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Elisabeth C. Miller Library

Archives: Plant Answer Line Q & A

Plant Answer Line questions and answers.

Rhododendron or Azalea?

I can’t always tell rhododendrons and azaleas apart. Is there a way to distinguish between them?

 

The answer is complicated because an azalea is also a rhododendron, but not all rhododendrons are azaleas! Furthermore, some azaleas are evergreen and others are deciduous. This information from the Azalea Society of America gives a few pointers.
Excerpt:
“Some small-leaved rhododendrons look like evergreen azaleas. To tell them apart, first look at a flower—most azaleas have only 5 or 6 stamens, while most rhododendrons have 10 stamens. Then look at a leaf—azalea leaves tend to be thinner, softer and more pointed than rhododendron leaves. Azalea leaves tend to have long straight hairs parallel to the leaf surface, usually along the midrib on the underside of the leaf. Finally, using a magnifying glass, look at the underside of a leaf for tiny round structures called scales. Azalea leaves never have scales, while small-leaved rhododendron leaves are always covered with scales. (The more correct name for small-leaved rhododendrons is ‘lepidote’ rhododendrons, where lepidote means ‘covered with scales’).”

In case you are curious, evergreen azaleas were first cultivated in Japan over 400 years ago (according to Christopher Fairweather, in his book Azaleas, Globe Pequot Press, 1988). These were the evergreen azaleas. The first azaleas in European gardens (starting in the 17th century in Holland) were deciduous, and they were endemic on three continents (particularly from Turkey, the Far East, and both coasts of North America).

There is a taxonomic explanation in Fred C. Galle’s book, Azaleas (Timber Press, 1987):
“The genus Rhododendron was first recognized by Linnaeus in Species Plantarum in 1753. Linnaeus created a separate genus Azalea containing six species. In 1834, George Don […] subdivided Rhododendron into eight sections which are botanically retained today. Azalea was included under the genus Rhododendron by Don. In 1870. Dr. C. J. Maximowicz made a major contribution in the classification of Rhododendron and many oriental Azalea based on the position of leaf bud in relation to flower buds. More recent classifications reflect refinement rather than major changes. […] The question of splitting Azalea into a separate genus came up as late as 1943, but it is hoped that this classification will never be accepted.”

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Toxicity of Nandina berries to birds

A rumor has been circulating among birders in our area (Puget Sound) regarding the toxicity of nandina berries to birds, specifically cedar waxwings. I use a fair amount of nandina in my landscape designs, so this is obviously a concern.

How toxic are nandina berries for wildlife? How often do birds or other critters eat enough of the fruit to be damaging?

 

I think that people are probably referring to this study:

Excerpt:
“Nandina domestica berries contain cyanide and other alkaloids. For most cultivars of N. domestica, cyanogenesis is the most important intoxication factor. Cyanide glycosides are substances present in many plants that can produce highly toxic hydrogen cyanide (HCN). At least 2000 plant species are known to contain cyanide glycosides with the potential to produce HCN poisoning. Generally, most parts of the plants contain cyanogenic glycocides [sic], the young rapidly growing portion of the plant and the seeds containing the highest concentration. At least 55 cyanogenic glycosides are known to occur in plants, many being synthesized from aminoacids as part of normal plant metabolism. Frost and drought conditions may increase cyanogenesis in some plant species. Cool moist growing conditions enhance the conversion of nitrate to aminoacids and cyanogenic glycosides instead of plant protein. Presumably, similar weather conditions during late winter and early spring in the study area might have favored increased cyanogenesis in N. domestica.”

Note that this is the first time a mass death of waxwings has been observed, studied, and related to Nandina. Also note that Nandina is invasive in southern states (which means there is probably a lot of it in Georgia, where the deaths were noticed). If there are diverse food sources for the birds in the landscapes you design, perhaps consumption of a few Nandina berries is less of an issue. Another thing to note is that there are a great many other plants whose fruit contains cyanogenic glycosides, and we are unlikely to be able to avoid planting every single genus with this characteristic.

You could aim to plant several plants in each landscape you create which are the preferred diet of local birds. Here is information about the cedar waxwing’s feeding habits.
Excerpt:

“Cedar Waxwings feed mainly on fruits year-round. In summer, they feed on fruits such as serviceberry, strawberry, mulberry, dogwood, and raspberries. The birds name derives from their appetite for cedar berries in winter; they also eat mistletoe, madrone, juniper, mountain ash, honeysuckle, crabapple, hawthorn, and Russian olive fruits. In summer Cedar Waxwings supplement their fruit diet with protein-rich insects including mayflies, dragonflies, and stoneflies, often caught on the wing. They also pick items such as scale insects, spruce budworm, and leaf beetles directly from vegetation.”

