Amarillo peanut: a perennial groundcover for subtropical orchards

Series: Agnote DPI-332  Edition: Second edition  Last updated: 01 Sep 2002

 

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Introduction

Amarillo peanut (Arachis pintoi cv. Amarillo) is a low-growing perennial legumenative to Brazil, and a relative of the commercial edible peanut.

Introduced into Australia for testing as a pasture legume in 1954, it is ideally suited to the high-rainfall (greater than 1000 mm) frost-free areas of the subtropical NSW northcoast — such as the elevated krasnozem soils of the Richmond–Tweed region.

TheAmarillo peanut has been assessed over a number of years in various horticulture crops and identified as a suitable groundcover species for macadamia. It meets most of the criteria of an ideal permanent orchard groundcover species:

  • permanent low-growing perennial, maximum height 30 cm, providing continuous year-round cover;
  • non-climbing;
  • tolerates shade;
  • tolerates competition from grass and effectively excludes broadleafed weeds;
  • some drought hardiness — does not die out in a dry spring (although growth is reduced);
  • some frost tolerance — leaves and stolons are killed by frost, but plants survive and shoot again in spring;
  • tolerates high levels of aluminium and manganese typical of the acid soils of the region;
  • can fix some atmospheric nitrogen in soil;
  • is not a host for common pests and diseases of orchard species;
  • will establish quickly with reasonable soil preparation and management.

Points of concern are as follows:

  •  may compete with some orchard species, particularly young trees;
  •  may attract or harbour undesirable vermin, such as rats and mice, when left unmown;
  •  requires regular mowing in summer/autumn.

Function in the orchard

Orchard tree crops are relatively long term, so any management program needs to ensure that both the physical and chemical status of the soil is maintained and erosion minimised.

A common orchard management program is to maintain a mown grass sward in the orchard interrow and a bare herbicide strip along the tree row.

Because some tropical grassesadapt poorly to low-light situations, they tend to die out as the orchard canopy enlarges. This leaves soil exposed to erosion, especially on steeper slopes. Groundcovers adapted to low light can protect the soil surface from erosion. The densestolon mat and leafy growth of Amarillo peanut helps reduce overland water flow anderosion. The destructive effects of large water droplets falling through the orchard canopy is minimised. In well-established peanut groundcover, soil structure is improved under the stolon mat. Soil compaction, as measured by penetrometer resistance, can be reduced down to 10 cm soil depth in 3 years.

In addition, Amarillo peanut, either alone or in combination with suitable grasses, promotes a more manageable orchard cover that requires less mowing in the interrow than some tall grass species.

Preparing for planting

Keep soil disturbance to a minimum to reduce potential erosion, especially on steeper sloping land. On reasonably level land, kill off vegetation with a registered herbicide before planting. A registered pre-emergence herbicide can also be used for residual weed control.

Where a mixed grass–legume stand is required and the desired grass is already established, use a registered desiccant herbicide to burn off the grass without killing it. This technique can also be used on steeper sloping land. Using a desiccant herbicide will give the legume time to germinate and establish with minimum competition.

Planting

Plant Amarillo peanut after the soil has warmed in late spring to early summer. In a very dry spring, delay planting until sufficient rain falls for germination and establishment.

Broadcast molybdenised superphosphate at a rate of 500 kg/ha, less if super has been applied in the past 2 years. You may incorporate it with seed if you are using a sodseeder. Alternatively, use ordinary superphosphate and coat the seed with molybdenumtrioxide to enhance rhizobium activity and correct possible shortages of molybdenum inseed.

Inoculate seed with the specific rhizobium inoculum available for Amarillo peanut, and then lime pellet before planting. Seed is currently sold in the shell and it is not necessary to remove the seed before inoculation. Amarillo peanut seed does not store well. Always check germination prior to planting.

