Many industrial regions have areas of derelict land which are gradually being landscaped. Additional costs are often incurred if substrate left on site is unsuitable for plant growth. This can be avoided if remediation and landscaping are budgeted and supervised in the site clearance contract when redundant structures are demolished. Involvement of a landscape consultant or project manager at an early stage of a redevelopment or reclamation project can save engineering costs and avoid the need to import topsoil for landscaping.

Similar considerations apply to the landscaping of areas surrounding industrial buildings and the establishment of low maintenance vegetation on road building projects.

The following specifications aim to ensure that a site is prepared for landscaping in the most cost effective manner.

Discuss a suitable landscaping plan with an ecologist or landscape consultant at the project planning stage.

Determine overall objectives and contours of the final landscape scheme before commencing the civil engineering stage.

Closely supervise the deposition of all material into which planting will be made to ensure that it complies with the specifications below.

Effective liason between the landscape manager and the civil engineers to achieve compliance with these principles will avoid costly mistakes.

The depth of rooting material should be a minimum of two metres for trees and a minimum of one metre for shrubs. These minima may be relaxed where they cover a basal structure conducive to root penetration. Subsoils such as clays, sands, well jointed chalk are readily penetrated by plant roots and require the minimum of soil covering. Old roadways, concrete floors, granite and other hard rock are unlikely to support mature trees unless covered to a depth of at least two metres.

Scattered concrete foundation blocks of up to five cubic metres can be tolerated, provided they occupy less than 20 per cent of the surface area and the rooting medium (infill) is mounded over them.

Topsoil, clay, sand, crushed stone, slag or crushed masonry are all acceptable materials. However, materials derived from former industrial sites, including slag tips and fuel ash should have been chemically analysed to ensure that they do not contain unacceptable contaminants, particularly heavy metals, before commencing the project.

· Crushed stone, masonry and slag should include a high proportion of sand-sized particles and have a volume mean particle diameter of less than 100 cubic centimetres.

· No more than 5 per cent should be decomposable organic matter such as wood or paper.

· No more than 10 per cent should be stones of housebrick size or greater.

· All of the above materials may be mixed as convenient in the same area.

Where there is a significant percentage of soil or clay in the infill, avoid soil compaction by refraining from driving loaded lorries over already tipped material. In contrast, if the infill is of unconsolidated large particles such as crushed masonry, compaction can be advantageous to soil moisture retention. The type of material being tipped should be monitored continuously and unsatisfactory loads rejected. In particular, guard against deposition of loads of uncrushed masonry.

As soon as tipping is complete, agree the intended final contours of the area with the site manager. Smooth the substrate to the agreed contours by bulldozer as soon as possible, working only when the substrate is sufficiently dry to avoid extensive compaction.

Do not cover areas to be used for tree planting with topsoil. Any loads of topsoil available should be reserved for planting pits or directed to areas intended for high quality grass. If perennial weeds appear, spot spray with a suitable systemic herbicide such as glyphosate for grasses and 2,4-D for broad leaved weeds. If annual weeds appear, spray with a suitable non-residual herbicide such as glyphosate before they flower. It is of the utmost importance that no weeds are permitted to seed on the site or on surrounding areas before planting.

Plant into a weed-free surface between October and March. Generally, trees and shrubs planted into in-situ topsoil can be slot-planted. Planting into poorly water retentive infilled substrates may require more expensive pit planting.

The required depth of rooting material depends on the standards of visual appearance desired of the vegetation and the quality of the material. Where a low aesthetic standard can be tolerated, a 10 cm layer of fine substrate will be acceptable. Where a high quality effect, with no summer browning of the vegetation is required, a 30 cm layer is necessary. Where the rooting medium is inhospitable clay or crushed stone, or the rooting material overlies an impenetrable layer such as concrete, these depths must be doubled.

For formal grass and lawns, a rooting depth of 20-30cm of well structured sand-sized particles or topsoil is recommended.

These are the same as those listed for planting trees and shrubs, with the following provisions:

• No particles of housebrick size or greater and no unsightly material such as plastic or paper should be present in the top 20 cm of infill.

• Where a mixture of substrates is used, an ecologist should be consulted to ensure suitable positioning of each substrate type.

• Where the material is crushed hard masonry or stone, it must contain a sufficiently high proportion of fines to retain moisture after several weeks drought and be well consolidated after deposition.

Soil, sand and particularly clay must on no account be excessively compacted by vehicles during any stage of the project.

As soon as tipping is complete, agree the contours of the area with the site manager. Smooth the substrate to the agreed contours by bulldozer as soon as possible, working only when the substrate is sufficiently dry to avoid extensive compaction.

If topsoil is to be spread on areas intended to grow high quality pure grass, do so immediately after levelling the site, without excessively compacting the subsoil. Do not use topsoil where wildflowers are to be sown except where the site is covered with clay. On pure clay, a 2-5 cm of weed free topsoil can greatly assist the establishment of grass and wildflowers. It is absolutely essential that weeds are not permitted to establish on the site or seed on adjacent areas before the site is fully planted.

If weeds appear, spot spray perennials with a suitable systemic herbicide. Spray annual weeds with a suitable non-residual herbicide before they flower.

Grass and wildflower mixtures establish best if sown in September. Sowing in February to the end of April can be successful, but soil moisture becomes critical for later sowing.

Recommendations for soil preparation, suitable seed mixtures and seed sowing are included in Herbiseed Brief Wild Flower Guides Numbers 2, 3 and 4. Nevertheless, for major projects you are advised to involve Herbiseed at the project planning stage to ensure that the most cost-effective land preparation and seed mixtures are chosen for the project.

Herbiseed, New Farm, Mire Lane, West End, Twyford, RG10 0NJ, England.
Tel: +44 (0) 1189 349 464  Fax: +44 (0) 1189 241 996  e-mail:[email protected]