NARGS Emerald Chapter

This article was originally published in the April 2008 Emerald Chapter Newsletter.

For most of human history, bogs (and such kin as marshes, swamps, mires, fens, morasses, muskeg, pokosins, seeps—“wetland” is a very modern eco-euphemism) have been associated with disease, discomfort, and bad metaphors, yielding little of note but peat moss, mosquitoes, and Bronze Age homicide victims. So why would a gardener (especially a rock gardener—we’re the drainage nuts, you know) build one? And why wouldn’t his friends and neighbors think he’s really gone over the edge when he starts bragging about it? It’s about plants, of course: some very nice plants are limited in nature to bogs. Even more need constant moisture during the growing season, and in our climate, these can best be accommodated in an artificial bog.

I built my first bog about 12 years ago as a transition between a waterfall-pond system and a dry-stream built to conceal land drains. Only 2 by 5 feet, the bog has been a refuge for a variety of neat plants, most living on for years while my finer alpines in troughs and rock gardens prove to be rather expensive annuals. When I put in a new front lawn a year ago, I installed a second, larger and sunnier bog as a transition to shrub beds.

How does one make a bog? I’ve read a number of how-tos in garden books and magazines. Most, in my opinion, illustrate a deadly mix of second-hand knowledge and plagiarism. Nearly all lose courage at some point and tell the gardener to puncture, slash, or perforate their pond liner “for drainage.” Er… sir, madam, this IS a bog-high water table, you know. “Bog drainage,” in my garden, would be another term for tree root invasion, desiccation, and wasted effort.

Here’s how I make a bog:

1. Location

Obviously, a bog is most natural when sited in a low spot. My bogs, one a marginal feature to a pond, the second in an artificial swale on my sloping lawn, are typical naturalistic sites. Vernally wet spots in the garden—especially hillside seeps—are good, and so are low spots or depressions caused by wind-throw in a woodland. It’s also possible to make container bogs—you can just fill the whiskey barrel “water feature” that didn’t work with suitable soil. A second consideration is sun. Perhaps surprisingly, the greatest variety of bog plants need at least several hours of direct sunlight, or at least bright filtered sun. Really hot exposures with afternoon sun can be a bit much though.

2. Construction

This reduces to hole, liner, soil, and water source. For a naturalistic bog, the outline should look right for natural drainage on a site. If a hillside seep, make the run-out go downhill; an isolated bog on flat ground would be rounded, a marginal planting of a stream or pond might suggest a bay with logs or rocks separating water from moist soil. The hole should be 12–18″ deep with vertical walls. Pond liner (butyl rubber) is usually recommended, but cheap PVC is quite adequate. Since the plastic is entirely buried, there’s no deterioration from exposure to sunlight. Both of my bogs are lined with 2 layers of 3 ml black PVC. Overlap the liner on all sides of the hole—you will trim it after back-filling with soil. Alternatively, you can bury a semi-rigid plastic or fiberglass pond liner, wading pool or container to its rim. In any case, don’t puncture it for drainage!

3. Soil

The soil for the bog may vary a bit, but generally you want to aim for a porous, rather acid soil that’s high in organic matter. My recommendation is to fill to about 6” below the final surface with clean, coarse sand. Builder’s sand is OK, but try to get a washed product with low fines content. Avoid beach sand or play sand-these are too fine. Since the sand acts as an inert water reservoir, you could probably use plastic sponges or old fiberglass insulation. To be safe, though, use sand! At this point, I add weathered oak or fir limbs or driftwood (fresh-water only) leaving the high points above the projected surface. The idea is to wick water to the surface and to provide planting sites above the water table for some plants. If the site slopes, place the wood across slope to slow percolation. Subsurface irrigation via a porous rubber “leaky hose” is quite convenient: just arrange the hose so that it runs within 18″ of any part of the surface, and cap off the end. The upper 6″ of soil should be a sand-organic mix. I used about 1 part sand/2 parts milled peat moss in my first bog, and a mix of oak leaf mold and fresh sawdust (1 part) and 1 part sand in my second bog. I’ve seen recommendations for more organic soils, e.g. 3 parts peat to one part sand, and these would likely suit the true acid bog specialists.

4. Final Steps

The final steps before planting are to trim the liner, adjusting height where necessary, concealing with wood or stones; check the irrigation system; and finally saturating the soil and tamping it down. The “next” bog I build will have planned hummocks and depressions—just 2 inches of surface relief can make a big difference when the water table is so close to the surface. I’d also like to grow live sphagnum as a medium for acid-lovers—in the Willamette Valley this would probably require a site with afternoon shade, and certainly with constant, soft water.

Some plants suitable for garden bogs

I’ve grown most of the following in my two bogs

Larger perennials, most suitable for bogs over 20 sq.ft.

Darmera peltata ~ Indian rhubarb or umbrella-leaf

Epipactis gigantea, e.g. ‘Serpentine Night ~ a runner, but lovely

Galtonia candicans, G. regalis ~ like many South African monocots, they like water

Gentiana sceptrum, G. andrewsii, G. makinoi ~ all upright bottle gentians

Lilium parryi, L. wigginsii ~ these and several other lilies like wet feet

Lobelia cardinalis, L. siphilitica ~ both do much better with constant moisture

Osmunda regalis ~ royal fern

Primula florindae ~ long summer bloom, very fragrant

Primula japonica, P. beesiana, other candelabra primroses; P. denticulata

Medium to small perennials, large and small bogs

Andromeda polifolia ~ bog rosemary is the one indispensable shrublet for bogs

Dactylorhiza spp. ~ All forms of this genus of European bog orchid are desirable. I have D. fuchsia, D. foliosa, and two other unnamed types.

Dodecatheon dentatum ~ white-flowered shooting-star usually found around waterfalls, but very garden worthy. This scorches in hot sun.

Gentiana sino-ornata, and Asiatic hybrids. G .acaulis, G. calycosa, G. newberryi, many others ~ These do very well when planted a bit above water table.

Graminoids ~ probably 90% of vegetation in natural bogs, but be careful. I can recommend red-leaved Uncinia uncinata, and probably Dichromena colorata

Hymenocallis traubii ~ this tropical looking amaryllid is hardy: too soon to tell if it will bloom for me

Iris setosa ‘Nana’ ~ beautiful, only 6–8″

Mimulus naiandinus, aka M. ‘Andean Nymph’ ~ pretty, long bloom, not weedy

Orchids, in var. I’m trying several Platanthera, Cypripedium californicum and C. reginae, among others. I’m passionately in pursuit of grass-pink, Pogonia ophioglossoides~ More later on the orchids!

Primula capitata, P. rosea, P. vialii

Rhodohypoxis baurii ~ fully hardy for me, blooms for six months

Sarracenia spp. and hybrids, Darlingtonia californica ~ pitcher plants alone will justify your bog!

Sisyrinchium ‘Devon Skies’, other sterile hybrids, almost ever-blooming—but avoid S. californicum!

Zephyranthes candida, Z. grandiflorum, other species and hybrids ~ At least some of the rain-lilies are hardy, I’m trying more of them in various garden habitats.

References

Fred Case wrote two articles on carnivorous plants and on bog plants in general for the Bulletin of the North American Rock Garden Society in 1992.

There’s a very useful article by Ron Determann on building a bog for Sarracenia pitcher plants posted on the Plant Delights nursery website: “Building a Pitcher Plant Bog”