Idaho’s Native Milkweeds (Updated)

As David Epstein said in an interview on Longform Podcast, “Any time you write about science, somethings is going to be wrong; the problem is you don’t know what it is yet, so you better be ready to update your beliefs as you learn more.” Thanks to the newly published Guide to the Native Milkweeds of Idaho by Cecilia Lynn Kinter, lead botanist for Idaho Department of Fish and Game, I’ve been made aware of some things I got wrong in the first version of this post. I appreciate being corrected though, because I want to get things right. What follows is an updated version of the original post. The most substantial change is that there are actually five milkweed species native to Idaho rather than six. Be sure to check out Kinter’s free guide to learn more about this remarkable group of plants.

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Concern for monarch butterflies has resulted in increased interest in milkweeds. Understandably so, as they are the host plants and food source for the larval stage of these migrating butterflies. But milkweeds are an impressive group of plants in their own right, and their ecological role extends far beyond a single charismatic insect. Work to save the monarch butterfly, which requires halting milkweed losses and restoring milkweed populations, will in turn provide habitat for countless other organisms. A patch of milkweed teems with life, and our pursuits to protect a single caterpillar invite us to explore that.

Asclepias – also known as the milkweeds – is a genus consisting of around 140 species, 72 of which are native to the United States and Canada. Alaska and Hawaii are the only states in the U.S. that don’t have a native species of milkweed. The ranges of some species native to the United States extend down into Mexico where there are numerous other milkweed species. Central America and South America are also home to many distinct milkweed species. Asclepias species found in southern Africa are considered by many to actually belong in the genus Gomphocarpus.

The habitats milkweeds occupy are about as diverse as the genus itself – from wetlands to prairies, from deserts to forests, and practically anywhere in between. Some species occupy disturbed and/or neglected sites like roadsides, agricultural fields, and vacant lots. For this reason they are frequently viewed as a weed; however, such populations are easily managed, and with such an important ecological role to play, they don’t deserve to be vilified in this way.

Milkweed species are not distributed across the United States evenly. Texas and Arizona are home to the highest diversity with 37 and 29 species respectively. Idaho, my home state, is on the low end with five native species. The most abundant species found in Idaho is Asclepias speciosa, commonly known as showy milkweed.

showy milkweed (Asclepias speciosa)

Showy milkweed is distributed from central U.S. westward and can be found in all western states. It occurs throughout Idaho and is easily the best place to look for monarch caterpillars. In fact, the monarch butterfly is Idaho’s state insect, thanks in part to the abundance of showy milkweed, which is frequently found growing in large colonies due to its ability to reproduce vegetatively via adventitious shoots produced on lateral roots or underground stems. Only a handful of milkweed species reproduce this way. Showy milkweed reaches up to five feet tall and has large ovate, gray-green leaves. Like all milkweed species except one (Asclepias tuberosa), its stems and leaves contain milky, latex sap. In early summer, the stems are topped with large umbrella-shaped inflorescences composed of pale pink to pink-purple flowers.

The flowers of milkweed deserve a close examination. Right away you will notice unique features not seen on most other flowers. The petals of milkweed flowers bend backwards, which would otherwise allow easy access to the flower’s sex parts if it wasn’t for a series of hoods and horns protecting them. Collectively, these hoods and horns are called the corona, which houses glands that produce abundant nectar and has a series of slits where the anthers are exposed. The pollen grains of milkweed are contained in waxy sacs called pollinia. Two pollinia are connected together by a corpusculum giving this structure a wishbone appearance. An insect visiting the flower for nectar slips its leg into the slit, and the pollen sacs become attached with the help of the corpusculum. When the insect leaves, the pollen sacs follow. Pollination is successful when the pollen sacs are inadvertently deposited on the stigmas of another flower.

Milkweed flowers are not self-fertile, so they require assistance by insects to sexually reproduce. They are not picky about who does it either, and their profuse nectar draws in all kinds of insects including bees, butterflies, moths, beetles, wasps, and ants. Certain insects – like bumble bees and other large bees – are more efficient pollinators than others. Once pollinated, seeds are formed inside a pod-like fruit called a follicle. The follicles of showy milkweed can be around 5 inches long and house dozens to hundreds of seeds. When the follicle matures, it splits open to release the seeds, which are small, brown, papery disks with a tuft of soft, white, silky hair attached. The seeds of showy milkweed go airborne in late summer.

follicles forming on showy milkweed (Asclepias speciosa)

Whorled or narrowleaf milkweed (Asclepias fascicularis) occurs across western and southern Idaho. Its distribution continues into neighboring states. It is adapted to dry locations, but can be found in a variety of habitats. Like showy milkweed, it spreads rhizomatously as well as by seed. It’s a whispy plant that reaches one to three feet tall and occasionally taller. It has long, narrow leaves and produces tight clusters of greenish-white to pink-purple flowers. Its seed pods are long and slender and its seeds are about 1/4 inch long.

