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Archives: Plant Answer Line Q & A

Plant Answer Line questions and answers.

What causes autumn leaves to stain concrete?

Each year, in early autumn, I notice tree leaf silhouettes staining the sidewalk. Do all trees do this? I mainly see maples and occasionally oak leaf shapes. What substance in the leaves causes this staining effect?

 

Tannin in the leaves is responsible for leaving behind those silhouettes. Rain rinses the tannins from the fallen foliage, and leaves a trace. The prints are most visible on lighter colored surfaces. This is similar to the way strong black tea leaves a tannin stain on porcelain cups and tooth enamel.

The very word ‘tannin’ has its origins in the Latin, tannum, meaning the bark of an oak tree (which was used to tan animal hides to make leather). Oak has especially high levels of tannins, but because its leaves are thick and take longer to decompose, you may notice fewer leaf prints from oak trees.

Tannins are widespread in many different plants, and may be present in many parts of a plant. They are especially common in leaf tissues, “particularly in the cells of the upper epidermis (on the top surface of a leaf).”

The substance may also be found in the bark and wood of trees, as well as the buds, stems, fruits, seeds, roots, and plant galls, where it may provide protection to the plant. For example, cottonwood trees can adjust their level of tannins to defend against beavers harvesting their wood.

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On the edibility of fiddleheads

Are Bracken fern fiddleheads edible? The very old Euell Gibbons edible plant books say it’s o.k. but I’ve heard rumors that it is toxic and shouldn’t be eaten. Do the edible Ostrich Ferns grow in western Washington? I have lots of sword ferns, but nobody seems to mention if they are edible.

 

The fern whose fiddleheads are most commonly (and perhaps most safely) consumed is the ostrich fern, Matteuccia struthiopteris. According to Sue Olsen’s Encyclopedia of Garden Ferns, this plant will grow “in the severe and forbidding climates of Newfoundland and Alaska,” but they do not do well in areas with hot summers. We do occasionally have hot summers in Washington State, so that could pose a challenge, but if you suite the plant in an area with some shade, it might survive a heat wave.

The University of Maine Extension has a factsheet entitled “Facts on Fiddleheads” which mentions the health risks associated with their consumption, and offers tips on how to avoid illness. Note also, this Centers for Disease Control and Prevention page on Ostrich Fern Poisoning.

An article in Fine Gardening discusses which fern fiddleheads are safe to eat.
Excerpt:
“Throughout the world, several types of fiddleheads are eaten, though most contain toxic compounds. The most commonly eaten and most esteemed fiddlehead is that of the ostrich fern (Matteuccia struthiopteris, USDA Hardiness Zones 2-8), often simply called fiddlehead fern. The ostrich fern is the safest fern to eat, even though it, too, can contain toxins. The fiddleheads of cinnamon fern (Osmunda cinnamomea), lady fern (Athyrium filix-femina), and bracken fern (Pteridium aquilinum) can also be eaten, but all are at least mildly toxic and can cause nausea, dizziness, and headache, so it’s probably best to avoid them. The safest way to eat fiddleheads is to stick to ostrich ferns and to eat them in small quantities.”

Below is information specifically about bracken fern, Pteridium aquilinum.

Handbook of Poisonous and Injurious Plants by Lewis Nelson et al. (New York Botanical Garden, 2007) says that all parts of the plant are toxic. The toxin is thought to be ptaquiloside, a sesquiterpene.

From the Earl J. S. Rook website (no longer available online):
“Most commonly used today as a food for humans. The newly emerging croziers or fiddleheads are picked in spring and may be consumed fresh or preserved by salting, pickling, or sun drying. Both fronds and rhizomes have been used in brewing beer, and rhizome starch has been used as a substitute for arrowroot. Bread can be made out of dried and powered rhizomes alone or with other flour. American Indians cooked the rhizomes, then peeled and ate them or pounded the starchy fiber into flour. In Japan starch from the rhizomes is used to make confections. Bracken fern is grown commercially for use as a food and herbal remedy in Canada, the United States, Siberia, China, Japan, and Brazil and is often listed as an edible wild plant. Powdered rhizome has been considered particularly effective against parasitic worms. American Indians ate raw rhizomes as a remedy for bronchitis.
Bracken fern has been found to be mutagenic and carcinogenic in rats and mice, usually causing stomach or intestinal cancer. It is implicated in some leukemias, bladder cancer, and cancer of the esophagus and stomach in humans. All parts of the plant, including the spores, are carcinogenic, and face masks are recommended for people working in dense bracken. The toxins in bracken fern pass into cow’s milk. The growing tips of the fronds are more carcinogenic than the stalks. If young fronds are boiled under alkaline conditions, they will be safer to eat and less bitter.”

