THE USE OF bog gardens to create novel habitats that support an incredible array of otherwise difficult to grow species is nothing new
to NARGS members. This style was championed by some of the great members in the history of NARGS. In his Orchids of the Western Great Lakes Region, Fred Case wrote about bog gardens and sand-peat beds as a primary means of cultivating otherwise finicky native orchids, carnivorous plants, and their associates found in the bogs of the U.S. Midwest and Coastal Plain. The recommendations given by Case in his book are still relevant and viable today. However, it is time to put a new lens on this style of gardening, not only to reexamine the range of plants that grow in these conditions but also increase understanding of the role this type of garden can play in the modern constructed landscape. Not only do bog gardens serve as a means of beautifying traditional rock gardens and enhancing plant collections, but they can also play a role in safeguarding rare plants and conserving the flora and fauna of globally rare ecosystems.
What is a bog garden?
In its simplest incarnation, the bog garden is a raised bed or excavated area lined with an impermeable layer (pond liner, roof liner, polyethylene, natural material, etc.) that is filled with a substrate that supports the growth of plants that need wet to damp, acidic conditions to thrive. Ideally, beds should range from 12 to 30 inches (30 - 76 cm) deep and the generally recommended medium is a roughly 50:50 mix of sphagnum peat (from the compressed rectangular bales) and silica sand (blasting sand is typically recommended, I use Quikcrete all-purpose sand). Intuition and previous information suggest that bog gardens should be built on level ground, but innovations in this style of gardening by Ron Determann at the Atlanta Botanical Garden recommend otherwise. One of the primary orchid and carnivorous plant habitats in the southeastern U.S. is a seepage slope bog. These bogs occur where water slowly seeps out of deep layers of sandy soil overlaying an impermeable layer of subsoil, typically clay. The water cannot drain through the impermeable layer and is forced
out through the sand layer, resulting in permanently damp or wet soil even on a hillside. There is constant, slow drainage of the water through the bog and the water and soil are well-aerated. Bearing this in mind, Determann suggests an alternative type of bog garden, like those described by Case, but built on a moderate slope to promote slow drainage of water through the medium and increase aeration. This also helps to prevent stagnation, which occurs when the medium becomes anaerobic and loses the ability to support the growth of unique plants. In my garden, I have adopted Determann’s approach with great success. Not only are the gardens built on a moderate slope in full sun, the depth of the medium and ratio of sand and peat mix is varied to create conditions more representative of natural bogs. This creates microhabitats for growing plants in close proximity that require different conditions and allows for some plants find their own preferred part of the garden through natural reseeding. In my primary bog garden, which is about 30 feet (9 m) long and of variable width, the upper part of the bog is 18 inches (46 cm) deep and filled with 50:50 sand:peat. Immediately next to it on a gradual slope is an area of pure, 12 inch (30 cm) deep sand that supports drainage and filtering of water from the upper part of the bog. In the summer, the top layer of sand is completely dry, but damp to wet 3 to 6 inches (7.6 to 15 cm) down. This is an ideal place for establishing unusual native milkweeds such as Asclepias cinerea, the incredible A. humistrata, and A. michauxii, in addition to xeric shrubs like Lyonia ferruginea and Asimina obovata. There is much room for experimentation in this bog garden microhabitat. Below the pure sand is an 18-inch (46 cm) layer of 50:50 peat:sand to support pitcher plants, orchids, and ericaceous plants, followed by an area that is only 6 inches (15 cm) deep and consists of pure wet sand. The last area is experimental, but so far has been ideal for establishing self-sowing seedlings of various Drosera species, live sphagnum, and Venus flytrap (Dionaea muscipula).
In southeastern Pennsylvania, the normal rainfall is enough to maintain adequate moisture in the bog garden. Periodic use of tap water is okay to irrigate with in the occasional exceptionally dry period, but if regular irrigation is needed, rainwater, or another water source free of dissolved solids and chlorination, should be used.
Plant selection for bog gardens: Sarracenia
Many bog gardens are designed and installed to accommodate a collection of the charismatic flora of the eastern North American Coastal Plain and sphagnum filled bogs of the recently glaciated Midwest and far north, with sarracenia featured prominently. Although there
is continued interest in hybridizing sarracenia amongst hobbyists, my collection is focused on growing species and hybrids of known wild origin to serve as a reference conservation collection and to showcase unique variants not typically seen in gardens. Some of the showiest have been seedlings of the well-known S. × catesbaei grown from seeds collected in Brunswick County, North Carolina. The almost glowing, bright red pigmentation of plants from this area is derived from the local forms of S. purpurea subsp. venosa that are distinct from all other populations of the species. Personal favorites also include S. pupurea subsp. venosa var. burkii in its many forms and hybrids, S. leucophylla, and especially S. × areloata. This hybrid between S. leucophylla and S. alata is extremely variable in the wild and many different forms can be found there and in nurseries. An added benefit of S. leucophylla and its hybrids is that they produce many pitchers in the later summer and fall, adding bright colors as the growing season for many other plants is coming to an end. Sarracenia alata is also worth growing, especially if you can find the forms with nearly black pitchers. Sarracenias are among the most commonly poached plants from the wild and most are now protected where they occur. There are many commercial sources, and they are extremely easy to grow from seeds. In addition, they are much more cold hardy than their southern origins would indicate, with most being reliably cold hardy into USDA hardiness Zone 5.