flowers of narrowleaf milkweed (Asclepias fascicularis)

seeds escaping from the follicle of narrowleaf milkweed (Asclepias fascicularis)

Swamp or rose milkweed (Asclepias incarnata) is more common east of Idaho, but occurs occasionally in southwestern Idaho. As its common name suggests, it prefers moist soils and is found in wetlands, wet meadows, and along streambanks. It can spread rhizomatously, but generally doesn’t spread very far. It reaches up to four feet tall, has deep green, lance-shaped leaves, and produces attractive, fragrant, pink to mauve, dome-shaped flower heads at the tops of its stems. Its seed pods are narrow and around 3 inches long.

swamp milkweed (Asclepias incarnata)

Asclepias cryptoceras ssp. davisii, or Davis’s milkweed, is a low-growing, drought-adapted, diminutive species that occurs in southwestern Idaho. It has round or oval-shaped leaves and produces flowers on a short stalk. The flowers have white or cream-colored petals and pink-purple hoods. The range of Asclepias cryptoceras – commonly known as pallid milkweed or jewel milkweed – extends beyond Idaho’s borders into Oregon and Nevada, creeping north into Washington and south into California. Another subspecies – cryptoceras – can be found in Nevada, Utah, and their bordering states.

Davis’s milkweed (Asclepias cryptoceras ssp. davisii)

The final species is rare in Idaho, as Idaho sits at the top of its native range. Asclepias asperula ssp. asperula, or spider milkweed, has a single documented location in Franklin County (southeastern Idaho). Keep your eyes peeled though, because this plant may occur elsewhere, either in Franklin County or neighboring counties. It grows up to two feet tall with an upright or sprawling habit and produces clusters of white to green-yellow flowers with maroon highlights. Its common name comes from the crab spiders frequently found hunting in its flower heads.

A sixth species, horsetail milkweed (Asclepias subverticillata), has been falsely reported in Idaho. Collections previously labeled as A. subverticillata have been determined to actually be the similar looking A. fascicularis.

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Using Weeds: Soapwort

Over the past year or so I have written about several edible weeds in an effort to highlight useful weeds. However, weeds don’t have to be edible to be useful. In fact, many weeds are most certainly not edible, but that doesn’t mean they are of no use to humans. Soapwort, for example, is poisonous, and while it does have a history of being used internally as medicine, ingesting it is not advised and should only be done under the direction of a doctor. A much less risky activity would be to make soap out of it.

soapwort (Saponaria officinalis)

Saponaria officinalis, commonly known as bouncing bet, hedge pink, fuller’s herb, scourwort, and soapweed or soapwort, is an herbaceous perennial native to Europe. It has been planted widely in flower beds and herb gardens outside of its native range, desired both for its beauty and utility. Capitalizing on our appreciation for it, soapwort has expanded beyond our garden borders and into natural areas, as well as vacant lots, roadsides, and other neglected spaces. Even in a garden setting it can be a bit of a bully, especially if ignored for a season or two.

The stems of soapwort grow to about two feet tall, are unbranched, and sometimes tinged with pink, purple, or red. The leaves are oblong and oppositely-arranged, and their bases form prominent collars around the stems. Showy clusters of flowers are found atop the stems throughout the summer. Like other flowers in the pink family (Caryophyllaceae), they are cigar-shaped at the base and opened wide at the end, showing off 5 distinct petals with notches at their tips. The petals of soapwort flowers bend backwards, with their sex parts protruding outwards. In his description of the flowers, John Eastman remarks in The Book of Field and Roadside that “the reflexed petals surrounding the sexual organs give the impression of flagrant thrust; this is a gaudy, unshy flower.”

collared stem of soapwort (Saponaria officinalis)

The fragrant flowers are pink to white in color. They open in the evening and remain open for a few short days. In an individual flower, pollen matures and is mostly shed before the stigma is ready to accept it. This helps reduce the chance of self-pollination. Cross pollination occurs with the assistance of moths who visit the flowers at night, as well as bees and other flower-visiting insects that come along during the daytime. Soapwort fruits are oval capsules containing as many as 500 kidney-shaped seeds. Seeds aren’t essential to the plants spread though, as much of its colonization occurs via vigorous rhizomes.

