Introduction
Jarrahdale, Japanese and butternut pumpkins are annual vegetables in the cucurbit family that includes cucumbers, melons and zucchini. They have vigorous, prostrate vines and produce fruit with a hard shell and yellow to orange flesh.
The traditional large round, slightly flattened, ribbed type of pumpkin (Cucurbita maxima) has strong, round stems, large roundish, not deeply lobed leaves and a round fruit stalk. The main variety is the Jarrahdale type, a selection from Queensland Blue.
Butternuts and Japanese pumpkins (both Cucurbita moschata) have lobed leaves with whitish blotches and angular stems. Japanese pumpkins are less vigorous than Jarrahdale and butternuts are the least vigorous of the trailing types. Japanese pumpkins have become popular because of their flavour, and production is now only slightly less than Jarrahdale.
Pumpkins are marketed from Carnarvon from late May to January and from Kununurra from late June to November. Production in summer and autumn is from Perth and the south-west. Kununurra is now the biggest growing area in Western Australia and also sends fruit to Eastern States markets.
Jarrahdale and Japanese pumpkins comprise about three-quarters of total production, with butternuts making up most of the remainder.
Pumpkins are often grown by farmers in south-western Australia as an opportunistic cash crop because labour and financial requirements are less than for most vegetables. The crop can also be harvested at one time and stored for many months. However, for these reasons, supply may exceed demand, leading to poor prices.
Climate
Pumpkins need temperatures greater than 22oC to grow and mature. They are killed by frost and need to be protected from strong winds.
Soil
Deep, well-drained loamy soils are preferred. Pumpkins will grow and yield well on heavy soils unsuitable for many other vegetables. Organic matter must be applied to sandy soils to produce good yields. The optimum pH (CaCl2) is 5.8 to 7.0. Pre-plant cultivation usually includes at least one deep cultivation to allow good root growth.
Varieties
Japanese pumpkins are slightly smaller and flatter than Jarrahdale, with a drier, sweeter flesh and a mottled yellow and green skin that is easier to peel. They do not store as well as Jarrahdale pumpkin and have slightly lower yields. Average fruit weight is 3 to 4kg.
Butternuts have smooth, beige skin. They are sweeter than Jarrahdale with a nutty flavour and are easier to peel. Fruit size varies from 0.5 to 2kg.
Jarrahdale has a tough, green-grey skin and has a diameter of about 25 to 30cm. Fruits average 4 to 8kg, with 5 to 6kg preferred at the markets.
Golden nugget, sweet dumpling and minikin are small-fruited (6 to 12cm diameter) bush types that set their fruit around the central stem. They have good flavour, but a short storage life and moderate yields.
Sowing
Seeds need a minimum soil temperature of 16oC to germinate after 7 to 12 days. The best air temperature for germination is 21 to 35oC.
A total of 1.5 to 3.0kg of fresh seed will be needed per hectare. Two to three seeds may be planted 2 to 4cm deep at each site for open-pollinated varieties, and one to two seeds per site for hybrid varieties. If there is sufficient labour, seedlings are thinned to one per site after germination. A precision seeder may be used to obtain the correct seed spacing.
In the south-west, plant from October to December. Many growers use a cup-type planter to sow seed with a band of fertiliser.
In Perth, pumpkins may also be transplanted in cell-packs from mid-August so that plants are ready for harvest before Christmas. Normal plantings are direct-sown from late August to early January.
In Carnarvon, sowing is from mid-January to early September. In Kununurra, seeds are sown from late March to August.
Large-fruited pumpkins are planted in rows 1.8 to 3.0m apart, with 1.0 to 3.0m between plants within the rows. Butternuts are planted at 1.5 to 2.5m between rows, with small-fruited pumpkins planted at 1.5m between rows. Both butternuts and small-fruited pumpkins are planted at 0.6 to 1.0m within the rows.
Fertilising
Pumpkins do not require much fertiliser as the roots are efficient in absorbing available soil nutrients.
On loam and heavy soils, apply a nitrogen, phosphorus, potassium (NPK) based fertiliser to supply 35kg/ha (banded) to 84kg/ha (broadcast) phosphorus. Apply one or two dressings of 46 to 57.5kg/ha nitrogen and 50kg/ha potassium at three and six weeks after planting.
