Constructing raised beds
See also Raised Beds for rents, the policy
A typical raised bed may be 1 x 4 metre rectangle, where the measurements are to outside of the 'box'.
While the shape (circular, rectangle or other) does not matter, it is important to be able to reach the ‘centre’ without compacting the soil or hurting self. A practical width is between 1 and 1 1/2 metre, however this will be less if the gardener is seated or has difficulty with balance.
The depth is more difficult to judge and will depend on the objective the beds and gardeners' needs.
A disabled gardener may need a bed 50 cm high to avoid bending too far or to enable working from a chair. A shallow bed may be on a raised table like structure.
For deep rooted plants, like parsnip, leek or potatoes, 30 cm may be insufficient for adventurous, competitive gardeners.
A depth of 30 cm (one foot) and one metre square will require almost 0.4 tonne of material as one tonne of soil is about 0.8 cu. metre.
However, despite the cautionary notes, bespoke beds should only be considered where there is a known demand.
Apart from the required shape, length and width, it is the height of border that can give most problems. While an obvious afterthought, a high border means more internal pressure of earth pushing out which requires substantial support all round.
Most often wooden stakes are used for support, but be aware they will rot quickly and require replacement. Better support is from metal posts like angle iron, scaffold poles or reinforcing rods, all cut to length that does not exceed height of border.
Recycled materials are very welcome if they are in good enough condition to last a few years. There are several options for the surrounding structure:
- Concrete slabs
- Purchase a precast-sections
- Needs robust support and best partially buried
- Casting in place
- Require steel reinforcing
- Tendency to break and deteriorate quickly if casting is not very well done
- Needs robust support and best partially buried
- Second hand bricks
- Building a surrounding wall needs skill
- Line inside with plastic to prevent lime leaching and brickwork becoming too wet
- Scaffold planks
- Gravel board, decking, preferably pressure treated (tantalised)
- Cheap compared to other materials
- May not be long lasting
- Best well treated with creosote or substitute
- Lining internally with plastic helps protect wood and prevent leaching of oils into soil
- Railway sleepers
- Long lasting
- Expensive, many required to gain height
- Can be purchased new and treated or not treated
- Used sleepers are usually creosoted which may leach into soil unless a plastic lining is used
- Laid edgeways they require supporting stakes and pegging together
- Each slab requires support, like metal frame
- Tendency to fall over under pressure from contents, unless partially buried
- Useful for small structures
- Can be sourced up to 40 cm width
- Tends to be too thin and lengths longer than one metre require good support
- Difficult to achieve raise height above width of material
- Weathering will deteriorate integrity causing fracture
- Difficult to repair
- Cannot be successfully painted, original colour only (white, black or grey)
Once again it is necessary to think far ahead about how the raised bed is to be used and by whom. Consider anyone with mobility and balance difficulties, or two people working to help each other.
Ensure there is sufficient space around the bed to allow easy access, and what, if any, surface covering is required.
For an agile person, about 60 cm (2 feet) is sufficient, but a person with walking aid or chair will need one metre.
Realistically, one metre path surrounding beds benefits everyone.
- Ideally grass would be used, in line with the general pathway policy for allotments
- Paving or bricks
- Probably the simplest solution, but can be expensive
- Best for chair access
- May require sand or gravel for bedding and level
- Regular weeding required
- Possibly use plastic underlay to reduce weeding
- Occasional relaying to avoid trips
- Consider raising above water level to reduce flooding or ice rink
- Lay paving or bricks without cement, fill gaps with sand
- Readily available
- Rots slowly into ground
- Requires replenishment
- Difficult for using chair
- Sand and Gravel
- Relatively low cost if purchased by tonne
- Tends to compress into the ground unless underlay is used
- Persists in ground which may be nuisance if to be cultivated
- Difficult to use chair
- Concrete or Tarmac
- Should not be used
- Not permitted on allotments in Ealing Borough
- Pollutes soil
- Cannot be laid to sufficient depth that maintains surface integrity
- Carpet or carpet tiles
- Useful quick temporary coverage
- Rots and leaves soil polluted as most carpets contain nylons and plastics
To counter effects of damp, use a layer of non-permeable plastic that covers the inside of the surrounding structure. The plastic need not cover the whole inside area, but should provide about 15 cm (6 inches) cover at ground level.
The plastic will help reduce effects of damp and rot, reduce leaching of chemicals from creosote or paint, and leaching lime from cement or concrete.
Buying soil is expensive. A tonne bag will fill about 0.8 cu. metre.
A good approach to filling the bed is to use layers of material from straw, old turf, rotted compost and manure, mixed up and topped over with 10 cm of soil. Dig up the top soil before adding the new filling.
Once filled, allow at least two weeks for ground to settle, and then refill as required.
Specialist soils, like ericaceous, are very expensive and hard to find in bulk supply. Consider using very large pots buried in the ground and fill them with the required soil type.
Square foot gardening is a very useful technique for raised beds. The general idea is to segment the growing area into small units and sow each unit with many seeds of the same crop like lettuce, carrot, beetroot, radish and similar salads. By taking young, small crop quickly and allowing a few to grow bigger each unit is filled with growing plants which suppresses weeds.
The concept of packing in as much as possible into your beds applies to most allotment crops. Unless you are looking to grow large plants, taking regular smaller crops keeps the ground in use, reduces weeds, pests and disease. Companion planting is worth utilising for best results.
There are many sources of information about raised beds. This article draws advice from those listed below, books, magazines and experience of our tenants.
Allotment and Gardens [http://www.allotment-garden.org/gardening-information/raised-beds/]
Thrive, Carry on Gardening [https://www.carryongardening.org.uk/raised-beds.aspx]