I originally built bog gardens as means for experimenting with growing some of the uncommonly cultivated lilies of the southeastern U.S. Lilium iridollae, Mary Gibson Henry’s pot-of-gold lily, thrives in peaty parts of the bog garden and flowers in August in southeastern Pennsylvania. Despite its southern origins, it is perfectly cold hardy in Zone 6. The flowers are born on characteristically long pedicels, culminating in a particularly graceful and long-lasting display. Lilium catesbaei, the pine lily, a plant with a reputation for being difficult, grows easily in a bog garden. If a seedling population can be grown, different individual plants flower from July to September, providing a long period of interest. It is probably more difficult to find seeds than it is to grow the plant. It is a variable species typically with orange flowers, but occasionally yellow or cream-colored flower variants occur, such as one I grew from a batch of seeds collected at the western edge of the species range in Louisiana. This species can be short-lived but is easily propagated from seed and one of the easiest lilies to propagate by scaling. The rare Lilium pyrophilum, L. grayi, and forms of L. superbum from the Coastal Plain and the Fall Line in Georgia also grow well in bog gardens. Eastern U.S. native lily plants are pollinated by various lepidoptera, and when they flower in my garden, the tiger and spicebush swallowtails are frequently seen visiting the flowers. A related star of the late-summer garden is Stenanthium gramineum var. robustum. Originally grown from seeds collected in Gallia County, Ohio, it took seven years to flower and produced enormous spikes of white flowers for a display that lasted for weeks.
A genus rarely discussed for sunny bog gardens is Asclepias. As mentioned above, many of these prefer more xeric habitats, but a couple of species prefer saturated, acidic soils in the wild and are excellent bog garden subjects. Asclepias rubra is of conservation concern throughout its range and considered extirpated in Pennsylvania. Plants grown from seeds collected in Delaware have thrived in my bog garden and flower every year in June and July, but rarely set seeds. Asclepias lanceolata prefers the same conditions and dazzles during the summer with its richly and variably colored, yellow, orange, and red flowers. Although the inflorescence of this species is few-flowered, it makes up for it by having unusually large individual flowers. These species are known to hybridize in the wild and give rise to some fantastically colored hybrids. Unfortunately, they do not appear to be in cultivation. Many other species from the Coastal Plain bogs of the Southeast are worth trying, but seeds can be difficult to obtain.
Shrubs are an important, but often overlooked component of bog gardens. In my experience, the root systems of shrubs are vital for helping circulate moisture through the medium and mitigating soil moisture during extremely wet periods as well as providing additional microsites for smaller, less vigorous plants such as pinguicula and orchids. Several native shrubs can be considered. Andromeda polifolia ‘Blue Ice’ has been a bog garden stalwart with me for the last ten years. It survives extremes of heat and cold, looks good throughout the year, and rarely needs pruning. Other native shrubs to consider include Chamaedaphne calyculata (can be aggressive); various kalmias including K. angustifolia, K. carolina, K. cuneata, and K. hirsuta; many other types of ericaceous plants including Rhododendron (Rhodora) canadense, and various Lyonia and Vaccinium. This is also an area for experimentation. The finicky Japanese native Leucothoe keiskei ‘Royal Ruby’ has thrived in a sunny bog garden for numerous years and some of the high elevation dwarf rhododendron from China, such as R. tsaii and R. campylogynum are showing promise after a couple of years of evaluation. The bog garden may be the only way to grow high elevation species of this genus in a hot, humid summer climate long term. The choice, dwarf shrubs of the genus Shortia (including Schizocodon) take patience to establish but are right at home in shady nooks of the bog garden. This is may be one of the only ways to grow them in regions where soils are not suitable.
