Revegetating black cracking clays PDF (191kb)

Introduction

The flats where black cracking clays occur are usually very open and windswept. The need for stock shelter and more wildlife habitat in these areas are prime reasons for revegetation, but the very openness and difficult qualities of the soil make tree-planting a challenging task. Species choice and ground preparation are crucial to achieve good results.

Soil description

Dark grey to black cracking clay soils (also known as ‘groundwater rendzina’) are mostly shallow, with only a few centimetres or up to half a metre of soil over a hard layer of limestone. The pH is mostly alkaline, around 8.5.

Traditionally these soils were found on extensive swampy flats and before drainage in the late 1800’s, were waterlogged for 6 to 8 months of the year. Drainage has meant the duration of waterlogging is now much less, sometimes only 1 to 2 months.

Although sticky and boggy when wet, the top-soil dries out very quickly, cracks open and turns crumbly with the onset of warm weather. The cracking nature can expose roots of new seedlings.

The original vegetation

The plains country was never heavily wooded but what vegetation did exist has now largely been cleared. Trees such as Eucalyptus ovata (white gum or swamp gum) and Allocasuarina verticillata (drooping sheoak) were scattered thinly, usually occuring where soils were slightly raised and hence less waterlogged, or transitional between the true black cracking clay and somewhat loamier clays.

Understorey plants consisted of shrubs such as Banksia marginata (silver banksia or honeysuckle), Allocasuarina paludosa (swamp sheoak) and Bursaria spinosa (Christmas bush) and sedges such as Gahnia trifida (cutting grass), Baumea juncea (bare twig-rush) and Isolepis nodosa (knobby clubrush). The shallower the soil over limestone and the more waterlogged the area, the more predominant sedges and grasses would have been compared to trees or shrubs. Melaleuca halmaturorum (salt paperbark) featured in brackish swampy country.

Species for revegetation

Table 1 shows species suitable for revegetation on black cracking clays. Plants should ideally be grown from seed collected from parent trees or shrubs that are growing in alkaline soils subject to periodic wet conditions to give the best chance of survival. Note that there are various degrees of alkalinity and severity of cracking at different sites - some sites have soils that are transitional between clay and loam and all of the species listed will do well on these soils, whereas sites that are extremely alkaline, very shallow over limestone and prone to extensive waterlogging will have limited success with some of the species (indicated by number of ticks given).

Table 1. Species suitable for revegetation on black cracking clay soils.

(more ticks = best options for true cracking clay soils; species with less ticks are better suited to loamier clays).

Site preparation

Weed control. Most black cracking clays have been sown with improved Phalaris and clover pastures and are prone to introduced weeds and thistles all of which compete strongly with new seedlings for moisture and light. To be successful with revegetation, you must get rid of all existing pasture and weeds well in advance of planting seedlings or direct seeding. This is usually achieved by spraying out the site with broad-spectrum knockdown herbicides such as glyphosate several times before planting or seeding.

Aim to start weed control the year before planting, in late spring and summer. Spray again in the year of planting/seeding just after the break of the season and again just prior to planting if a new flush of germination has occurred. Residual herbicides (eg Oust ®, Ally ®, Goal ®, simazine) can be used for more effective longer-term weed control but they can harm seedlings if not used appropriately (note that some are not registered for use with natives). Seek advice from revegetation consultants or contractors prior to use.

Weed control should be carried out again in the year after planting/seeding. This can be done with shielded sprays but special over-spray mixtures can also be used (consult revegetation experts).

Ripping to allow better root penetration should be considered if limestone is within 10 to 15 cm of the soil surface. Riplines in cracking clay can tend to open up later in summer so only rip if absolutely necessary. Rip in summer or early autumn when soils are dry so that the ripper shatters the soil rather than slices through it. Slicing glazes the soil surface and prevents lateral root penetration.

Ripping should be at least 50 cm deep or more if possible. Use a concrete roller (at least 60 cm wide) over the rip line afterwards to help crush rocks, remove air pockets and settle the soil. Avoid rolling with narrow tractor tyres as this can cause compaction and guttering. Where planting or spraying machinery is to be used, very large limestone pieces brought to the surface by ripping should be removed if they cannot be crushed.

