A bog garden can be built from scratch, as a conversion from
a redundant pond or a naturally waterlogged depression, to
give a pond an informal edge or to create a wildlife garden
as a stand-alone soft feature.
Larger expanses are difficult to maintain and should include stepping stones for access. Mark out the area with sand or string and calculate approximately how much soil will need to be moved given that you will need to excavate to a minimum depth of 45cm (18in). If the figure arrived at seems excessive, double check your calculations and perhaps reduce the size of the proposed bog - remember that you will be double handling all the soil.
Line the hole with a plastic or butyl liner of at least 0.5mm thick. Under-laying or lining with sand is not necessary. Bog plants do not like stagnant anaerobic conditions or drying out so lay a length of Leaky-Hose or porous pipe in the bottom and seal the submerged end. Peg or weigh down the hose and lining edges before covering the hose with a 2.5cm (1in) layer of crocks and coarse grit to prevent soil blocking the pipe. Ensure that the inlet connector is a few inches clear of your final soil level and in an accessible position.
Pierce the liner, with a garden fork, every few feet and replace the excavated soil removing all perennial weeds and large stones. Depending on soil condition it may be necessary to incorporate well-rotted but aerated leafmould or garden compost.
The soil level will now be substantially above where you started but it will settle back down and will be assisted by regular watering. Any remaining persistent perennial weeds and germinating dormant seeds should be removed during this settling period. Do not tread down or artificially hasten this process as soil structure will be lost and compaction will result.
Once the original level has been regained a wide variety of native, ornamental and wildlife friendly specimens can be planted in late winter or early spring. Plan carefully for differing levels of sun, shade and soil acidity/alkalinity as many bog plants are quite particular.