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Plant Answer Line Question

Keywords: Sheet mulching, Yucca

Our home in Seattle came with a large number of established Yucca plants, and we would like to get rid of them. However, they are quite stubborn. We've tried a few things, including digging them up, but the root system seems quite deep and extensive and they always come back, and quickly! Any suggestions? I've thought they were non-native, but I guess they could be the sort that are found in eastern WA. Are there invasive species here in western Washington?


As you have observed, Yucca is very difficult to eradicate completely. Most of the literature on the subject suggests using herbicide, but even this may be ineffective, which makes the risk of using harmful chemicals to control the plant seem even less worthwhile. There are quite a few informal discussions on how to get rid of this plant on various online gardening forums, and one mentions local gardening expert Ciscoe Morris's method for getting rid of unwanted Yucca:

"...he cut it back to ground level and put a couple of squares of heavy cardboard over it, piled on some compost/bark to hide the cardboard. I'm not growing yucca, but he said it really worked for killing it without breaking your back. Leave in place for a year."

The technique described here is called sheet mulching. This involves laying down overlapping layers of cardboard and then covering thickly with leaves, compost, and other materials. Agroforestry.net offers information on how to do this. StopWaste.org provides additional helpful information.

Here is an excerpt from an answer to a question similar to yours, written by one of my colleagues here at the Miller Library:

"The general idea is you spread out a layer of cardboard or newspaper (about 4-6 sheets) and then cover that with a layer of organic mulch (compost, straw, alfalfa hay--available at feed stores, wood chips, coffee grounds, etc.). Then wait 6-8 months. This is not an exact science because there are many variables, such as thickness of newspaper, type of mulch and what type of plant you're trying to kill. Perennial weeds and especially coarse grass will push through the cardboard once it starts to break down so it is critical that if and when this happens you pull the mulch back and put down more newspaper/cardboard, and then replace the mulch."

Yucca is not (yet) considered invasive in our state. Here are links to information on locally noxious weeds.
Washington Noxious Weed Control Board.
King County's Noxious Weed Control Program.

Date 2018-09-12
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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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