The book Ecosystems and Human Health by Richard Philp (CRC Press, 2001) states that “considerable evidence exists that bracken fern produces bladder cancer in cattle that eat excessive amounts when better fodder is unavailable, and in rats fed large amounts of it. Because the young shoots, called fiddleheads because of their curled shape, are eaten as a delicacy in many parts of the world, including Canada and Japan, there has been concern over potential for carcinogenic effects in humans. At one point, it was suggested that the relatively high incidence of bladder cancer in Japan might be related to consumption of bracken fern. Epidemiological studies, however, have failed to demonstrate such an association, and it is now felt that eating fiddleheads does not constitute a risk factor for cancer.”

This Northwest gardener, Paghat, also discusses the toxicity and edibility of bracken fern:

Excerpt:

“While causality for human illness from eating bracken is not proven, plausibility is present. Toxins break down in cooking, but the traditional light frying or quick parboiling is insufficient to break down potentially harmful chemical components. Bracken should be cooked at high temperatures to be safe, and are quite easy to prepare correctly in woks.

It is not recommended to eat rare bracken under any circumstances because of the statistical increase in cancers in countries where brackens are a consumed in high numbers. Ostrich Ferns are of such low toxicity as to be far preferable to meet the dietary interest in fiddleheads. But as a well-cooked food item eaten only occasionally, there is no indication of risk from bracken. Plausible risk is restricted to the accumulative effects over time from consumptions of high amounts of bracken parboiled or so briefly cooked as to still contain toxins.”

Sword fern (Polystichum munitum) growing in the wild is seldom browsed by herbivorous animals because the rough foliage is fairly repellent. That specific epithet ‘munitum’ in the scientific name means ‘armed.’ You may have seen information about Native Americans roasting the rhizomes and eating them, but this was a famine food resorted to when other resources were scarce. (The leaves were used to line fire pits for cooking, according to Frank Tozer’s book, The Uses of Wild Plants, 2007). I would not take unnecessary risks experimenting with plants that are not typically considered edible.

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Historic plantings in Washington Park Arboretum

I am researching the early years of the Washington Park Arboretum for a historical narrative I’m writing. Can you tell me what is growing near the footbridge from E. Lynn Street into the Arboretum in a January 1912 photograph? Do you know when the bridge was constructed?

I did find a 1935 plan from the Olmsted archives which seems to have a full inventory of plants, but some of the abbreviations escape me.

I am seeking any other online sources that might help in my research.

 

The bridge, designed by the architects William Sayward and Walter Willcox, was built in 1910, and at the time was referred to as the Arboretum Sewer Trestle or the Arboretum Aqueduct. Because the photo was taken in winter, it does not provide useful clues about the bare shrubs and trees. However, the 1935 plan does yield information, assuming some of those plants would have been there at the time of your photo. I can decode some of the abbreviations (such as Syc for sycamore, V.M. for vine maple, and Cot for cottonwood, W for willow, and A could be alder), and an awful lot of “Cat,” which turns out to be Catalpa, according to the 1936 plan’s Legend of Trees in the high resolution version of the image.

In addition to the Olmsted archives online, you may find these links useful:


Click the image to view close-up.

Leafy goosefoot

While taking a walk in the neighborhood, a weedy-looking plant caught my eye. It had nubbly, deep red fruit all along the stems and was quite dramatic by contrast with the tattered-looking leaves. The gardener said it was leafy goosefoot. What can you tell me about it?

