Elisabeth C. Miller Library

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Plant Answer Line Question

Keywords: Lolium, Ajuga, Vinca, Hedera helix, Festuca, Arctostaphylos, Achillea, Land treatment of wastewater, Ornamental grasses, Lawns and turfgrasses, Landscaping drain fields

We have a new house that we have to landscape around. The biggest problem is that we have to be careful what we plan due to the septic system. It is an evaporation system, with two huge cement tanks buried under the ground in the front of the house and plastic pipes running through the side yard. We are planting grass in a rectangle right above the biggest bunch of the plastic pipes, but what can go around it or by the cement tanks that will not grow long roots and dig into it? In looking at the planting information on the packages and in my Western Garden Book, nothing seems to mention root depth.


Below is an article entitled What to Plant Over the Septic System by Mary Robson (originally published in her Regional Garden Column for Washington State University Extension, December 6, 1998):

"As more and more people move into rural areas, questions about septic systems and landscaping have become quite common. This column deals with some of the basics. A new brochure from Washington Sea Grant called: Landscaping your Septic System, offers considerable detail on the subject and provided much of this material.

"First, be sure that the septic field is clearly identified, and you know where the reserve area is. Keep all construction away from these areas. Understanding the functioning of the system is vital. Get information. (Some of it is available in video form.) The drainfield will not work well if overloaded with extra surface water, so be certain that it is not in the path of downspout run off or irrigation systems.

"Sunlight and air circulation also help the drainfield perform properly. Avoid surrounding it with tall trees. (Some shade is fine, but you would not plant an oak on the edge of a drainfield.) Set up some barriers so that it is not compacted by frequent foot traffic. Occasional mowing or moving through the field to check the system is certainly fine, but you do not want the drainfield in the middle of a heavily used path.

"There are advantages to using plants over the drainfield. Plants do help provide oxygen exchange and contribute to evaporation necessary in the drainfield area. Choose plants with shallow, non-invasive roots. You do not want breakage or damage in pipes from root intrusions.

"Grasses are most commonly recommended for the septic area. Lawn can be attractive. Do not overload the system by watering it a lot. Meadow grasses or a mixture of turf grasses like perennial rye and some broadleaf flowers (such as yarrow) can also look good and require little maintenance. Several mixes sold as Eco-Turf or Fleur de Lawn have these components.
"Small, shallow-rooted ornamental grasses (for instance, Festuca ovina 'Glauca' 4-10 inches) can also be good looking. Very tall grasses like Stipa gigantea are not appropriate. Avoid over-active plants like English ivy (Hedera helix), which is becoming a menace in forested areas by moving in and stifling trees.

"Edible crops are not suggested. Vegetable gardening requires frequent cultivation, and digging in the drainfield area is inadvisable. Also, the brochure notes that: Sewage effluent is distributed through the soil in the drainfield area. Any root vegetables planted in this area may be directly exposed to septic tank effluent.

"Other possibilities are low-growing ground covers. Some, such as bugle weed (Ajuga reptans) and vinca (Vinca minor) grow vigorously and would fill in quickly. The native kinnickinnick (Arctostaphylos uva-ursi) grows well in full sun but is slow to establish. A mulch around the plants may help with weed control while the plants spread.

"The green growing layer over the septic tank helps the system to function, adds to the appearance of the landscape, and should, ideally, be set up to allow easy monitoring and maintenance. Keep landscaping simple and straightforward, remembering that the object is the good performance of the system."

To get more information on septic systems, contact your local health department. The brochure Landscaping Your Septic System (pdf) is available through the Sea Grant Program.

Here are links to publications that might also be helpful:
Mounds: A Septic System Alternative
Understanding and Caring for Your Sand Filter System
Care and Feeding of Septic Tanks

Date 2019-07-17
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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: Chionodoxa, Vancouveria hexandra, Tiarella, Pulmonaria, Galium, Brunnera, Vinca, Epimedium, Lamium, Platanus, Narcissus, Liliaceae, Geranium

We have a very large beautiful sycamore in our back yard. My roommate thought it would be nice to build a flower garden around the base of the tree, but something tells me that doing so would be harmful to the tree's root system. Is this true? I would love to hear your thoughts.


I think it should be safe to plant shallow-rooted, shade- and drought-tolerant perennials and small bulbs under your sycamore (I'm assuming you mean Platanus species, and not sycamore maple, which is Acer pseudoplatanus). You just need to be careful not to pile soil on top of any exposed roots, and try not to scrape or scuff any roots when you are planting. This tree does have spreading roots so they may extend out some distance. More information about the tree can be found on the pages of the U.S. Forest Service.

Some of the plants which may work well in your garden are:

Brunnera macrophylla
Galium odoratum
Geranium phaeum
Lamium (but not the invasive Lamium galeobdolon)
Vancouveria hexandra
Vinca minor

Date 2019-05-15
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Plant Answer Line Question

Keywords: Reversion, Vinca, Variegation

The variegated vinca growing in light shade in my garden has lost its white borders. Should I move it to a sunnier location?


The technical term for a loss of variegation is reversion. Variegation can be affected by light conditions as well as other factors, as this information from the Royal Horticultural Society suggests: "Variegation may vary during the year and is often less apparent in late summer. Where there is a permanent loss of colour inadequate light is often the cause. Waterlogging can also lead to loss of leaf colour. Reversion may also occur where more vigorous green shoots outgrow variegated shoots."

Here is a link to some helpful information about variegated plants, from a now-defunct website which was called Enjoy Gardening:
"Understanding all the science behind variegation is quite complicated, so here is the nutshell version. For variegation to be stable, the variegation trait must be caused by a mutated cell or a group of cells that are fit enough to grow and multiply, and those cells must originate in a region of the plant where they are involved in generating new variegated gene cells. For example, if the mutation originates in a section of a leaf or stem where normal tissue can outgrow the mutation, the variegation pattern will often be unstable and disappear. However, when there is an 'island' of mutated cells within a growing point or tip, the variegation has a greater chance of being stable... Of course, to every set of rules there are always exceptions, which is why it's quite common for completely stable varieties of plants to lose their patterns over time. And that's exactly what happens with some variegated Euonymus shrubs when the 'normal' buds produce leaves that have more chlorophyll than the variegated leaves have. Having more chlorophyll allows the normal leaves to grow faster and to out-compete the variegated foliage. In this case, the solution to preserving variegation is to get out the pruners and to lop off the emerging threat of normalcy."

As suggested above, you can try cutting off the non-variegated ('normal') foliage and hope that the variegated foliage will return to dominance.

Date 2018-12-29
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