How do I make a bog garden?
How to build permanently moist ground deliberately — with a lined basin and a controlled leak, or by working with a wet corner you already have — and what to plant in it.
- Difficulty
- intermediate
- Time
- 4 hr
- Read
- 5 min
- Safety
- caution
Short answer
Dig a basin roughly a spade and a half deep, line it with pond liner punctured with a few holes so it holds moisture but never becomes a pond, put a layer of gravel and a perforated pipe in the base so you can top it up in summer, and refill with the excavated soil mixed with a large volume of organic matter. Then plant moisture-lovers. If you already have a permanently wet corner, you can often skip the liner entirely.
A bog garden solves two problems at once: it gives you somewhere to grow a group of plants that nothing else in a garden suits, and it turns a wet corner from a liability into the most interesting part of the plot. It is also a great deal easier and safer than a pond, and it needs no pump, no filter and no maintenance regime.
Safety
What you'll need
- A spade
- Pond liner, or a thick polythene sheet (optional)
- Well-rotted organic matter — compost or composted bark
- Gravel (optional)
- A length of perforated pipe or land drain
- Gloves
Step by step
- Choose the position honestly.The lowest, dampest part of the garden is the obvious place, and working with existing conditions is far less effort than creating them. Sun or part shade both work; deep dry shade under a tree does not, because the tree takes the water.
- Check for services before you dig.You are excavating a substantial hole. Check for drains, water pipes, electrical supplies to sheds and anything else buried before you start.
- Excavate a basin.Roughly a spade and a half deep, with sloping rather than vertical sides, and as large as you can reasonably manage — a small bog dries out far faster than a large one. Keep the excavated soil, you will use it.
- Line it, and then puncture the liner.This is the step people get wrong in both directions. An unpunctured liner gives you a pond that goes stagnant; no liner at all and a free-draining site means the water simply leaves. Pierce the liner with a fork every half metre or so across the base, so it holds moisture and drains slowly.
- Put a watering system in the base.A layer of gravel across the bottom with a perforated pipe running through it and one end brought up to the surface lets you top the bog up in a dry summer without waiting for water to soak down from above. It is the difference between a bog garden that works in August and one that does not.
- Backfill with soil and a great deal of organic matter.Mix the excavated soil with a large proportion of well-rotted compost or composted bark. The organic content is what holds the moisture. Do not use peat, and do not use fresh manure.
- Fill it with water and let it settle.Soak it thoroughly, leave it a week or two and top it up. It will settle, so expect to add more of the mix before planting.
- Plant generously and mulch.Moisture-lovers grow fast and large, so plant with their eventual size in mind. Mulch the surface after planting to slow evaporation, which is the main way a bog garden dries out.
- Top it up through the summer.The one maintenance job. Water through the pipe in dry spells — rainwater from a butt is ideal — and keep an eye on it in a heatwave, when a bog garden can dry out surprisingly fast.
Tips
- Plants to look at: astilbe, hosta, rodgersia, ligularia, primula candelabra types, iris ensata and sibirica, filipendula, lysimachia, trollius, ferns of many kinds, marsh marigold and gunnera if you have the space for it.
- A bog garden alongside a pond is the classic arrangement and gives a natural transition, but the two must be separated — a bog that drains into a pond loses its water and a pond that floods the bog turns it into a shallow pond.
- Slugs love a bog garden. So do frogs, toads and newts, which is a large part of the answer.
- Puncturing the liner is what stops the water going stagnant. A sealed basin with no through-flow smells and kills plant roots.
- This is one of the very few garden features that gets easier rather than harder as the planting establishes, because the foliage shades the ground and slows the drying.
Common mistakes
- Using an unpunctured liner — You have built a shallow pond with soil in it. The water stagnates, the soil goes anaerobic and sour, and bog plants — which want moisture with some air movement — rot.
- Making it too small — A small volume of moist soil dries out quickly and needs constant attention. A larger bog is genuinely less work than a small one.
- Building it under a large tree — The tree's roots will colonise the moist soil within a season or two and take the water. Bog gardens fail fastest in the exact place people are tempted to put them.
- Forgetting the top-up pipe — Watering a bog garden from the surface in a dry spell wets the top and runs off. A pipe into the gravel base recharges it from below, which is how it is designed to work.
Questions people ask
Can I make a bog garden without a liner?
If your ground is naturally wet all year — heavy clay in a hollow, a spring line, a permanently damp corner — then yes, and you simply improve the soil with organic matter and plant into it. On any free-draining site you need a liner or the water leaves.
Is it the same as a rain garden?
No. A rain garden is designed to take runoff from a roof or a hard surface, hold it briefly and let it soak away, so it is wet intermittently. A bog garden is permanently moist. Some plants suit both.
How much maintenance does it need?
Very little: topping up in dry spells, cutting back in late winter, and dividing the more vigorous plants every few years because they grow strongly in these conditions.
Will it breed mosquitoes?
Not significantly — mosquito larvae need standing open water, and a properly built bog garden has soil to the surface with no open water. That is also why it is far safer around small children than a pond.