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Plant Answer Line Question

Keywords: Land treatment of wastewater, Ornamental grasses, Landscaping drain fields, Ground cover plants, Drought-tolerant plants

I am looking for plants suitable for a septic drain field site. I have a very large north facing slope in open sun with a drain field running along the top half. I would like to plant low to no maintenance ground covers and low growing shrubs to cover this area. This is a focal point when driving up to my house so I want it to be eye catching and interesting year round.

I thought of heaths and heathers as a possibility, but I'm not sure if the root system is shallow enough. I also would like to include native ground covers such as ferns, Gaultheria shallon and any others that you might think would work, as well as ornamental grasses and perennial flowers for interest. Can you please offer a resource for planting over drain fields or a list of plants that you think would work?


Trees or large shrubs should be kept at least 30 feet away from your drain field. If you do plan to plant trees near a drain field, consult an expert to discuss your ideas and needs. Trees and shrubs generally have extensive root systems that seek out and grow into wet areas like drain fields. Grass is the ideal cover for drain fields. Grasses can be ornamental, mowed in a traditional lawn, or left as an unmowed meadow. You can also try groundcovers and ferns.

The key to planting over the drain field is to select shallow-rooted, low-maintenance, low-water-use plants. When tank covers are buried, keep in mind that plantings over the tank--from inlet to outlet--will have to be removed every three or four years for inspection and pumping.

Planting your drain field will be much different from other experiences you may have had landscaping. First, it is unwise to work the soil, which means no rototilling. Parts of the system may be only six inches under the surface. Adding 2 to 3 inches of topsoil should be fine, but more could be a problem. Second, the plants need to be relatively low-maintenance and low-water use. You will be best off if you select plants for your drain field that, once established, will not require routine watering.

SOURCE: WSU Cooperative Extension - Clallam County

Information can be found here.

Thurston County, Washington, has some information about landscaping a drain field, including plant suggestions, here.

Additionally, the Pacific Northwest Gardener's Book of Lists (1997, by R. & J. McNeilan) offers a number of groundcover lists for various situations, including groundcovers for dry sites, slopes, and sun and shade. The Miller Library has this book.

Date 2018-08-16
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Plant Answer Line Question

Keywords: Native plants--Washington, Ground cover plants

I am looking for a native groundcover to grow as a walkway along side my house. It is a shady spot and it would need to be a hardy plant that could be walked on.


Regrettably, there are not any native, shade-loving, walkable ground covers available unless you are interested in mosses. If that is appealing, you can check in the book Plants of the Pacific Northwest Coast by J. Pojar. The following websites may help you find native plants:
King Conservation District
King County Native Plant Guide
Washington Native Plant Society

For information on growing mosses, see "Encouraging Mosses" from Oregon State University, based on the writing of George Schenk.

If your heart isn't definitely set on natives, there are some good alternatives:

  1. In her book Big Ideas for Northwest Small Gardens, Marty Wingate recommends Mazus reptans. It is semi-evergreen to evergreen with tiny blue flowers from late spring through summer. It takes full sun to part shade and is delicate looking, but takes foot traffic. It requires some fertilizer to stay perky. (Note: I use it in my garden--it is versatile and pretty)
  2. Another source of ideas is the website http://stepables.com/
    Click on plant info, then plant search.
  3. A ground cover that I have found useful (it can take car traffic a couple times a day) is Leptinella gruveri Miniature. You're almost certain to find it at the website above.

Date 2019-04-10
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Plant Answer Line Question

Keywords: Vinca, Lamium, Lavandula, Ground cover plants, Ceanothus

Our house is on a corner lot. The side yard has a very small slope with big rocks along the edge. Presently it has a variety of flowers such as lavender that bloomed last summer. However, my question is what kind of ground cover can I put there, other than grass, that would look good and be evergreen.

Secondly, there are two big pine trees at the corner. What are my options for plantings beneath these trees that would give it a pulled-together look?


I am guessing that the spot receives a good amount of sun, since you have lavender Lavandula that flowered there in the summer. Were you looking for a groundcover that will tolerate people walking on it, or did you want somewhat taller plants that will blend well with the lavender?