In Carnarvon, apply sufficient phosphorus-based fertiliser to supply 21kg/ha phosphorus before planting and two dressings of nitrogen-based fertiliser to supply 32kg/ha nitrogen at three and six weeks after planting.
On the sandy soils of the Swan Coastal Plain, apply phosphorus-based fertiliser to supply up to 87kg/ha phosphorus before planting. Apply nitrogen and potassium weekly from a week after planting to two weeks before harvest, building to a peak at early fruit setting stage and declining towards harvest. Apply to a maximum total of 299kg/ha nitrogen and 300kg/ha of potassium per crop. Also, apply 5kg/ha magnesium five weeks after planting.
Trace elements are generally not needed if they have been applied in the last two years. Heavy soils need only copper, zinc and molybdenum. Trace elements are usually not needed in the Carnarvon region. On the Swan Coastal Plain, apply a general trace element mix to vegetables every 12 months. This would include manganese sulphate (25kg/ha), copper sulphate, ferrous sulphate, zinc sulphate and borax (each applied at 18kg/ha) and sodium molybdate (2kg/ha).
Sodium molybdate at 1kg/L may be needed as a foliar spray three weeks after planting, especially on acidic soils. Plants with molybdenum deficiency are stunted with pale leaves, and scorching on the margins of the leaves.
Most fertiliser can be applied through the irrigation system if required. If irrigation water contains some nitrogen and potassium, reduce fertiliser rates.
Soil analysis before planting can give information on the nutrient status of the soil. Leaf analysis of the youngest mature leaf a few weeks after planting will provide a guide to whether the fertiliser program needs to be modified.
Irrigation
Pumpkins have a deep root system but need large amounts of water when they are growing vigorously. Irrigation is needed on all soils when rainfall is insufficient. Water stress will substantially reduce fruit set and yield.
A full canopy crop will need to be irrigated to 100% of evaporation replacement on sandy soils. Water can be applied by sprinkler, drip system or furrow. Furrow irrigation is the main system in Kununurra. Trickle irrigation requires the least water, especially when combined with polythene mulch.
Tensiometers can be used to measure soil moisture levels and provide a guide on when to water.
Pumpkins have medium tolerance to saline water. Yields decrease as the electrical conductivity (salinity) increases above 130mS/m.
Allow the soils to dry out as the vines mature to allow the fruit to ripen with less risk of rots.
Weed and pest control
Weeds
Weed control is especially important for the first six weeks, after which pumpkins are vigorous and will suppress weeds. Weeds can be controlled by cultivation, plastic mulches, hand weeding and herbicides.
Perennial weeds such as couch should be controlled the year before a crop is planted with a herbicide that contains glyphosate.
Control weeds by shallow cultivation (less than 8cm) between the rows until the vines start to run. Hand weed between the plants in the rows.
The use of polythene mulch (90cm wide) with trickle irrigation gives good control of weeds.
Pests
There should be a break of at least two years between cucurbit crops on the same ground. This will reduce pests such as nematodes and Fusarium fungi, which can cause the disease brown etch.
Caterpillars, mites, rats, snails, two-spotted mites and thrips may attack pumpkins but are usually not serious. Heliothis caterpillars may burrow into the fruit. Aphids should be controlled as they are a vector for many viruses, transferring the virus between plants within a crop.
The registration and availability of chemicals for pest, disease and weed control changes 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 readily accessible.
Cockatoos occasionally damage fruits severely in northern areas, up to one month before maturity.
Mice may eat the seeds, especially when planted on polythene mulch.
In the Kimberley, aphids, pumpkin beetle and 28-spotted ladybird beetle are the main insect pests.
Root knot nematode (Meloidogyne sp.) is damaging to pumpkins. Roots appear with small lumps. The pest is worst on sandy soils and irrigated areas that are cropped repeatedly.
The major leaf disease is powdery mildew which shows first as whitish to greyish areas on older leaves, mainly after flowering. It will also affect the stems. Bad infections result in small fruits. If the leaves wither, this can also result in sunburnt fruit. The disease must be controlled with fungicides every 7 to 14 days, as soon as it is seen on the leaves. Black spot (Alternaria cucumerina) is seen as small brown spots on the leaves and caused by a fungus.
Fusarium wilt causes plants to become yellow, stunted and wilted but is not often seen in well drained soils or where a rotation is practised.