Seed must be buried in the soil for reliable germination. Small rills up to 5 cm deep need to be opened up with a sodseeder or tined cultivator at a row spacing to suit the desired sowing rate. A row width of about 50 cm ensures rapid soil cover by the plant. Asowing rate of 20–25 kg/ha is recommended. An appropriate seed spacing for this rate is seven rows, 50–70 cm apart between orchard tree rows. Plant seeds 2–3 cm deep at intervals of 30 cm along the row.

Amarillo peanut will establish easily from cuttings. Cuttings 15–20 cm long buried 2–3 cm in soil, with a few centimetres protruding, quickly establish and spread if planted in showery weather in late summer. Dip the cuttings in an inoculum solution to ensure effective nodulation.

Species mixes

Amarillo competes with grasses. The combination with grasses provides fibre and bulk for use in under-tree mulching. Species such as bahia grass (Paspalum notatum), perennial rye, kikuyu and Rhodes grass are suitable.

It should combine well with grass species recently assessed in macadamias including Shadegro (Panicum laxum) and Microlaena stipoides. However, persistence when grown with sweet smothergrass (Dactyloctenium australe), which grows best in the shade of mature macadamias, has not been tested.

Establishment

With enough moisture, seeds germinate in 2–3 weeks. In good growing conditions, weeds and unwanted grasses will compete — unless residual pre-emergence herbicide has been used before planting.

When a mixed legume–grass stand is required, suppress broadleaf weeds. Broad-spectrum herbicides cannot be used. Control weeds by slashing them off about 10 cm above the height of the peanut, which should be spreading close to ground level in the first few weeks after emergence. Spot spray or use a ropewick applicator on difficult-to-control weeds.

If grass tends to dominate early, apply a selective registered herbicide to kill grass.

With little competition, good growing conditions and the recommended sowing rate, Amarillo peanut will cover the ground area within 3 months. Wider row spacings may take longer. The soil will be completely covered in 4 or 5 months.

Management of established stands

Management will vary according to the planting mix, amount of weed growth present, and the depth of groundcover required.

Spring and summer management normally requires mowings at monthly intervals in the interrow area to keep the stand about 10 cm high. Growth in the shade under the tree canopy is less vigorous and can be controlled with an offset slasher.

In macadamia orchards, mow to ground level under the trees, or use a desiccant herbicide in late January. This will suppress growth before the first harvest in late February.

Reducing covercrop canopy at this time also reduces rat harbours in the orchards. Rats are attracted to the underground nuts (tubers) when the peanut is left unmown for long periods. However, an active baiting program and elimination of perimeter harbour areas reduces rat problems and the likelihood of the peanut acting as an extended harbour.

Reduced growth rates have been observed in young macadamia trees where the peanut has been established for 2 years. It is likely that strategic ‘knockdown’ with a herbicide will be necessary under young, unirrigated trees in a dry spring. The benefits of a legume can still be obtained by suppressing growth for short periods in the year.

Amarillo peanut is best left as an unmown canopy in the interrow from late summer to early spring in the first year. This allows sufficient depth of groundcover to build up, which will prevent weed growth in the dormant season.

Performance compared with other species

Recent trials in macadamias show that Amarillo peanut is still a useful legume species, although it is not as shade-tolerant as sweet smothergrass, which grows better in very low light near tree trunks in mature orchards. However, sweet smothergrass may be more competitive with young trees.

Summary

In summary, Amarillo peanut is a useful perennial legume for the orchard floor, either alone or in combination with grasses: 

  • It establishes quickly compared with other useful groundcover legumes, such as Maku lotus, and tolerates grass competition better.
  • It forms a dense mat that withstands the passage of machinery and helps protect the soil from compaction.
  • It is adapted to low light in orchards, although sweet smothergrass provides better ground cover in very low light under mature macadamias.
  • It is one of the most effective species for protecting soil from erosion. In addition, because it is a legume, it will add nitrogen to the soil when effectively nodulated.

Its major limitations are: 

  • its tendency to compete with orchard species if growth is not controlled near the stem;
  • its potential to harbour vermin in unmown stands within the orchard.

Further information

For further information contact your local District Horticulturist or agriculturalenquiries officer.