11. Seeding and planting
Where roadside planting or sustainable drainage system is proposed for adoption, it is important to ensure planting and biodiversity will be established.
Topsoil
Topsoil is the top layer of soil which can support plant growth and contains a mixture of organic material and nutrients, and shall be removed from the area the road or footpath is built on and should be stockpiled for re-use. Imported topsoil shall be Class A multipurpose in accordance with BS 3882 Clause 4.1.
For sustainable drainage systems a mixture of 70% sand and 30% topsoil shall be at the base, known as the root zone. The topsoil shall in accordance with BS 3882:2015 and the sand shall meet the following requirements:
- The rate at which water passes through when saturated, known as saturated hydraulic conductivity, to be greater than 220mm per hour.
- The ratio of voids to the total volume to be greater than 30%.
- An acidic value of 6.5 to 8.5 pH.
- Graded to:
- 5.00mm sieve with 89% to 100% passing.
- 2.36mm sieve with 65% to 100% passing.
- 0.30mm sieve with 5% to 50% passing.
- 0.063mm sieve with less than 4% passing.
Topsoil shall be added to the side slopes of cuttings, embankments, and verges and compacted to create a level surface, ensuring a minimum compacted thickness of 150mm. This will serve as a preparation for seeding or turfing and the process involves treading and raking the area in various directions.
Wildflower and Grass Seed Mix
Grass seed from an approved manufacturer, shall be spread at a total rate of greater than one kilogram to 50 square metres. Grassed areas shall be maintained in order to control weed growth.
SuDS embankments and bases should include a seed mix with an appropriate wildflower perennial and a minimum of 70% native wildflower.
SuDS filter strips and non-SuDS applications should be made up from 25% Barkoel Crested Hairgrass, 20% Hardtop Hard Fescue, 35% Barpearl Slender Creeping Red Fescue and 20% Bargreen Chewings Fescue.
Before sowing, all surfaces to be grassed should be reduced to a fine state, known as tilth. Stones larger than 20mm in diameter and all harmful materials shall be removed. Fertiliser from an approved manufacturer may be used, made up of part nitrogen, potassium and phosphorous to a ratio of 10:15:10.
Turfing
Turf in accordance with BS 3969:1998+A1:2013 shall be laid on the topsoil within three days prior to cutting between the 1st of April and 31st of August, or within one week prior to cutting during the rest of the year. The turf shall be clean, strong, well-bonded, and lightly beaten. Turf should be laid diagonally across the side slopes.
Planting
Additional evergreen or perennial planting along the roadside such as areas of bulbs and low maintenance shrubs will be assessed on a site-by-site basis.
New Trees
Trees shall be over 1.5m in height and a minimum 8cm girth, with planting carried out between in early November and March.
The species and variety should be diverse and suited to the soil type, space available and growing conditions. An indicative list is provided by us however alternative species may be proposed.
A tree pit shall have a minimum size of 450mm long, 450mm wide, and 300mm depth with loose soil in the bottom of the pit and 50cm deep mixture of organic material at the surface.
The positioning of tree planting should consider the height and spread. Trees should not be planted within two metres of buildings or five metres of street lighting columns, if this is the case the street lighting column should be repositioned.
Root Barriers shall be required around tree pits where trees are planted within five metres of roads and footpaths. A proprietary, high strength, photo and bio degradation resistant root barrier should be used. This shall be two millimetres thick with a minimum width of one metre.
Where individual trees are to be planted, the use of preformed root directors may be more appropriate.
Retained Trees
Existing trees that are retained alongside the road or footpath shall be protected in accordance BS 5837 for the duration of construction.
The rooting area shall be protected from compaction, excavations and contamination so that they remain viable and in good condition.