I’ve encountered many questions lately about how to turn a septic mound into an attractive garden feature. Frankly, I’ve never dealt with this personally.
However, I have cultivated rock gardens for most of my adult life, and I think that they offer some of the same opportunities and challenges.
When people talk of septic mounds, they refer to Glendon Biofilter and other mound systems installed usually when a high water table doesn’t allow for safe underground sewage distribution. In these systems, the sewage is pumped into one or more above-surface mounds filled with sand held in place by a biodegradable net. The sand filters the sewage so that it is innocuous by the time it reaches the water table.
The gardening challenge is that these mounds are frankly hideous, yet there are restrictions on what can be grown in them. Moreover, you can’t cover them with a depth of soil or mulch. Therefore, the gardener is restricted to shallow-rooted drought-tolerant plants that will not infiltrate the sand mound deeply yet will thrive in the fast-draining sand. That’s where the similarity to rockery plants comes in, since so many of these are shallow-rooted plants that survive with excellent drainage, thriving in the small pockets of soil that the gardener inserts at the time of planting for their benefit. They also tend to be low-growing plants, which is perfect because you really don’t want to stack a tall plant on the top of a mound anyway.
So let’s start with the lowest-growing plants that should work on a septic mound. Woolly thyme is the first that comes to mind because I’ve actually seen it cultivated happily on a septic mound. It can be grown on a rockery, mound or between pavers as a lovely, gray, slowly-spreading ground cover for sunny areas. There are number of succulents, particularly creeping sedums, that fit the bill also, but my favorite is sedum ‘Angelina’ for its brilliant yellow summer color and autumn bronze.
In the slightly taller category, Lithodora’s true-blue late spring blossoms are a big hit with early pollinators like bumble bees. Like woolly thyme and low sedums, it will spread gradually and is not terribly fond of pruning, so give it elbow room. Helianthemum (sometimes called rock rose or sun rose) is another low, flowering plant. It comes in a variety of warm colors. While it spreads, it does not grow self-rooting branches like Lithodora does, so it doesn’t need as much room. I think that it would look lovely cascading down a sunny septic mound. I’ve grown it in a part-shade rockery, but it is definitely happier in full sun.
If we move up to taller plants – those appropriate for planting probably in the lower third of a septic mound – there are lots to choose from. Lavender comes in many varieties and, as I described in a long-ago column, it requires an annual shearing back, which is more maintenance than the plants described earlier. It is well worth the effort for both its visual and aromatic beauty. Like Lithodora, pollinators love it, but it will be frequented by the midsummer crew. If you want to provide late summer pollinator attractants, then Sedum ‘Autumn Joy’ and its cultivar cousins will continue to feed pollinators into early autumn. Echinaceas also provide late summer and early autumn interest, and are now available in a variety of colors.
Two medium-sized soft purple-flowered plants that might work are Russian sage and Nepeta (catmint). The reason I hesitate is that the recommended planting hole depth for these plants is about the maximum planting depth that is recommended for septic mounds. I would worry that the roots would invade the mound.
Many ornamental grasses work well on or around septic mounds. But you would need to consider root depth in these too. Some of the large Miscanthus or Calamagrostis like ‘Karl Foerster’ might work well as a backdrop for a septic mound. But for planting right on the mound, I think it would work better to use Mexican feathergrass (Stipa), blue oat grass, Elijah blue fescue or bronze mop-head sedge. Many of these are not long-lived in dry sunny conditions, but some self-seed. A great guide to ornamental grasses is available at bluestem.ca/grass-comparison-chart.htm. I’ve also found a number of lovely photographs of grass and mixed grass-perennial plantings on Pinterest and the Four Winds Gardens and Walls Facebook page.
If you prefer a wilder, freer look, it is worth considering spreading a thin soil layer on top of the mound and pressing in wildflower seeds. You will need to keep the seeds watered for weeks. Just please make sure that the seed mix you use does not have invasive species in it. West Coast Seeds and American Meadows are both good at assembling seed mixes that work well in our area.
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