Growing rhubarb in Western Australia

Introduction

Rhubarb (Rheum rhaponticum) originates from China and is a member of the Polygonaceae or dock family. The main part of the plant is the crown which is semi-woody and perennial. It is valued for its long, thickened, red stalks — up to 75cm long — which arise from the crown and carry the large leaves. The stalks are used as a dessert — mostly in pies — and have an acidic taste. They contain useful levels of acids (pH 3.0–3.6), sugars, fibre, protein, potassium, calcium, magnesium and vitamin C. The leaves are poisonous and may contain high levels of oxalic acid.

In Western Australia, a small amount is grown commercially throughout the year for the domestic market, mainly in the Wanneroo area. Local production is lower in the cooler months but imports from Queensland supply the market from April to October.

Climate and soils

Rhubarb grows best in cool to warm conditions and yields and quality decrease as the temperature rises above 27°C. The stalks colour best in cool temperatures. It will withstand slight frosts. Heavy frosts may cause dieback but the plants will resume growth with the onset of favourable conditions.

Rhubarb is a deciduous plant in Europe. In Western Australia it retains its leaves throughout the year but is less vigorous in winter.

Rhubarb is adapted to a wide range of soils in the pH range of 5.5 to 6.5 as long as they are well drained and prepared with large amounts of added organic matter such as conditioned poultry manure or compost.

Propagation and planting

Growers normally plant a new area each year. Rhubarb can be propagated by division of crowns into planting pieces or from seed. Crown division is preferred where suitable good quality material of proven varieties is available.

New areas are established, preferably from summer to early winter or from August to September, by planting vigorous, disease-free crowns. Growers have selected plants with desirable characteristics from seedlings, which can be reproduced by division of crowns. These are divided into planting pieces by cutting between the ‘eyes’ so that a piece of the crown has at least one eye. Six to ten cuttings can be made from a crown older than two years.

Named varieties are difficult to obtain in Western Australia and they are expensive. Growers may not supply their own varieties to other growers, and bulking-up of new varieties may be slow.

Variable plant types result from seedling plantings, but this serves a useful purpose in selecting superior types for subsequent multiplication by crown subdivision. This is the main method used by new growers entering the industry as it is cheap and seed is readily available.

Careful selection of superior plant types over a period of years has resulted in the development of local types with marked improvement in yield, quality and uniformity.

Plantings can be established from seed by transplanting seedlings raised either in open seedbeds or in containers, preferably in autumn. Transplants are put out in the field at six to eight weeks from seeding at 30cm apart and assessed for yield and quality. Many of the seedlings will be inferior, with stems which are too green. The best selections should be established and can be bulked up after two to three years by dividing the crowns.

The main plant characteristics selected for are high yield, regular cropping (five to six picks per year), straight, erect, evenly red-coloured stalks of uniform thickness and length, ease of harvesting and adaptability to seasons.

Green-stalked plants are as suitable for cooking as red-stalked varieties but the market perception is that these are unripe and therefore only red-stalked varieties are marketed. Green-stalked varieties are preferred in some countries.

The most popular plant spacings are 90 to 120cm between rows and 60 to 75cm between plants within rows. When dividing crowns, take care not to damage shoots or buds. Plant crowns and seedlings at the correct depth; the buds of the crowns should just be above soil level. Do not plant too wide apart, as this may result in increased plant flowering.

Growing the crop

Fertiliser

Rhubarb is a heavy feeder. Apply compost at around 50 cubic metres per hectare before planting and every year after planting. This adds organic matter to the soil, supplies nutrients and helps retain moisture.

Apply a fertiliser to supply 105kg/ha of phosphorus. This rate can be reduced on old vegetable ground or if compost is regularly used. The crop will respond to balanced fertilising of nitrogen and potassium every one to three weeks. Apply these nutrients as 80kg/ha of urea and 75kg/ha of muriate of potash. Do not apply too much nitrogen as this will result in excessive flowering and elongated, poor quality stalks. Apply 50kg/ha magnesium sulphate every three months.