If you plan to walk on the area, you might want to consider chamomile (Chamaemelum nobile) or creeping thyme (Thymus serpyllum).

There are many great choices for plants not intended to be walked on, and I recommend that you take a look at some of the resources we have in the Miller Library so you can find the plants that most appeal to you. I recommend the books Gardening with Groundcovers and Vines by Allen Lacy (HarperCollins, 1993), and Perennial Groundcovers by David MacKenzie (Timber Press, 1997) as starting points.

Plants that are evergreen (or 'ever-grey') and might go well with lavender are Santolina, Helianthemum (sun rose), Teucrium chamaedrys (germander), and Ceanothus thyrsiflorus (creeping blue blossom ceanothus).

For the spot under your pine trees, you will need plants that tolerate shade and do not have large root systems. I would try Lamium (dead nettle), which comes in several foliage and flower colors, and I would avoid Lamium galeobdolon, a species which is considered a noxious weed in King County. Vinca (periwinkle) might also work. The Lady Bird Johnson Wildflower Center has information on planting beneath pine trees.

Date 2019-05-15
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Plant Answer Line Question

Keywords: Mosses and moss gardening, Lawn alternatives, Ground cover plants

What is the best way to encourage moss to take over and cover large surface areas in a relatively short amount of time? My goal is to replace my lawn with a moss garden.


Here are some links to information which may be useful to you:

Primitive Plants: Bryophytes, Ferns, and Fern Allies

Moss cultivation:

Encouraging Mosses

Mad About Moss: The Simple Art of Moss Gardening

There are two books I would recommend, Moss Gardening by George Schenk (Timber Press, 1997), particularly the chapter on "Moss Carpets," and How to Get Your Lawn Off Grass by Carole Rubin (Harbour Publishing, 2002). Rubin gives directions for preparing your site, which involve digging out existing plants or--in your case--smothering the lawn with mulches of leaves (12 inches), bark (3 inches), or newspaper (10 sheets thick). Schenk offers several different methods for creating a moss garden. Briefly paraphrasing, these are:

  1. Work with nature, allowing self-sown spores of moss to take hold. (Prepare the site by weeding, raking, and perhaps rolling the surface smooth.)
  2. Encourage the moss in an existing lawn by weeding out grass. You can plant what the author calls "weed mosses" which will spread, such as Atrichum, Brachythecium, Calliergonella, Mnium, Plagiothecium, Polytrichum, and others.
  3. Instant carpet: you can moss about 75 square feet if you have access to woods from which large amounts of moss can be removed legally.
  4. Plant moss sods at spaced intervals (about one foot apart) and wait for them to grow into a solid carpet.Choose plants that match your soil and site conditions.
  5. Grow a moss carpet from crumbled fragments. This is rarely done, and only a few kinds of moss will grow this way, including Leucobryum, Racomitrium, and Dicranoweisia.

Another approach is to change the soil pH. Sulphur should be beneficial to moss and detrimental to lawn grass. The reason for this lies in the fact that moss grows best with a soil pH of 5.0-6.0, while lawns grow best with soil pH between 6.0 and 7.0 (according to The Lawn Bible by David Mellor, 2003). Added sulphur lowers the soil pH, creating a more acidic environment.

Date 2019-05-15
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Plant Answer Line Question

Keywords: Brunnera, Stachys, Liriope, Epimedium, Lamium, Alpine and rock gardening, Shade gardening, Ground cover plants, Geranium

I'm looking to plant in a narrow strip on our retaining walls some "spiller" plants which will overhang the walls (which face north).

I'd prefer evergreen plants which would fill in fairly quickly, but I could also mix in slower-growing and deciduous plants. There's great drainage since I have gravel reservoirs behind each wall, and the part of the plant above the wall will get part to full sun, though I could overplant them if necessary for a plant that couldn't handle full sun.

I would like plants with interesting foliage and form to soften the look of the walls, and so would prefer a furry look to a spiny one. Flowers and fragrance are less important though always nice, and I'm hoping to have at least 2 or 3 different plant types with different colored foliage (shades of green are fine).