The rumor continues to arise from time to time, because of social media. This article by Mike Darcy, from the April 2014 issue of Digger, is helpful. In it, he quotes Nikkie West, the backyard habitat coordinator for Audubon in Portland, Oregon:
“We have not taken in any birds at the Wildlife Care Center that have displayed the symptoms associated with the Nandina berry, nor have our wildlife veterinarians heard about the issue within rehabilitation circles and professional affiliations in the Pacific Northwest. […] Of the approximately 3,000 birds we take in at the Audubon Wildlife Care Center each year, domestic house cats are by far the largest cause of injuries — about 40 percent. Due to the types of injuries sustained, these birds
have a low survival rate.”

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Carob to carat

As an avid plant person who is also a metalsmith and jeweler, I was surprised to learn that we get the weight measure for stones (carat) from the ancients who used carob beans as a standard weight. Because “carob beans are unusually consistent in size. This means that carob beans usually all weigh the same, no matter when or where harvested!”

My mind is reeling! How can that be? Nature doesn’t do that! Every plant is unique, I thought, weather, soil, location should change the harvest, I thought. Am I mistaken?

 

On that last point, you are not mistaken: there is variability in the weight of carob seeds, but it is relatively small. Ceratonia siliqua is in the bean/legume family [Fabaceae]. It is not the elongated carob pod that was used as a standard, it’s the seeds contained in that pod ( a typical pod contains about 10 seeds). Seeds from female trees are relatively consistent (0.197 grams or 1/150th of an ounce). This weight was standardized to 200 milligrams in 1907, and continues to be in use.

The scientific paper Seed size variability: from carob to carats (Turnbull et al., 2006, Biology Letters: The Royal Society, published online 2006 May 2) attempts to explain the “myth of constant seed weight.” As far back as ancient Greece, there was a weight called a kerat (which is echoed in carob’s Latin genus name, Ceratonia). Keration was the Greek word for carob (possibly a Semitic loan word from Aramaic/Syriac karta meaning pod or husk), and its literal meaning was ‘little horn,’ which describes the shape of the pods (not the seeds). Siliqua, carob’s species name, was the Latin word for carob, and used to refer to the smallest subdivision of the Roman pound. Carob seeds were no more consistent in mass than the other 63 species the article’s authors measured. They theorize that seeds used for weighing were a product of human selection.

Carob thrives in its native Mediterranean and Middle Eastern regions, and would simply have been readily available as a counterweight for precious substances, like gemstones and spices. Humans can perceive mere 5% differences in carob seed mass. In the ancient world, measurement of weight based on carob seeds would have been fairly dependable, based on the accuracy of the human eye—unless an unscrupulous vendor also kept sets of seeds that were heavier or lighter than the standard, either to shortchange or overcharge!

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Safety of gardening in arsenic-affected surfaces

This is about composting from raised beds constructed of railroad ties. I went to a workshop on growing edible plants, and was informed that one cannot eat anything grown in a railroad tie bed because of arsenic and other nasties, and if one has such beds, they should ONLY BE USED for ornamentals.

I try to compost everything in my garden, so I need to know if it is safe to use compost made from plants growing in railroad beds on the beds where I am growing edibles. If it is not safe, would time, weather, or decomposition EVER make it safe? I’m willing from now on to put all the soil-contaminated clippings in the city yard waste bin that goes to Cedar Grove, but I’d rather be able to make use of them in my own garden.

 

Your question about the safety of compost made from plant matter grown in a railroad-tie bed is complex. Railroad ties are treated with wood preservative that contains arsenic. Arsenic never goes away entirely, but the amount may be at lower levels than Washington State’s law on clean-up, based on parts-per-million. I would definitely recommend a soil test. Here is information from the Northwest Coalition for Alternatives to Pesticides, which has a page on chromated copper arsenate.

The Center for Disease Control has published a Public Health Statement on arsenic, excerpted below:
“About 90% of all arsenic produced is used as a preservative for wood to make it resistant to rotting and decay. The preservative is copper chromated arsenate (CCA) and the treated wood is referred to as ‘pressure-treated.’