 

Leafy goosefoot refers to Chenopodium foliosum (or its synonym Blitum virgatum, as well as a related species, Blitum capitatum). Another common name is ‘strawberry blite,’ not to be confused with blight of any kind, but derived from the genus name Blitum. The leaves do resemble the footprint of a goose. Those nubbly strawberry-like fruits that are produced from summer to early autumn are edible, with a mildly sweet flavor or–according to a 1794 issue of Curtis’s Botanical Magazine–“in their taste they have nothing to recommend them, though not pleasant they are harmless.” The leaves are also edible and similar in both appearance and flavor to spinach, which is why another name for the plant is ‘strawberry spinach.’ It has value as a beneficial plant for wildlife, and it has a history of being used to make dye and ink.

Strawberry blite is a wild and weedy plant that can tolerate harsh conditions, as demonstrated by its ability to grow in landscapes ravaged by wildfire.

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Can watermelon and cantaloupe cross-pollinate?

Will watermelon and cantaloupe cross-pollinate and produce bad-tasting melons? Is it possible for a vine that had cucumbers growing on it earlier in the season to produce a cantaloupe? I could swear that we now have what looks like a melon on a vine that had cukes before…

 

The short answer is, no. It’s fine to grow watermelons and cantaloupe side by side. Cross-pollination between melon varieties may occur, but not between watermelons (Citrullus lanatus v. lanatus) and cantaloupes (Cucumis melo ssp. melo v. cantalupo), as they are two different species. In addition, cross-pollination affects not the melon produced that year, but the melons one might grow from any seeds produced inside that melon. According to Sue Stickland’s Back Garden Seed Saving (Chelsea Green, 2001), “commercial seed growers are recommended to isolate melon varieties by 500-1000 meters” or “bag and hand-pollinate the flowers” to keep unwanted hybridization from happening.

The same principle holds true for cantaloupe (Cucumis melo and cucumber (Cucumis sativus): they are indeed in the same plant family (Cucurbitaceae), but they are different species. If your vines were planted close together, you might not have realized there was a melon developing in among the cucumbers–and if you planted the vines from seed, it’s very possible the seed packet contained a surprise cantaloupe!

You may find this information from Iowa State University Extension about cross-pollination among vine crops interesting:
“Since they have a similar flowering habit, bloom about the same time, and are members of the same plant family, it is logical that gardeners might assume that squash, melons, and cucumbers will cross-pollinate. Fortunately, however, this is not true. The female flowers of each crop can be fertilized only by pollen from male flowers of the same species. Cross pollination, however, can occur between varieties within a species.”

An article on fruit set in the
Cucurbit family from University of California, Davis (which also has information on how to hand-pollinate plants when necessary) says much the same thing:
“A common misconception is that squash, melons, and cucumbers will cross-pollinate. This is not true; the female flowers of each can be fertilized only by pollen from that same species. Varieties within each species, however, will cross-pollinate.”

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Hydrangea pruning

It’s January, and my hydrangeas look bedraggled and terrible. When should I prune them?

 

The answer will depend on which species of hydrangea you are growing. According to the American Horticultural Society Pruning & Training edited by Christopher Brickell (DK Publishing, 1996), Hydrangea macrophylla (bigleaf or mophead hydrangea) should be pruned after flowering in warm climates but in colder climates it is best to leave the old flower heads on the plant over the winter, and prune in spring. This rule also applies to Hydrangea serrata. If your hydrangea is blue-flowered, it is probably H. macrophylla. In her Guide to Pruning (Sasquatch Books, 2006), local pruning expert Cass Turnbull of Plant Amnesty says this plant should not need much pruning, but if you want to remove the faded blooms, you can do this in February by looking for four or five pairs of plump buds below the old flowers, and cutting just above the lowest or second lowest set of buds.

Hydrangea paniculata should be pruned in early spring, before active growth begins.

Hydrangea arborescens needs little pruning, and any pruning at all should be done in early spring.

Oakleaf hydrangea (Hydrangea quercifolia) also needs little pruning, but may be pruned in spring.