Some of the plants that occur to me, based on the description of your site, are Brunnera macrophylla, Epimedium, Geranium phaeum, Stachys byzantina, Lamium maculatum, and Liriope. Of these, the Geranium and Lamium will trail somewhat, while the others are essentially upright.

Graham Rice's article on the Royal Horticultural Society site features a selection of recommended trailing (or spilling) plants. Here is another good list of trailing plants for walls.

You could also try entering your site requirements into the plant-finding and plant selection web pages below:

Great Plant Picks (a local site)

King County's native plant guide

Missouri Botanic Garden Plant Finder

Royal Horticultural Society Plant Finder

The Miller Library has many books on gardening in the shade, so you may wish to come in and do some research to help you in your plant selection. Here is a booklist that may be of interest.

Date 2019-02-23
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Plant Answer Line Question

Keywords: Chamaemelum, Thymus, Ground cover plants

I am building a stone walkway. What can I plant in the cracks that will take the sun and that I can walk on?


Creeping thyme would be ideal for your needs. It will do well in sun, and can withstand foot traffic. Colorado State University Extension has useful information on groundcover plants for dry conditions.

Leptinella, or brass buttons, is another option.

Chamaemelum nobile likes sun and will withstand light foot traffic.

The website for Stepables, a company specializing in groundcover plants, allows a search by plant characteristics.

Date 2019-05-09
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Plant Answer Line Question

Keywords: Aegopodium, Convallaria, Oxalis oregana, Lobularia, Pachysandra, Galium, Lamium galeobdolon, Sheet mulching, Euonymus, Shade-tolerant plants, Polystichum munitum, Native plants--Washington, Fragaria, Soils, Ground cover plants, Geranium

What is a good way to deal with a gravelly area with a lot of shade? Are there good groundcovers that would be low maintenance? Can the plants grow right in the gravel, or do I need to do something to the soil?


If it's pure gravel, you can just make a border (with rocks and/or wood, preferably non-treated) and fill it with 9-12" of soil. (No need to remove the gravel.) You buy soil by the cubic yard, so to figure out how much, multiply the length (feet) x width (feet) x depth (.75 or 1), then divide by 27 to get the number of yards. One yard of soil is 3' x 3' x 3', or 27 cubic feet. My guess is that you need less than a yard, but it settles.

You can save money by buying the soil in bulk. Otherwise, you have to buy it by the bag, and they might come in cubic feet. If there is only some gravel, you may be able to get by with the soil/gravel mix that you have. See how much hardpan there is by digging around a little.

If you have lots of weeds in the gravel, cover the whole area with large sheets of cardboard or multiple layers of newspaper (about 10 sheets), overlapped to prevent light from getting through. Then put down a border and fill the area with soil. Smothering weeds depends upon complete darkness more than anything. Therefore, overlapping biodegradable stuff and deep soil is key.

Once you've done that, you can plant right away. Here are some plant suggestions. I've included links to pictures, but you can always find more on Google images or the Missouri Botanical Garden's PlantFinder.

  1. Lobularia maritima, known as sweet alyssum: You can plant seeds of this and it will come up this year. It's best to mix it with something else, since it dies down in winter (but self-seeds vigorously and will return). The white seeds the fastest (year to year), but it's nice to mix with purple. Both varieties smell good and attract beneficial insects.

  2. Fragaria x ananassa 'Pink Panda': A strawberry-potentilla hybrid that grows fast and spreads easily, is good weed suppresser, and blooms twice a year with pink flowers. This is an excellent groundcover, will probably be evergreen.

  3. Pachysandra: This plant is evergreen, and though it is not as fast growing as some groundcovers, it does spread.

  4. Hardy Geranium spp.: Geranium x oxonianum 'Claridge Druce' is a variety that spreads well. Another good variety is Geranium endressii 'Wargrave's Pink'; in particular, it seeds itself well. Geranium macrorrhizum has many cultivars, a pleasant scent, and self-seeds readily.