Arsenic cannot be destroyed in the environment. It can only change its form, or become attached to or separated from particles. It may change its form by reacting with oxygen or other molecules present in air, water, or soil, or by the action of bacteria that live in soil or sediment. Arsenic released from power plants and other combustion processes is usually attached to very small particles. Arsenic contained in wind-borne soil is generally found in larger particles. These particles settle to the ground or are washed out of the air by rain. Arsenic that is attached to very small particles may stay in the air for many days and travel long distances. Many common arsenic compounds can dissolve in water. Thus, arsenic can get into lakes, rivers, or underground water by dissolving in rain or snow or through the discharge of industrial wastes. Some of the arsenic will stick to particles in the water or sediment on the bottom of lakes or rivers, and some will be carried along by the water. Ultimately, most arsenic ends up in the soil or sediment. Although some fish and shellfish take in arsenic, which may build up in tissues, most of this arsenic is in an organic form called arsenobetaine (commonly called ‘fish arsenic’) that is much less harmful.”

Washington State University has information on gardening on arsenic- or lead-affected soil which may be of interest to you.

To be cautious, you should keep the compost from these beds separate from your other compost, and only use it on your ornamental plants already being grown in those beds. I don’t recommend putting even slightly arsenic-contaminated yard waste into the city compost, since that means the problem is being spread farther afield. It would be worthwhile testing every so often for soil contaminants. Here is King County Public Health’s guide on arsenic, and testing information.

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Syrup-producing maples

I want to know what trees produce maple syrup, and if they’ll grow in the Pacific Northwest. Also, will I be able to get syrup from them in our climate?

 

I believe that any of the syrup-producing maples will grow here, but syrup production might not be possible as it depends on specific weather conditions. Massachusetts Maple Producers Association’s website lists and describes the main species of maple used for syrup (including Acer saccharum, Acer rubrum, Acer saccharinum, and Acer negundo.

An article in Wikipedia describes the type of weather conditions needed for syrup production.

Excerpt:
“Production is concentrated in February, March, and April, depending on local weather conditions. Freezing nights and warm days are needed to induce sap flows. The change in temperature from above to below freezing causes water uptake from the soil, and temperatures above freezing cause a stem pressure to develop, which, along with gravity, causes sap to flow out of tapholes or other wounds in the stem or branches.”

Massachusetts Maple Producers Association describes temperature’s effect on sap flow:

Excerpt:
“Sap flow from sugar maples is entirely temperature dependent. A rise in temperature of the sapwood to above 32 degrees F. causes a positive pressure within the wood. This pressure produces the sap flow. Many people assume that maple sap flows up from the tree’s roots on warm days. Actually, on warm spring days which follow cold nights, sap can flow down from the maple tree’s branches and then out the spout. The sap can also flows back and forth laterally within the tree. It will flow out a hole drilled into the tree or out through a broken or cut branch. The internal pressure of the tree, when it is greater than the atmospheric pressure, causes the sap to flow out, much the same way blood flows out of a cut. If you visualize a portion of a tree trunk as being under positive pressure, a taphole is like a leak, sap moves towards the point of lowest pressure from all directions.”

There is a tree description for Acer saccharum from the University of Washington’s Campus Tree Tour:

“The chief attributes of this species are its major role as an important component of forests in much of eastern North America, its warm orange fall color, its highly useful wood, and its sweet sap. When the trees are leafless in late winter, their sap rises and descends with the temperature, and people extract it to use in making syrup or sugar, whose maple flavor is one of the unique delights of life. Our climate is too warm in winter for commercially worthwhile sap harvest, but the trees grow well here.”

I think that our winters and early springs are not usually cold enough to be optimal for maple sugaring. However, I did find an archived article in Mother Earth News (1979) about tapping trees for syrup in the Pacific Northwest. There is also research being done at the University of Washington’s School of Environmental and Forest Sciences to explore the feasibility of producing maple syrup from bigleaf maples (Acer macrophyllum).

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Distinguishing among Weigela, Kolkwitzia, and Dipelta

I’m ready to call it quits…ia! How can I tell the difference between Weigela, Dipelta, and Kolkwitzia?

 

All three of these opposite-leaved shrubs are in the honeysuckle family, Caprifoliceae. However, some taxonomists historically split Kolkwitzia and Dipelta (along with Abelia and Linnaea) into Linnaeaceae, while Weigela joins Diervilla in Diervillaceae.