Cucumber mosaic virus and zucchini yellow mosaic virus often cause severe damage to the fruit and plants at Carnarvon and Kununurra, and sometimes in the Perth area. Infected fruit has large knobby outgrowths and the leaves are mottled. There is no treatment for infected plants. However, planting on reflective mulch (to repel the aphid vector) and watering by trickle irrigation will help to reduce infection.
Fruit rots and brown etch of butternuts can be severe on the rind of the fruit in contact with the soil, especially after wet weather. They may be caused by several soil-borne fungi. Brown etch consists of concentric marks on the rind of the fruit and often with small cracks. It is reduced by restricting water in the last month of growth, followed by harvesting as soon as fruits mature.
Too much rain may cause rotting of young pumpkin fruits and corky outgrowths in butternuts. Scarring on the fruit may be caused by too much wind.
Woodiness or ‘boning’ in the flesh of the fruit may be caused by high temperatures and poor watering.
Pollination
Pumpkins have separate male and female flowers on the same plant. The male flower is large, yellow and has a long thin stalk and a spike in the middle of the flower. The female flowers are large and have a small swelling at the base.
There are normally sufficient wild bees for pollination in a commercial planting. If pollination is poor, especially in Kununurra, introduce one to four hives per hectare. An indication of poor pollination is small, deformed fruits with a small number of seeds or fruits that turn yellow and do not develop.
Pesticides should be sprayed in late afternoon when bee pollinators are less active.
Harvesting
Fruit mature five to six weeks after pollination. Poor fruit set may be due to poor pollination, disease, low temperatures, dry or wet conditions, low potassium, high nitrogen levels or too many established fruits.
Pumpkins mature about 14 to 22 weeks after planting, with the shortest time for butternuts and the small-fruited types, and the longest time for the large-fruited pumpkins.
An indication of maturity is cracking and corking on the stalks. The skin of the fruit changes to a dull colour, the flesh is deep yellow to orange and fingernails cannot penetrate the rind of the fruit.
Early crops of butternut and Japanese pumpkins may be harvested two to three times by selecting the most forward fruit, which is fully coloured with no trace of green on the skin. This may be sent straight to market. For most crops and for storage, the crop is allowed to mature on the vine and harvested when the vine has completely died off.
Storage enables marketing when prices rise to an economic level. The stems must be cut so they are 20 to 30mm long. Jarrahdale types will store for four to six months in winter/spring. Butternuts store for less than four months. Usually, they are stored at ambient temperatures.
The pumpkins must be mature and placed unwashed in a dry, ventilated and shaded area. If loose, they should preferably be placed on a slatted floor and stacked less than 1.0m high or less than four to five deep. They are often stored in shade in bulk bins and sometimes loose under trees. Remove rotten fruit regularly. They may also be kept in a coolroom at 10 to 13°C and 70 to 90% relative humidity.
Pumpkins yield from 12 to 40t/ha. Jarrahdale is usually the highest yielding, followed by Japanese, butternuts and bush pumpkins. About two to four fruits are obtained from each plant.
Marketing
Jarrahdale and Japanese pumpkins can be sold in half tonne or one tonne plastic bins or collapsible cardboard containers. Butternuts are mainly sold in 36L plastic containers or cartons. Small-fruited pumpkins are mainly sold in 22L containers.
Large pumpkins are usually cut into pieces and wrapped in clear plastic for retail sale. Butternuts are sold entire or are cut into halves.
Pumpkins may also be marketed as skinned, diced pieces wrapped in clear plastic for ready use.
Seed production from open-pollinated varieties
Seed may be saved from open-pollinated crops of butternuts and Jarrahdale pumpkins. Do not save seed from areas of mixed pumpkin varieties or from hybrid varieties, as it will not grow true to type. Crops must be more than 500m away from other pumpkin varieties, zucchini or squash if harvested for seed.
Giant pumpkin competitions
Competitions to find the heaviest ‘novelty’ pumpkins are held annually in every state. These are conducted in Western Australia in April. Special varieties such as Atlantic Giant are used, but are of poor eating quality.
The Australian record was established for a pumpkin weighing 300kg grown at Port Lincoln, South Australia in 2002. The record in Western Australia is 231kg for a pumpkin grown in Albany in 2000.
Acknowledgement
The original version of this material was authored by John Burt.