  5. Galium odoratum: Also called sweet woodruff, this plant is prettily scented, probably evergreen here, and spreads fairly rapidly. It produces white flowers in early spring, and it would be particularly good to mix with something taller, like Geranium species.

  6. Oxalis oregana: This native plant looks like a shamrock, and though it is slow to establish, once it has it's very tough and spreads. If you don't get the native Oxalis oregana be careful, as the other species are very aggressive.

  7. Euonymus spp.: These woody groundcover plants are evergreen, and come in lots of varieties like E. fortunei 'Emerald 'n'Gold' and 'Emerald Gaiety'. Do be sure to get a groundcover and not a shrub version of the plant. 'Emerald and Gold' is the most robust choice.

  8. Convallaria majalis: Also known as lily of the valley, this is a vigorous (aggressive!) groundcover.

  9. Maianthemum dilatatum: Called false lily of the valley, this native plant is a good choice for shade groundcover.

  10. Polystichum munitum: The native swordfern (or another fern species) might work. P. munitum is basically evergreen, though you might need to cut out some dead fronds in late winter, and makes a good mix with something else. Other deciduous ferns are higher maintenance.

There are also a couple of plants to avoid!

  1. DON'T plant Aegopodium podagraria 'Variegatum': Commonly called bishop's weed, and frequently used as a groundcover, this plant is very invasive.

  2. DON'T plant Lamium galeobdolon (formerly known as Lamiastrum), either: Yellow archangel is very invasive in Pacific Northwest forests.

Date 2018-09-26
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Plant Answer Line Question

Keywords: Liriope, Ground cover plants

A client, who planted a small Liriope "lawn" heard from a different landscape professional that this plant does poorly in our climate because we don't have enough summer humidity. I haven't used it a lot in my designs, and haven't grown it in my own yard, but have considered it a tough and versatile plant. Is it true it does poorly here? (Why have I not heard this before?)

They used small plants for their lawn. I've suggested they give it two years since I think it is an interesting idea. We all realize it won't take the kind of foot traffic regular turf will take. What do you think?


My personal experience with Liriope is not altogether positive. It pokes along in partial shade in my garden, looking rather ratty most of the time. It may be that I don't provide it with enough water to make it happy. It's hard to say, based on one person's garden, whether the same will hold true in other soils, and other light and irrigation patterns. I don't see it planted in large public spaces, or even in large quantity in home gardens in our area. And I agree with you, it's not a turf substitute--its common name 'lilyturf' is a misnomer, as it's not a lily and neither is it a turf plant.

Missouri Botanical Garden's information about two commonly grown species suggests it does well in the South (where it's humid in summer).

Liriope is included in Perennials: The Gardener's Reference by local authors Susan Carter, Carrie Becker, and Bob Lilly (Timber Press, 2007). There are several species. Liriope muscari forms clumps a foot and a half wide; Liriope spicata "spreads rapidly by underground stems and will cover a wide area; it is therefore not suitable for edging but is excellent for groundcover." It grows 8-12 inches tall by a possibly infinite spread, meaning it can be aggressive if the conditions are right. The authors say all Liriope flowers best in sun, and prefers moist, well-drained soil though it may be drought-tolerant once established. "Ragged with neglect" accurately describes the way my own plants look, so perhaps I'm just negligent. The authors say it may be cut back to the ground in spring before new growth begins, but "if there's no winter damage, do not cut back."

Date 2019-04-27
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Plant Answer Line Question

Keywords: Trachystemon orientalis, Invasive plants, Ground cover plants

A mail-order nursery catalog I recently browsed is touting Trachystemon orientalis as the plant for problem areas of the garden where other plants would struggle. They say it will rapidly cover blank or weedy areas in the garden, whether they are sunny, shady, wet, or dry. What can you tell me about it? It sounds intriguing, but I'm worried it might be invasive!