Although spring is the time when most plant lovers notice these flowering shrubs, it is easier to tell them apart when they develop dry fruit. Dipelta floribunda and Kolkwitzia produce achenes (dry, single-seeded fruits that do not split open), whereas Weigela florida fruits are capsules (clustered together like a tiny bunch of bananas, and developing from green or red to brown as they mature). Dipelta bracts are ornamental, papery, and colorful before they dry to brown. Kolkwitzia bracts are weird-looking, like bristly chicken feet.

If you are eager to know what you are looking at while the shrub is still in flower, it is easy to rule out Dipelta and Kolkwitzia if the flowers are red or yellow, in which case, it’s Weigela. Bear in mind that Kolkwitzia and Weigela are common in home gardens, while Dipelta is much less so. Dipelta and Kolkwitzia (Linnaea amabilis) have pale pink to whitish flowers with markings on the petals; Weigela lacks such markings. Weigela petals also differ from the other two genera in that they are all about the same size (radially symmetrical, or actinomorphic), while in Dipelta and Kolkwitzia, the lower petals are larger than the upper two (that is, bilaterally symmetrical, or zygomorphic). The bristly characteristics of Kolkwitzia that are so notable in the dried fruit are also visible in the white bristles at the flower’s base. The Dipelta flower’s base is concealed between a pair of circular bracts. To summarize: Bracts? Dipelta. Bristles? Kolkwitzia.

Sources:

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Propagating Kolkwitzia amabilis

How do I propagate Kolkwitzia amabilis?

 

There are a couple of methods of propagating Kolkwitzia amabilis, sometimes known as Linnaea amabilis. Fine Gardening says to take greenwood cuttings in late spring or early summer, or remove suckers in spring.

The American Horticultural Society’s Plant Propagation, edited by Alan Toogood (DK Publishing, 1999) says to take softwood and greenwood cuttings in late spring or early summer. Kolkwitzia amabilis is known to root easily from cuttings, and the new plants should flower in three years. The cuttings should be “two internodes or about 3 inches long; avoid thick, pithy water shoots and look out for tips distorted by aphids. Root semi-ripe cuttings in a tray or directly in pots. Rooting takes 4-6 weeks.”

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Water chestnuts and edible tubers

What is the botanical name for water chestnut? Will it grow here? Are there other water plants that have edible tubers which will thrive in the Pacific Northwest? What about edible lotus root, from Chinese lotus?

 

Chinese or Sacred lotus (Nelumbo nucifera) is hardy in zones 4-10 and is considered invasive in some parts of parts of the midwestern and southeastern U.S. This article in The Guardian by Mark Griffiths, author of The Lotus Quest, suggests growing it in a container in a conservatory or on a sunny deck. However, you may not want to harvest tubers from a lotus grown in a relatively small container, as the plant needs to be large enough to have a substantial system of linear growth in order to sacrifice some of its tubers for human consumption. According to the Colorado Water Garden Society, “Lotus grow in a linear fashion, with a sequence of a tuber producing a leaf and perhaps a flower, then beginning another tuber to repeat the cycle . . . Tuber, leaf, flower, tuber, leaf, flower, etc. Each terminal point produces a single leaf and flower and then sends out the next, new growth. Beneath the soil, lotus growth takes on one of two forms: runners and tubers. The Summer “runner” growth is thin and long (to 24″+).”

American yellow lotus (Nelumbo lutea) also has edible tubers, but it can be an aggressive grower. If you are seeking out plants, be aware that there is sometimes identity confusion among Nelumbo, Nymphaea, Nymphoides, and Nuphar. In King County, there are two common invasive water lilies that are sometimes mistaken for lotuses, Nymphaea odorata and Nymphoides peltata. While some of these water lilies have tubers that have been considered edible in times of famine, they are not a desirable food source.

The common name ‘water chestnut’ may refer to the edible corms of the Chinese water chestnut familiar from Asian cuisine (Eleocharis dulcis), which is in the sedge family (Cyperaceae), or to European water chestnut (Trapa natans), which is in the loosestrife family (Lythraceae). Eleocharis is not winter-hardy in our area (it requires zones 9-11). Trapa natans is a noxious weed in Oregon and is on the Washington State Noxious Weed quarantine list, so it is not a good choice if you are planning to grow your own aquatic plants. Green Deane’s Eat the Weeds webpage describes the differences between these plants.