The U.S. National Arboretum offered a good introduction, no longer available on their website, but excerpted here:
“Established bigleaf, panicle, oakleaf and smooth hydrangea plants can often benefit from regular pruning. Removing about one-third of the oldest stems each year will result in a fuller, healthier plant. This type of pruning is easiest to do in winter, since the absence of leaves makes it easier to see and reach inside plants.
Gardeners may also want to prune to control height or to remove old flower heads. The best time for this type of pruning differs between species. Bigleaf and oakleaf hydrangea, which flower on previous year’s growth, should be pruned shortly after flowering is complete. Panicle and smooth hydrangea flower on current year’s growth and can be pruned anytime from late summer until early spring. If pruning these two species in the spring, try to prune before leaves appear. Plants of H. arborescens ‘Annabelle’ have been known to produce a second flush of flowers if pruned lightly after the first flowering.
Stems of bigleaf hydrangea that have been damaged by cold should be pruned as soon as it is determined that they are dead. Watch for new growth at the base of the plant. If your plant has basal shoots that are 6 to 8 inches in length, but the upper parts of the stems are still bare, then the bare stems need to be removed. For bigleaf hydrangea plants that are subject to frequent weather-related dieback, other than removing the dead stems, you probably won’t ever need to do any other pruning–Mother Nature has been doing the work for you.”

Kitsap County Master Gardener Peg Tillery, in an article formerly available on the WSU Extension Kitsap County web site, recommends waiting until March to prune hydrangeas: “In our climate we need to wait until early March to prune roses and summer blooming hydrangeas. This way the tender new growth won’t be harmed by frosts.”

The Royal Horticultural Society provides general pruning recommendations.

Here is an excerpt from a Seattle area gardener’s response to a question about hydrangea pruning on Garden Banter, a now-defunct British gardening forum:

“Different species of hydrangeas have different criteria for pruning. Some need very little pruning at all, other than to shape, as with H. quercifolia and H. anomala petiolaris. H. arborescens does well with the dramatic pruning you describe. Pruning of H. paniculata would depend on if you were training it to be upright like a tree or as a broad shrub, and need not be dramatic pruning, just barely enough to induce new growth on which flowers occur, though in your zone a more dramatic pruning might be needed because of winter damage. Most hydrangeas prefer late winter pruning, but H. macrophylla is better done in late summer when flowers are getting scruffy and new shoots are developing.”

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LECA balls

What are LECA balls, and should I use them for growing my indoor plants? I have been seeing them for sale at garden centers.

 

LECA stands for ‘lightweight expanded clay aggregate,’ and is made from clay, brick dust, and waste from the processing of albite (a sodium-rich mineral derived from feldspar). The primary use of the clay balls is as a substrate in hydroponic growing. A similar product is sold under the brand name Hydroton. From the point of view of hydroponics, LECA may be beneficial because the spaces between the clay balls offer more airflow and ease of root development, but the LECA balls have “limited water holding capacity (only a problem if you forget to water or let the water level drop).” Their absorption rate varies based on the make-up of the aggregate; the more pulverized brick and albite, the less they absorb.

Houseplant enthusiasts may mistakenly assume that using LECA balls will free them from being attentive to watering and drainage concerns. Some promoters of the clay balls suggest that you can soak them and grow your indoor plants in a container without drainage holes because the balls somehow magically provide the roots with just the right amount of moisture. It may be a stylish (if expensive) look, but it is still best to grow your indoor plants in the appropriate potting soil for their needs, and in containers with drainage. Definitely do not mix clay balls with potting soil, and do not use them in the bottom of containers. The myth of improving drainage by putting various items in the bottom of a pot (whether an indoor or outdoor container) has been debunked. Don’t create a perched water table by putting anything—clay balls, broken pottery, rocks, etc.—in the bottom of the pot. When we do this, “water percolates through the soil and, upon encountering the different layer, the water moves sideways, creating a saturated zone. Water in this saturated zone gets ‘hung up’ [or ‘perched’] on the layer that is different.”

