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Ground rules: bog gardens (Filed: 09/06/2001)John Cushnie advises on how to work with swamp plants
PLANTS that thrive in damp soil are among the most beautiful in the garden. Many are native and the planting may be supplemented with permanent visitors from Asia and northern Europe. Adding a bog area to the garden increases the range of plants and it will soon be teeming with wildlife. Where it can be put close to a water feature, the overflow should be directed into the bog. Marginal plants will spread, joining the two features and disguising the edge of a pond. As always, if you have children, remember to ensure that they are safe.
Gunnera manicata
What you will need
Flexible liner or heavy-duty black polythene; organic rich soil; stepping stones.
Tools Spirit level; spade; digging fork; wheelbarrow; knife.
The natural bog garden
If there is a naturally wet area in your garden it may provide the ideal site but you will have to be familiar with the conditions for at least a full season. Areas that are boggy in winter and spring, because of bad drainage, often dry out in summer, leaving the plants suffering in hard, cracked soil. Where an underground spring is keeping the soil wet, take care when excavating not to divert the flow or to break through the impermeable lower crust.
Natural bogs may be overrun with weeds, such as rushes and reeds. These must be removed or thinned out to allow other plants space to grow. Spraying with weedkillers, including glyphosate, has some effect but several applications are needed. Once the worst of the weed has been dealt with, dig the area over and incorporate organic material in the form of compost, leaf mould and used compost from grow-bags. Spent mushroom compost is a good bulky material which contains a little lime; allow that to leach out before digging in by stockpiling outside for six months.
Constructing a bog garden
It is easy to make a wet area in your garden, either beside a pond or as a separate feature. For bog gardens that benefit from a pond overflow, excavate a hole at least 18in deep. Where it isn't fed from any runoff, increase the depth to 24in. Remove stones from the base and sides and spread a 1in layer of sand, geo-textile or old carpet to protect the liner from sharp stones.
Where the boggy area is being constructed beside a pond, and at the same time, one large flexible liner may be used for both. Spread the flexible liner over the hole, temporarily holding it in place with stones. Wrinkles in the liner won't be seen. Use a spirit level and timber straight edge to level the rim of the bog garden.
Cover the base of the liner with 6in of river gravel for drainage and fill the hole to the top with soil. Add compost to enrich the soil and soak with water. Leave for a few days until it has settled, add more soil and wet again. Large bog gardens will require regular watering in summer to ensure the soil is kept moist. It might be worth laying a hose underground to the site. It can be turned on as necessary.
To make it easier to work without having to bend, an independent area of bog may be raised above the surrounding garden. Use wooden railway sleepers, peat blocks or dry-stone walling to raise the sides to 24in. Attach the liner to the inside of the wall and fill with a layer of gravel and the enriched soil.
Miniature bog
Provided it is waterproof, almost any container may be used to make a small bog garden. Timber half-barrels look good on the patio when lined, filled with soil and planted with moisture-loving dwarf plants. An old salvaged sink sunk into the soil is ideal for small plants but remember to plug the waste pipe.
Planting
Planting into soil that is sticky and saturated with water is difficult: the soil sits in lumps, leaving air pockets around the roots. Planting before the soil is wet allows the roots to be spread out and the soil firmed. After planting the water may be turned on and the garden flooded. The best season for planting is the spring when the last of the ground frosts are over. If the root ball is tightly matted, tease out the roots before planting.
Small groups of the same variety, especially candelabra primulas, are more impressive than single plants. Hostas with different leaf colours also clump together well. Moisture-loving plants such as Houttuynia cordata will become rampant and are best confined to a plunged pot or planted with their roots wrapped in fine mesh netting.
When planting is complete and the soil has been soaked, apply a 2in layer of fine composted bark or leaf mould as a mulch to prevent the soil surface from drying out.
Larger areas of bog require access for weeding, pruning and dividing clumps of plants. Large flat stones may be used as stepping stones. A wooden platform of decking boards may be laid on logs above the soil surface.
Feeding
Bog plants benefit from regular feeding to help sustain the leafy growth that many of them produce. Nutrients are quickly diluted in wet conditions and are immediately available to the plant through its roots. Feed in late spring and again in mid-summer, using a slow-release fertiliser.
Pests and diseases
Slugs and snails love damp conditions and your five-star bog garden will encourage activity for most of the year. In spring, they do enormous damage to emerging shoots and by summer they will have shredded the leaves of hosta and ligularia.
Beer and citrus skins are effective baits for trapping. The best line of defence is nature and predators, such as frogs, hedgehogs and birds. They should be offered every inducement to become garden residents.
Frogs enjoy leafy shade. Hedgehogs, one of the few animals tolerant of slug mucus, need a drier area close by with lots of old leaves and woody debris. Song birds get snails out of their shells by dropping them on to a hard surface. Flat stepping stones are ideal.
-- Anonymous, June 15, 2001