Trachystemon orientalis is in the same family as borage (Boraginaceae), with similar blue flowers and rough leaves (to my eye, it's a plant for a wild garden, with its resemblance to borage and comfrey). According to Perennials by Roger Phillips and Martyn Rix (Firefly Books, 2002), it is a native of eastern Bulgaria, northern Turkey, and the western Caucasus region. It is naturalized in parts of England. The authors say it thrives "in wet beech forest on shady river banks and on damp rocks at up to 1000 meters, flowering in March-May." It spreads by rhizomes and is tolerant of neglect. Its preferred site is moist shade, but it adapts to a wide variety of conditions, and excels at smothering weeds. There is additional information from Missouri Botanical Garden.

You may be right to wonder when a plant seems to be just the thing for almost any spot, and when it is promoted for its ability to spread easily. Some ground cover plants hover on the brink between covering the ground you want covered, and exceeding those boundaries. Trachystemon's common name is Abraham-Isaac-Jacob. The origin of that name is unclear, but my interpretation is that a plant named for the patriarchs of Genesis has a connection to generational continuity, a fitting name for a plant which is skilled at self-propagation!

See garden writer Margaret Roach's blog post entitled "A Plant I'd Order," and notice what she says about the likely behavior of this plant in the Pacific Northwest.

Trachystemon orientalis is included in the Manual of the Alien Plants of Belgium and is listed in the Invasive Species Compendium. You could test its behavior in your garden for a season or two, and if it shows signs of aggression, you should still be able to eradicate it. If your aim is to be cautious, you may want to avoid planting it.

Date 2019-04-04
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Plant Answer Line Question

Keywords: Shade-tolerant plants, Thuja plicata, Drought-tolerant plants, Ground cover plants

I have a large Western Red Cedar (Thuja plicata, I think!) under which grow a few weeds but not much else. I would like to find a low maintenance solution for the ground that won't do any harm to it. Ground cover? Pea gravel? I worry that the shallow root system can be easily smothered so that adding soil and plants is not a good idea, plus the roots are a dense mat and difficult to dig through. Do you have any ideas or suggestions? What native plants might grow under the tree and how can I get them established?


As you've realized, Western red cedars have a dense mat of roots close to the surface. It's not a good idea to add soil on top of the roots of trees because their root flare should remain above the soil -- and even if you did, the roots of cedars would spread into that soil in a short period of time. It's also important to keep in mind that under natural conditions the ground beneath Thuja plicata is usually bare of other plants.

If the area beneath your tree isn't in deep shade and has at least some sun, you could plant spring ephemerals, including bulbs. They emerge in spring when the soil has plenty of moisture, then most die back before our summer droughts. They're not difficult to plant under large trees because you don't need to dig a large hole for seeds, bulbs, or small bare-root perennials. Good choices are Anemone blanda, Aquilegia (Columbine), Corydalis lutea, Crocus, Galanthus (snowdrops), Iris reticulata, and various kinds of Narcissus, including daffodils. Most tulips are not long-lived in our area. Hardy Cyclamen emerge and flower at other times of the year, but they're also an excellent choice. Unfortunately, most of these plants are summer-dormant, when you'll probably be out in your garden, and some self-seed prolifically under ideal conditions. A valuable resource, available for checkout at the Miller Library, is Planting the Dry Shade Garden, by Graham Rice (2011). Some of the plants he recommends will require regular watering under cedars.

Another option, also feasible if your area has some sun, is to plant our native Western sword fern (Polystichum munitum). It's evergreen, so has a presence all year, and is the most sun and drought tolerant of our native ferns. I have a 60 foot red cedar in my garden, and have successfully maintained sword ferns beneath it in partly sunny areas, but not in deep shade, where they've died out. However, because they won't survive our summer droughts in nature under these conditions, I've needed to water them about every 3 weeks in order to keep them alive. I've planted fairly small plants and watered them more often than that during their first year. If your soil is very sandy, sword ferns might not do well.

If you require a reliably showy solution, staging large planters planted with annuals or perennials, shrubs and/or trees might be best. The plants you choose will depend upon how much sun the area receives. Of course, they'll need to be watered regularly, but large planters don't need watering as often as small ones. If your hose-bib is not too far away, installing drip irrigation on a timer will ensure that your plants survive when you're away from home.

Date 2019-02-16
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