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Black bamboo flowering

Our stand of black bamboo that has flowered is dying back. It was already escaping its barrier, but now I am concerned about the flowers producing seed that take hold everywhere. When we cut it down, should we be careful about not letting seeds scatter? I’ve heard that when one bamboo flowers, it coincides with other bamboos flowering. Is there going to be a massive die-off of bamboo?

 

You can certainly lay out a tarp for your cut bamboo, its flowers, and any potential seeds. If you are curious about seed viability, you can put some in containers and wait to see if they germinate. Some bamboo species have larger seeds that are easier to see, while others are small and easily obscured by decaying flower parts. Seeds collected before they are mature are unlikely to germinate. Based on all of these details, I don’t think your Phyllostachys nigra will be sowing itself all over the garden or the compost pile.

According to The Gardener’s Guide to Growing Temperate Bamboos (Michael Bell, Timber Press, 2000), flowering may be partial or complete. With partial flowering (which sometimes precedes full flowering), some culms will keep on going and not die. “When a bamboo flowers completely, most of the leaves are replaced by flowers, transpiration is largely interrupted, and this triggers natural responses that hasten the aging of the culm,” eventually resulting in death.

Depending on the species of bamboo, flowering is a fairly infrequent occurrence, and there are multiple theories about what prompts it. It can happen once every 30-60 years up to intervals of over 120 years. (There are just a few unusual species that flower yearly.) Bell says anecdotal accounts suggest that bamboos rarely if ever set seeds but, in his experience, “it is very rare that bamboos flowering in earnest do not set some seed during one of the years of their flowering cycle.” Clumping (sympodial) bamboo species will flower in winter and produce seed in spring, while running (monopodial) species like your black bamboo will flower in summer and produce seed in fall.

You mention the phenomenon of many bamboos flowering in unison. This is sometimes called mass synchronous flowering, or gregarious flowering, and can occur across the globe. According to Bamboo by Robert Austin and Koichiro Ueda (Weatherhill, 1970), “practically every bamboo of the same species, young or old and however widely separated they may be […] will flower in or about the same year.” Flowering in bamboo is complex and incompletely understood. A more recent scientific article, “The Bamboo Flowering Cycle Sheds Light on Flowering Diversity” by Xiao Zheng et al., classifies flowering into four categories: sporadic, massive synchronized, combined massive synchronized and sporadic, and partial flowering. Depending on the species of bamboo, regeneration can occur through sexual reproduction (seeds) or asexual reproduction (rhizomes forming small, weak shoots at first, as “the proportion of flowering bamboo generally first rises and then falls, while the proportion of non-flowering bamboo falls and then rises.” If your black bamboo dies, it is still possible you may observe some regeneration that follows this pattern.

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Ribbons of ice from plant stems

This winter, I was walking past a neighbor’s garden and noticed a few dried stalks (not sure what the plant was) that were sprouting a candy floss-like substance. When I got down close, I could see it was ice coming out of cracks in the stems. What causes this phenomenon? Does it happen to only certain types of plants?

 

You were fortunate enough to witness an example of frost flowers, also called ice flowers, or crystallofolia, a term coined by Bob Harms (University of Texas, Austin) to distinguish the phenomenon from “frost flowers” which are sea ice. Not all plants exhibit these fanciful formations of ice crystals, but sometimes their common names will hint at this potential. Verbesina virginica, native to most of the central and southern reaches of the eastern United States, is known by the names frostweed and white crownbeard; Helianthemum canadense is sometimes called rock frost or frostwort. This is not a widespread occurrence, and there is no clear pattern dictating which plant families or genera are likely to produce these ribbon-like excrescences. A few others which do this include American dittany (Cunila origanoides), Isodon excisa and I. rubescens.

In a column (from December 18, 2013) called The Buzz [no longer available online], Memphis Botanic Garden’s website explains the formation of flowers as follows: “When the ground is warm enough for the plants’ roots to still be active, but the air temperature drops below freezing […] juices from the plant are expelled through slits in the stems […] This may happen multiple times over the winter since our ground rarely freezes far down, but once the moisture is gone, so are the frost flowers.” The theories and explanations of why certain plants do this are far more complex. It may have to do with the xylem rays which carry sap from the center to the periphery of the stems, according to James R. Carter of Illinois State University. Plants with prominent rays are more likely to have ice flowers, which may be using the xylem rays as a source of fluid.

