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PAL Questions: 2 - Garden Tools: 1
Keywords: Aegopodium, Convallaria, Oxalis oregana, Lobularia, Pachysandra, Galium, Lamium galeobdolon, Sheet mulching, Euonymus, Shade-tolerant plants, Polystichum munitum, Native plants--Washington, Fragaria, Garden 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.
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.
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.
Pachysandra: This plant is evergreen, and though it is not as fast growing as some groundcovers, it does spread.
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.
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.
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.
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.
Convallaria majalis: Also known as lily of the valley, this is a vigorous (aggressive!) groundcover.
Maianthemum dilatatum: Called false lily of the valley, this native plant is a good choice for shade groundcover.
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!
DON'T plant Aegopodium podagraria 'Variegatum': Commonly called bishop's weed, and frequently used as a groundcover, this plant is very invasive.
DON'T plant Lamium galeobdolon (formerly known as Lamiastrum), either: Yellow archangel is very invasive in Pacific Northwest forests.
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Part of my yard has clay close to the surface. In addition to heavy and yearly amending with compost and other organic materials, I have heard that adding gypsum is helpful for breaking up heavy clay. Any thoughts or suggestions on this?
One of the most reliable resources I've found on how to amend soil is Prof. Linda Chalker-Scott, of Washington State University Extension. According to her very useful website on horticultural myths, gypsum does improve clay soil but is not the best choice for most home gardens. Here is an excerpt:
"With the exception of arid and coastal regions (where soil salts are high) and the southeastern United States (where heavy clay soils are common), gypsum amendment is just not necessary in non-agricultural areas. Urban soils are generally amalgamations of subsoils, native and non-native topsoils, and--in home landscapes--high levels of organic and non-organic chemical additives. They are also heavily compacted and layered (and gypsum does not work well on layered soils). In such landscapes, it is pointless to add yet more chemicals in the form of gypsum unless you need to increase soil calcium levels. This nutrient deficiency can be quickly identified by any soil testing laboratory for less than a bag of gypsum costs. (If you need to improve sulfur nutrition, it's wiser to use ammonium sulfate). To reduce compaction and improve aeration in nearly any landscape, application of an organic mulch is more economically and environmentally sustainable."
Colorado State University says the following about gypsum as a soil additive:
"The belief persists that adding gypsum can 'break up' [...] compact clay soils [...] Calcium sulfate or gypsum when added to our clay soils only increases the already high calcium content. Since plants growing in our soils already have all the calcium they need, the added gypsum does nothing to improve plant growth. Spending money and time to add gypsum to a soil that is already high in calcium is a waste of money and effort.
"As far as relieving soil compaction, gypsum has no effect. Loosening soils is a physical process, not a chemical one. The way to break up tight, clay soils is through adding and mixing in organic amendments. The amendment holds the clay particles apart creating more space for the air so critical to plant root growth."
It sounds to me as if your practice of amending the soil with compost and organic matter is the best approach.
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Successful gardeners know that healthy soil translates to healthy plants. See the guide to soils and fertilizers for the home gardener from WSU Cooperative Extension.
Season: All Season
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March 22 2017 13:26:25