One commonly grown native plant with edible tubers is Sagittaria latifolia (common arrowhead, wapato, duck potato). It is an attractive ornamental in a water garden. According to Missouri Botanical Garden, the starchy golf-ball sized tubers that develop at the ends of the rhizomes (underground runners) “are edible, and may be boiled or baked and eaten as a potato-like food. Native Americans harvested and consumed these tubers, which in some areas were known as wapato. The tubers are also an important food source for waterfowl, hence the name duck potato.” According to Eat the Weeds, only Sagittaria latifolia is of edible interest to humans because the size of the tubers or corms is more significant than in other species. Generally, the larger the leaf size, the larger the edible tuber. In any case, avoid planting the two species of Sagittaria on the Washington State Noxious Weed list: S. platyphylla (quarantine list) and S. graminea (class B).

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What’s an Amarine?

What’s an Amarine and should I grow it? Also, when do you plant the bulbs?

 

x Amarine is a cross of the South African bulb Nerine and Amaryllis belladonna (Naked Ladies). According to the Pacific Bulb Society, the plants have larger flowers than Nerine. The cross was developed in the Netherlands in 1940, according to this article by Graham Duncan of Kirstenbosch National Botanical Garden in South Africa.

This British commercial gardening site says that x Amarine improves upon Nerine’s reluctant and unpredictable blooming habits. x Amarine keeps its foliage while it is blooming. Nerine, on the other hand, produces foliage in the spring and flowers when the foliage dies back in autumn.

In his article on Belles of the Autumn Border, Graham Rice says that × Amarine tubergenii is in between its parents in flower numbers and flower size. For a Pacific Northwest perspective on growing Nerine (and its cousins), you may find Susan Calhoun’s article in Fine Gardening useful. As far as when to plant these fall-flowering bulbs in the Northwest, we suggest doing it in May when all danger of frost is long past.

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Aquatic plants as mulch?

I noticed a Seattle P-Patch garden that was using milfoil from Lake Washington as a mulch in the vegetable beds. That made me wonder about using other aquatic plants as mulch, such as seaweed. Would this be beneficial to the plants? Or would it add salt to the soil and cause problems?

 

Milfoil (Eurasian watermilfoil, or Myriophyllum spicatum) is a Class B noxious weed in Washington State, and it is on the quarantine list. I am not sure whether moving milfoil dredged from the lake into a garden as mulch violates the quarantine’s prohibition on ‘transport of plants,’ but presumably it had died back before being spread on the beds. When the plants decay, they do impart nutrients (potentially beneficial to the soil, but a detriment to the lake because they cause algae growth), but Lake Washington is not a pristine body of water, and I would be somewhat concerned about pollutants.

As for using seaweed as mulch in the garden, the book Seaweeds: Edible, Available & Sustainable by Ole Mouritsen (University of Chicago Press, 2013) notes that seaweed has been used as fertilizer for centuries in coastal regions. “In France and on Iceland, this practice goes back at least as far as the 14th century.” In Scotland and Ireland particularly, scraps of seaweed that wash ashore have been added to soil to form raised beds for potatoes and other crops. Such beds hold moisture well, but there is a concern about soil salinity (harmful to earthworms and some plants) and pollutants from contaminated water, so it is best to wash the seaweed in rainwater before use. Plants that were originally shore plants, like asparagus, cabbage, and celery are more salt-tolerant.

There is an enlightening discussion on the University of British Columbia Botanical Garden forum about using seaweed in the garden. A biologist urges rinsing the seaweed at the beach to free any creatures that might be attached to it. Even desiccated seaweed higher up on the beach harbors living things that will not survive if you unwittingly transport them with the plants you are collecting. Additionally, there may be seeds and roots of other plants you might not want to introduce into your garden.

It Is worth noting that you must have a license to harvest seaweed from Washington beaches; it is not permitted everywhere, and where it is allowed there is a ten-pound wet-weight limit. There are specific guidelines on what tools to use, and how to leave behind the base of the plant so it can continue to flourish. Be mindful that seaweed is an integral part of a complex ecosystem, and you do not want to disrupt habitat and food sources when gathering plants to use as mulch. Also heed any notices posted about pollutants that may have been released in the water where you are harvesting.

All of this being said, it does not make much sense to collect seaweed for mulch unless it is ‘in your own backyard,’ that is to say, you live near the beach. There are more sustainable mulch options (feed a compost pile with materials already in your garden, and use that as mulch; obtain free wood chips from a local arborist) that do not come with so many environmental factors to consider.

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