If you would like to increase the odds of witnessing these fascinating ice formations again in your own garden, you could try growing some of the plants on Carter’s list [no longer available online] (avoiding any which are invasive in our area!).

  • Anemone halleri
  • Ceratostigma willmottianum
  • Echinacea species
  • Eupatorium cannabinum
  • Helleborus argutifolius
  • Origanum vulgare
  • Plumbago auriculata
  • Salvia coccinea

To this list, I would add Monarda didyma, the only plant on which I have ever seen frost flowers in Seattle.

Should you wish to read more, and see additional illustrations, there is an article by James R. Carter entitled “Flowers and Ribbons of Ice” in American Scientist (September/October 2013 ). The website Kuriositas also has a page of photographs of “Frost Flowers: Nature’s Exquisite Ice Extrusion.”

This question and answer were featured in the Washington Park Arboretum Bulletin’s Winter 2016 issue, available here.

Leaf-out date for deciduous trees

Can you direct me to a list of deciduous trees whose leaves generally emerge in early spring, or a list of trees ranked in order of their leaf emergence? I know this will vary from year to year and from individual tree to individual tree due to climate and genes, but if there is a list out there with a general sequenced time schedule, it would be a great tool for design.

 

An early American observer of the varying timing of leaf emergence was Henry David Thoreau, whose journals list leaf-out dates for the trees and shrubs he saw in Massachusetts in 1854. In fact, his data is now being used in climate change research. Though it’s a subject that hasn’t been much approached from a garden design standpoint, the increased interest in climate change means that more research on phenology and the leafing out sequence is becoming available. There are several articles by Richard Primack in the Arnold Arboretum newsletter, Arnoldia. Primack (of Boston University) is a specialist in this topic. Primack and Caroline Polgar co-authored “Leaf-out phenology of temperate woody plants: from trees to ecosystems” (Arnoldia, Volume 68, Issue 4,2011) which states that “maples (Acer spp.), birches, alders (Alnus spp.), and poplars” tend to leaf out earlier, while “oaks, ashes (Fraxinus spp.), and hickories (Carya spp.)” are among the later-leafing trees.

The article “Why Do Temperate Deciduous Trees Leaf Out at Different Times? Adaptation and Ecology of Forest Communities,” (The American Naturalist December 1984, Martin J. Lechowicz) has a chart (p. 825) showing the tree species the author studied leafing out in this order:

 

  • Acer rubrum
  • Populus tremuloides
  • Betula papyrifera
  • Sorbus americana
  • Acer saccharinum
  • Betula alleghaniensis
  • Ulmus americana
  • Tilia americana
  • Quercus macrocarpa
  • Fraxinus pennsylvanica
  • Populus grandidentata
  • Fraxinus nigra

 

 

Another chart on the same page compares 1980 and 1981 leafout dates, with Populus tremuloides and P. balsamifera and Betula species consistently leafing early, followed by Acer and Prunus, then Fagus and Populus grandidentata, then Fraxinus and Tilia, and finally Carya and Juglans.

You may want to read a short article in the March 11, 2015 online version of Conservation Magazine on predicting the future of forests, based on two centuries of data from citizen scientists in England. Here is an excerpt:
“It is likely that the variation in each species’ sensitivities to both spring forcing and winter chilling will mean that forests will look quite different in the future. Those species for whom spring forcing is most important will grow leaves earlier in the year; those for whom the autumn and winter chill is more critical could leaf later in the year. Eventually, a late-leafing species like oak might wind up growing its leaves earlier than an early-leafing species like birch.”

Another resource that may help you determine the leaf-out date of specific trees is The Botanical Garden: volume 1: Trees & Shrubs by Roger Phillips and Martyn Rix (Firefly, 2002). The book presents photos of branch samples from many tree species, often showing the young leaves associated with a date (though not with the geographical location; bear in mind that the authors reside in England). While it doesn’t have such a photo for every tree, it might have enough trees for you to get useful data.

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