Orchids are among the stars of the bog garden and one of the primary reasons I started to experiment with them. Some of the fringed or bog orchids of the genus Platanthera are readily grown in bog gardens. The orange fringed orchid, Platanthera ciliaris, and its white- flowered counterpart, P. blephariglottis thrive and reliably flower in mid-summer along with many of the lilies and provide another source of nectar for swallowtail butterflies. Perhaps the showiest is Platanthera peramoena, the purple fringeless orchid. It grows readily in a bog garden but is one of the more difficult Platanthera to source and difficult to grow from seeds. Other species, including P. clavellata and P. lacera have been successful and as seed propagation protocols for these species continue to develop, others will be available in the future. Other native orchids for bog gardens include the well-known grass pink, Calopogon tuberosus, and its relatives. I find that these naturally reseed if conditions are right and tend to place themselves among shrubs. There are now dozens of them throughout the gardens. Other calopogons, such as the hybrid ‘Fluffy’, have been just as easy to grow. Pogonia ophioglossoides, the rose pogonia, requires consistently wet conditions, but prefers live sphagnum. Bogs gardens have been one of the only ways to support long term growth of the western European marsh orchids of the genus Dactylorhiza. Hybrids involving D. elata, D. fuchsii, D. majalis, D. purpurea, and others have been steadily growing and increasing in size for the past five years. Many other species thrive here as well including Eleorchis japonica, Neottia bifolia, Spiranthes ochroleuca, and others also thrive. The bog garden is an ideal place to try a wide variety of temperate terrestrial orchids.
Bog Garden Weeds
Bog gardens come with their fair share of weeds, both good and bad. One of the most rewarding bog garden weeds is Polygala lutea. Affectionately known as “bog Cheetos” in some gardening circles, it is one of the hallmarks of the sunny bog garden. This biennial produces short dense heads of golden-yellow flowers from June until frost in my garden and seeds itself into appropriate areas of the garden, but never aggressively so. Other polygalas such as P. brevifolia and P. cruciata are beautiful summer flowering plants that have been slow to establish but are starting to show promise. In my experience, nearly all the native Drosera species are individually short-lived and need to renew themselves from seeds. Drosera filiformis is among the most charismatic, but the small rosettes of D. rotundifolia and D. intermedia, and even smaller rosettes of D. brevifolia beg close inspection. Many other species are worth trying, both native and non-native. Perhaps the worst weed of bog gardens is Rhexia virginica, the meadowsweet. While a lovely native plant deserving of wider cultivation, it can become noxious in a bog garden, where it sends out long runners that rapidly grow and outcompete slower growing plants. I am trying Rhexia aristosa and so far appears to be much less aggressive with much larger, deeper colored flowers than the former. The cranberry, Vaccinium macrocarpum, can also be a weed, forming an impenetrable mat and rapidly choking out other plants. There is one exception, the slow-growing cultivar ‘Hamilton’ can be used in bog gardens without fear that it will become weedy.
Bog gardens in the shade
More recently I have been developing bog gardens in a protected, shady part of the garden. It is constructed the same way as previously described, but allows for experimentation with a completely new array of plants from around the world that had proven intractable in other parts of the garden. So far, the results have been extremely encouraging. Polygala is a favored genus, and the shady bog garden is proving to be an ideal way to cultivate the desirable native spring wildflower Polygala paucifolia in my hot, humid southeastern Pennsylvania climate. This species is common in the cooler, mountainous parts of central Pennsylvania and study of its native habitat confirms its need for a cool root-run and consistently moist, organic soil. Another group of native plants rarely seen in gardens and ideally suited for a shady bog garden is the genus Parnassia, grass of Parnassus. All native parnassia are late summer to fall flowering plants known for their white flowers stenciled with intricate green veins. They have a reputation for being difficult to grow, but Parnassia asarifolia and a unique form of P. grandiflora from central Tennessee have proven to be an easy-growing, and stunning addition to the shady bog garden. There are many Asian species, including Parnassia foliosa, with remarkably fimbriate flower petals, that are well worth trying ,too, but rarely available. Other choice Asian plants thrive in bog gardens. Several heloniopsis thrive and are among the earliest plants to flower. Heloniopsis orientalis, H. orientalis var. breviscapa ‘A-so’, H. tubiflora ‘Temple Blue’, and the related Ypsilandra thibetica have been favorites, in addition the native Helonias bullata, which also thrives here. The diminutive Trillium pusillum var. virginianum, which occurs in damp or wet conditions in the wild, has grown extremely well in shadier parts of the bog garden along with the associated Arisaema pusillum. Several Asian primula species are also being tried here in addition to a wide variety of plants from Asia that require similar conditions, several native lycophytes, and a variety of orchids.
Sourcing Bog Garden Plants
Many of the plants discussed here are rare, threatened, or endangered in the wild. Plants should be sourced from nurseries with responsible and sustainable propagation and production practices, although many will be difficult to find commercially. The best and most rewarding way to obtain these plants is through seed propagation. Many of them are easily grown from seed and seeds are often found through various seed exchanges produced by special interest plant societies. Some plants, such as drosera and polygala are short-lived, and the only reliable way to keep them in the garden is to establish reseeding populations. Although there may be a substantial initial effort to install bog gardens, the ease with which such choice plants can be grown is instantly rewarding and long-lasting.