Rhubarb is often affected by manganese deficiency on alkaline soils. This shows as a mottled chlorosis (yellowing) on the young and medium-aged leaves.
Apply the following rates of trace elements to the soil every 18 months:

• 20kg/ha manganese sulphate to supply manganese
• 18kg/ha borax to supply boron
• 18kg/ha ferrous sulphate to supply iron
• 18kg/ha copper sulphate to supply copper
• 18kg/ha zinc sulphate to supply zinc
• 2kg/ha sodium molybdate to supply molybdenum.

Analyse the soil and irrigation water for nutrients before planting, plus one to two analyses of the youngest mature leaves after planting. This will enable you to adjust the fertiliser program and provide information on nutrients that are deficient or toxic. Some of the suggested nutrients may be deleted or reduced if they are sufficiently high in the irrigation water and 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 groundwater pollution in rivers and estuaries.

Irrigation

Rhubarb requires a higher level of watering than most vegetables.

Plants must be kept well watered, with one watering per day in early morning from April to October — dependent on rainfall — and two waterings per day, half between 7 and 9am and half between 2 and 3pm from November to March.

Table 1 is a guide for watering, but there has been no research on rhubarb to obtain the precise watering needs of the crop. Over-watering after planting crown divisions may cause rotting.

Evaporation data from Medina Research Station, Western Australia represents average conditions and adjustments must be made for marked changes in temperatures, humidities, effective rainfall and wind speeds. Use evaporation data from the nearest meteorological station if your property is not situated near Medina.

Typical butterfly sprinklers are spaced at 277/ha with an output of 15L/minute or 4.15kL/ha/minute. Typical knocker sprinklers are spaced at 69/ha with an output of 22L/minute or 1.52kL/ha/minute. Irrigation time has been adjusted to compensate for the efficiency rating of butterfly (85%) and knocker (80%) sprinklers.

Disease, pest and weed control

The main disease of rhubarb is downy mildew (Peronospora destructor) which appears as brown patches on the leaves, especially after storms from May to October.

Rhubarb can be killed by crown rot caused by Rhizoctonia or Phytophthora diseases. These diseases may be difficult to control. Ensure that rhubarb is rotated every few years with other crops and that plants are grown on soils with good drainage.

Rhubarb may be affected slightly by Alternaria and Phoma species, which cause black spots on the leaves.

Few pests damage rhubarb but keep a look out for aphids, African black beetle, grubs, mites, slugs and snails, vegetable weevil and wingless grasshoppers. Sugar beet nematode (Heterodera schactii) may cause damage. The cysts on the roots look like sugar grains. Root-knot nematode will also damage rhubarb.

Control of weeds in a mature crop is difficult as deep mechanical hoeing may damage the root system. Shallow hand-hoeing may be the best means of weed control.

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 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 readily available.

Flowers

Remove flowers as they will cause difficulties with boom spraying against pests and will deplete nutrients required for future stalk production. Flowering is most common after cold winters and hot summers.

Harvesting the crop

Picking

Pull the stalks away from the plant with a downward motion so that the entire stalk is removed. Cutting the stems will result in rotting. Growers usually pick the mature stems and leave the four youngest stems. The plant is ready for picking just before the sides of the oldest leaves begin to turn down.

New plantings are ready for picking after nine to twelve months. When established, rhubarb may be picked every six to ten weeks throughout the year.

Yields

The best crops are produced between April and July. The stalks show most colour from May to December.

There may be a shortage of rhubarb in late winter and early spring when demand is highest and production is affected by lower temperatures. Prices are usually highest from June to September and lowest from November to March. A good yield is 15t/ha per year.

Packing and storage

The crop is washed, trimmed, graded and packed loose or in 0.5–1.0kg bunches (five to ten stalks), held by rubber bands, in 18 or 48L (10kg) plastic crates or cartons. Discard damaged and split stalks and trim the leaves to within a few centimetres of the main stalk.

Rhubarb can be stored at 0°C and over 90% relative humidity for up to two weeks.

Life of the crop

Rhubarb can be picked for 10 years, but it is preferable for a commercial crop to be kept for three to six years.

Acknowledgement

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