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Gardening Answers Knowledgebase

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I am setting up a native plant garden in Seattle this fall. Since the plants I'm choosing are adapted to our dry summers, is it OK for my landscape to go without supplemental water next summer?


Expect that your new garden will still need some supplemental water next July and August. People often ask why it is necessary to water plants that can grow with just rainfall in the wild. Depending on the situation, though, it can be essential for several reasons. Our city gardens often have hot areas of paving nearby, and soil that's compacted, sandy, or poor compared to forest or meadow soils. Wild plants spring up from seed or spread underground to where they can find water and other necessary conditions for their species, while the roots of transplants must suddenly support a whole plant in a new environment that is likely quite different than the native plant nursery where they were grown, and may also differ from the conditions for which they are ideally adapted. Climate change is a factor, too.

A great book that can help you with planning and maintaining landscapes with our native plants is April Pettinger and Brenda Constanzo's Native Plants in the Coastal Garden. The authors emphasize the importance of siting your native plants well, in communities of plants that are suitable for the conditions you have. Their list of plants for dry places may be helpful to you if you are still choosing plants.


I attended a garden lecture where the speaker recommended using wine corks in the garden, either ground up as a mulch, or whole in containers. She said cork would help with moisture retention in the soil. Do you agree?


The primary source of wine corks is the cork oak tree, Quercus suber. The species name is a clue to the fact that cork is largely made of suberin, a waxy hydrophobic (water-repelling) substance found in other woody plants.

It does not make sense to use a hydrophobic substance as a mulch, since mulch is meant to allow moisture to reach the roots of your plants, not to repel water. For the same reason, it does not make sense to add cork to your containers, either. Professor Linda Chalker-Scott of Washington State University says that cork will not in any way help the soil retain water.

Chalker-Scott has long advocated using arborists' wood chips as a mulch rather than bark mulch, for similar reasons:
"Bark does not function like wood chips in its water holding capacity. Bark is the outer covering of the tree and is heavily suberized to prevent water loss. Suberin is a waxy substance that will repel water, and in fact helps explain why fresh bark mulch always seems dry. Wood chips, on the other hand, consist primarily of the inner wood, which is not suberized and has the capacity to absorb and hold moisture."


I was looking at the Great Plant Picks list of recommended grasses, and I wonder why they exclude Carex, which I find is such a useful plant in the garden.


Great Plant Picks does in fact list two different Carex species but they are not on the list of grasses because Carex is a type of sedge, and sedges are not grasses. An article entitled "Sedges Have Edges" from Hilton Pond Center for Piedmont Natural History has a detailed explanation of the differences among grasses, sedges, and rushes. All three are graminoids, but grasses are in the Family Poaceae, sedges are Cyperaceae, and rushes are Juncaceae. According to the article, "a simple 'touch test' is the giveaway for sedges, whose stems when rotated have a very noticeable triangular shape--hence a total of three 'edges.'"


While reading a book on British woodlands, I came across a reference to a flower whose common name is 'town hall clock.' Can you tell me what plant it is, and why it has this name?


Townhall Clock's scientific name is Adoxa moschatellina, in the Adoxaceae family. Other common names for this native British wildflower are Moschatel, Five-Faced Bishop, Muskroot, Tuberous Crowfoot, and Hollowroot. All of those names are descriptive of various aspects of the plant's appearance, but the name Townhall Clock is evocative of the way this pale yellow-green early spring flower rises up on a slender stem. According to Sarah Raven (in her book Wild Flowers, Bloomsbury, 2012), four faces of the flower are "arranged as if on the surface of a cube, the fifth facing upwards." The aroma of the flowers is musky, or like "elder blossom with a bit of almond."

There are some detailed photos of it on this Finnish plant identification website. Despite the name Townhall Clock, it's a subtle flower that might easily be overlooked. In fact, its genus name means "without glory," due to its unshowy blooms.


My local nursery is selling bags of bat guano, and enriched compost that includes it. What is it good for, and is it safe to use? The company describes all their products as organic.


No matter what is in your compost, it is always a good idea to wear a dust mask when opening bags of soil amendments, and when spreading them in the garden. A mask will help protect you from breathing in airborne fungal spores.

Bat guano is used as a fertilizer, and provides supplemental nitrogen, according to this information from Oregon State University. It contains about 12 percent nitrogen. The ratio of N-P-K (nitrogen-phosphorus-potassium) is approximately 12-4-2.

A recent news story on National Public Radio highlighted the human health risks of exposure to bat waste (guano) in caves in Borneo. Both world travelers visiting bat caves and local harvesters of guano may be at risk of contracting very serious viruses, unless they take precautions (masks, gloves, and scrupulous hygiene). In parts of the United States (particularly the Ohio and Mississippi River valleys), there is a fungus called Histoplasma that is found in soil which contains bat or bird droppings. Gardeners who wear masks when digging in affected areas can avoid contracting histoplasmosis.

Bat and bird guano are allowed as soil amendments "with restrictions" imposed by the U.S. Department of Agriculture. They must be decomposed and dried according to the USDA Organic Regulations requirements for raw manure. I recommend contacting the manufacturer of the products and asking them about where they obtain their bat guano, and whether they meet NOP (National Organic Program) and OMRI (Organic Materials Review Institute) standards. You can also ask about their veterinary and phytosanitary certificates for these products, and whether they make certain the guano is harvested sustainably and without harm to the bats and their ecosystem or to the health of harvesters (particularly in countries without strong worker protection laws).