Mounding of the soil to 50 cm high and 50 cm wide can help plant survival in waterlogged areas. Mounding is carried out with an offset grader blade or special forestry mounding machinery with two opposing off-set discs. Bear in mind though that the surface of mounds will dry out quicker in summer so this technique should only be used in very wet areas. When mounding in rows, consider leaving breaks every now and then to allow for natural water flows.

Cultivation is not advisable as it can lead to the soil drying out too quickly and may encourage wind erosion. A light scratching of the soil can improve germination for direct seeding.

Methods of revegetation

Plants can be established either by planting seedlings or direct seeding (where seed is spread out directly onto the soil after the site is prepared). Direct seeding can be effective where the soil is transitional to loam, but planting seedlings is much more reliable on the true heavy cracking clays.

Seedlings

Members of the organisation "Trees for Life" can obtain free native seedlings grown by volunteers (call 08 8406 0500) for membership details, a small fee is involved). Should you like to grow your own seedlings Trees for Life can provide materials and instructions. PIRSA’s Struan office also has propagation instruction booklets. To buy native seedlings it is best to place an order by November in the year preceding planting. Nurseries specialising in species for farm revegetation:

• Mimosa Farm Trees (Peter Feast), Eight Mile Creek: 8738 8070
• Reedy Creek Nursery (Mike Quarmby), near Kingston: 8768 7220
• Eucaleuca Native Services (Ralph Scheel) Naracoorte: 8762 2061
• Hardy's Propagating Nursery (Andrew Hardy) Mt Burr: 8734 8365
• State Flora (wholesale), Murray Bridge: 8539 2100.

Direct seeding

Direct seeding has not been a reliable method on black cracking clays unless the soil is transitional to loam. Seed can be either sown by hand (see “Hand direct seeding of native plants” Fact Sheet) or spread by special direct seeding machinery for large areas. Machinery working with press-wheel attachments, however, may get too clogged up on this sticky soil type. It is better to let seed fall on top of the soil surface, lightly scratched to form a good seed bed.

Autumn sowing results in new seedlings being smothered by weeds. Native seed needs warmth to germinate so there is no point in sowing before late August when soils are too cold. It is better to control weeds over winter and sow in early spring. Black cracking clays dry rapidly and seed sown too late in the year often fail. On the other hand the soil is sometimes too waterlogged to get on before October. Careful judgement is required depending on how the season is progressing as to when seeding is carried out.

Look out for red-legged earth-mite and if present, spray out. Newly germinated seedlings are vulnerable up until the four- to six-leaf stage.

Direct seeding contractors:

• Lower-Mid SE: Mimosa Native Farm Trees (Peter Feast), Ph. 8738 8070.
• Mid-Upper SE: SE Direct Seeding Service (Josie and Jamie Jackson),
  Ph. 8757 8276.
• General (large areas only): Greening Australia
  Ph. 8372 0100 or
  Trees For Life 8406 0500.

Further information

Peter Tucker
Environmental Consultant
Rural Solutions SA, Struan Office
Ph. 8762 9100.

Acknowledgments

This fact sheet is based on an original draft produced by Greening Australia with input from Darryl E. Kraehenbuehl, Neville Bonney, Jeff Whittaker and Ralph Scheel.

Last update: January, 2008

Agdex: 301/31

Author: Peter Tucker, Environmental Consultant and Zita Stokes, Revegetation Consultant, Rural Solutions SA Struan & Neville Bonney, (formerly of) Greening Australia SA.

Disclaimer

Use of the information in this Fact Sheet is at your own risk.  The Department of Primary Industries and Resources and its employees do not warrant or make any representation regarding the use, or results of the use, of the information contained herein in terms of its suitability, correctness, accuracy, reliability, currency or otherwise.  The entire risk of the implementation of the information which has been provided to you is assumed by you.  All liability or responsibility to any person using the information is expressly disclaimed by the Department of Primary Industries and Resources and its employees.