality.

Harvest with care by hand or machine to avoid damage, possibly with a modified carrot harvester (top-puller) or potato harvester (digger-elevation). If these are not available, use a modified blade to loosen the soil 30cm below the soil surface and harvest the crop manually.

The standard system is to slash half the tops and then use a carrot harvester to lift the leaf-stalks and pull up the roots. A good yield is 25 to 35t/ha.

Parsnip roots and leaves contain compounds (furano coumarins) which may cause inflammation of the skin so wear protective clothing.

After lifting, wash parsnips gently to avoid bruising the roots. Do not allow them to dry out at any stage as this can result in discolouration. Grade according to condition and length.

Remove leaves and market straight, sound, white roots. Pack in 22L or 36L crates which have a plastic liner or pack in 10kg cartons.

Parsnips can be successfully stored for two to six months at 0^°C and a relative humidity of 90 to 95%. At 0 to 2^°C starch in the roots converts to sugars and results in a high quality product after two weeks of storage. Do not store with ethylene-producing crops such as apples, bananas and tomatoes as this may impart a bitter flavour to the roots.

Acknowledgement

The original version of this material was authored by John Burt.