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Plant Answer Line Question

Keywords: Soils, Soil amendments, Mulching, Compost

Help! My clay soil is stunting the growth of my plants. I've amended the soil with compost and manure. Is there a another method of conditioning the soil that you can recommend?


First and most important, it appears mulching is the best organic solution for conditioning clay and heavy soils. Organic soil conditioners include compost, well-rotted animal manures, and natural fertilizers. Planting green manures such as clover, rye grass, or vetch are also effective for breaking up large clods in clay soil over time.

Sheet composting - laying compost over the entire area to be worked and using a fork (or rototiller) to work it into the soil to a depth of 2-4 inches - is cited by the resources listed below as an efficient method of soil conditioning. Both books listed below recommend repeating this process at least twice a year, in early spring and in late fall.

Secrets to Great Soil [by Elizabeth P. Stell, 1998, (pbk)] and
The Gardener's Guide to Better Soil [by Gene Logsdon, 1975, (pbk)]

The Saving Water Partnership (the City of Seattle and other government entities) has a website full of information about improving soil.

The site includes Growing Healthy Soil.

Current thinking contradicts the notion of working compost or other amendments into the soil, as explained in a March 31, 2010 Garden Professors blog post by Professor Linda Chalker-Scott of Washington State University Extension Horticulture. She specifically takes issue with the "Growing Healthy Soil" information linked above. Here is an excerpt:
"Not only will extensive digging or rototilling destroy any soil structure you might have, it will also take out the roots of any desirable plants in the vicinity). [...] improper soil amendment can cause serious problems such as soil subsidence, perched water tables, and nutrient overloads. This last point is especially important to anyone living near aquatic ecosystems, since excess nutrients always end up in the water. Before you plant this year, find out what your soil needs before amending it. And remember that mulching is the natural (and sustainable) way to add organic matter to the soil."

Date 2017-05-11
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Plant Answer Line Question

Keywords: Soils, Aegopodium, Convallaria, Oxalis oregana, Lobularia, Pachysandra, Galium, Lamium galeobdolon, Sheet mulching, Euonymus, Shade-tolerant plants, Polystichum munitum, Native plants--Washington, Fragaria, 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 2017-05-25
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Plant Answer Line Question

Keywords: Soils, Gypsum, Soil amendments

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.

Date 2017-05-25
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Plant Answer Line Question

Keywords: Soils, Cornus canadensis

What are the best soil conditions for bunchberry, Cornus canadensis?


In his book, Gardening with Native Plants of the Pacific Northwest (University of Washington Press, 1996) Arthur Kruckeberg talks about "an acid, gritty soil, somewhat damp." Paghat's Garden, a local gardener's web page, says that "being near rotting deadfall helps bunchberry a great deal, as it is frequently found growing naturally around rotting stumps and logs. To meet its desire for rotting wood in its vicinity, I started it in soil into which I had buried many whole twigs before planting the bunchberries, and I worked deeply into the soil a considerable amount of wood shavings in which our pet rats had pooped, besides organic matter from finished compost. After it is established it is a good idea to mulch every two or three years with pine or fir needles gathered from the woodlands. The wood particles worked into the soil seem really to do the trick and we've experienced none of the troubles some people have when attempting to establish a healthy patch of bunchberry. After a couple years, shavings or woodchips will have broken down into good organic compost, so to keep the quotient of decaying wood current, every couple years I pound a few sticks (mostly sundry bush trimmings) flush into the soil, as I also do for huckleberries and other such plants that have a strong symbiotic relationship with the sorts of beneficial fungus that break down wood."

Date 2017-05-25
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Plant Answer Line Question

Keywords: Effect of storms, Soils

We had quite a lightning storm today, and it made me think of something a friend had told me. He used to farm in Eastern Washington, and he said that lightning was good for the soil and the crops. Is this pure folklore or does it have some scientific basis?


This isn't just folklore. According to the Indiana Public Media's Moment of Science, "there is enough electrical energy in lightning to separate the nitrogen atoms in the air. Once the atoms are separated they can fall to earth with rain water, and combine with minerals in the soil to form nitrates, a type of fertilizer."

Date 2018-04-04
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Garden Tip

Keywords: Soils, Soil testing, Plant-soil relationships, Soil amendments, Fertilizers

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.

Date: 2007-07-12
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May 31 2018 13:14:08