In fact, vegetative reproduction is the means by which soapwort forms such expansive, thick patches. It also helps that it’s poisonous. The saponins – its soap making compounds – that it produces in its roots, shoots, and leaves deter most insects and other animals from eating it. It has a reputation for poisoning horses, cows, and other livestock, and so is unwelcome in pastures and rangelands. Saponins are also poisonous to fish, so growing soapwort near fish ponds is not advised.

soapwort (Saponaria officinalis)

Soapwort occurs in a variety of soils including sandy, dry, and rocky sites and is surprisingly drough-tolerant, fine qualities to have when colonizing neglected sites. While most other organisms ignore soapwort, it has a friend in humans. Eastman sums this up well: “Soapwort’s most important associate – as is true of most plants we label weeds – is undoubtedly humankind, without whose helpful interventions the plant would surely be much rarer than it is.”

I made a soapy liquid out of soapwort by following a recipe that can be found on various blogs and websites by searching “saponaria soap recipe.” Basically it’s a cup of fresh leaves and stems along with a cup of dried leaves and stems added to a quart of distilled water brought to a boil. After simmering for 15 minutes and then allowing it to cool, strain the mixture through cheese cloth, and it’s ready to go.

This gentle but effective soap can be used for cleaning countertops and other surfaces, as well as dishes, fabrics, and skin. Several sources say it is particularly useful for cleaning delicate fabrics. Sierra and I both found it to have a cooked cabbage or spinach scent to it. This can be masked by adding a few drops of essential oil. Despite its odd aroma, both Sierra and I were impressed by its cleansing power and plan to use it more often.

dried leaves of soapwort

soapwort soap

Selections from the Boise Biophilia Archives

For a little over a year now, I’ve been doing a tiny radio show with a friend of mine named Casey O’leary. The show is called Boise Biophilia and airs weekly on Radio Boise. On the show we each take about a minute to talk about something biology or ecology related that listeners in our local area can relate to. Our goal is to encourage listeners to get outside and explore the natural world. It’s fascinating after all! After the shows air, I post them on our website and Soundcloud page for all to hear.

We are not professional broadcasters by any means. Heck, I’m not a huge fan of talking in general, much less when a microphone is involved and a recording is being made. But Casey and I both love spreading the word about nerdy nature topics, and Casey’s enthusiasm for the project helps keep me involved. We’ve recorded nearly 70 episodes so far and are thrilled to know that they are out there in the world for people to experience. What follows is a sampling of some of the episodes we have recorded over the last 16 months. Some of our topics and comments are inside baseball for people living in the Treasure Valley, but there’s plenty there for outsiders to enjoy as well.

Something you will surely note upon your first listen is the scattering of interesting sound effects and off the wall edits in each of the episodes. Those come thanks to Speedy of Radio Boise who helps us edit our show. Without Speedy, the show wouldn’t be nearly as fun to listen to, so we are grateful for the work he does.

Boise Biophilia logo designed by Sierra Laverty

In this episode, Casey and I explore the world of leaf litter. Where do all the leaves go after they fall? Who are the players involved in decomposition, and what are they up to out there?

 

In this episode, Casey gets into our region’s complicated system of water rights, while I dive into something equally complex and intense – life inside of a sagebrush gall.

 

In this episode, I talk about dead bees and other insects trapped and dangling from milkweed flowers, and Casey discusses goatheads (a.k.a. puncture vine or Tribulus terrestris) in honor of Boise’s nascent summer celebration, Goathead Fest.

 

As much as I love plants, I have to admit that some of our best episodes are insect themed. Their lives are so dramatic, and this episode illustrates that.

 

The insect drama continues in this episode in which I describe how ant lions capture and consume their prey. Since we recorded this around Halloween, Casey offers a particularly spooky bit about garlic.

 

If you follow Awkward Botany, you know that one of my favorite topics is weeds. In this episode, I cover tumbleweeds, an iconic western weed that has been known to do some real damage. Casey introduces us to Canada geese, which are similar to weeds in their, at times, overabundance and ability to spawn strong opinions in the people they share space with.

 

In this episode, I explain the phenomenon of marcescence, and Casey gives some great advice about growing onions from seed.

 

And finally, in the spring you can’t get by without talking about bulbs at some point. This episode is an introduction to geophytes. Casey breaks down the basics, while I list some specific geophytes native to our Boise Foothills.

 

Field Trip: Orton Botanical Garden

In the inaugural year of this blog, I wrote a short post about a visit to Plantasia Cactus Gardens, a botanical garden in Twin Falls, Idaho that specializes in cold hardy cactus and other succulents. I finally made a return visit all these years later (thanks to a co-worker who organized the trip). Back in 2013, the garden was private but open to the public by appointment. Today, the garden is still open by appointment but is now a 501(c)(3) non-profit organization with a new name: Orton Botanical Garden.

With the name change and non-profit status comes a new mission statement. The garden has been an impressive display of cold hardy cactus and succulents along with native and drought-tolerant plants for many years now. It has also long been a resource for educating visitors on the importance of these plants, as well as the importance of water conservation through water efficient landscaping. So the mission statement isn’t necessarily a new direction, but rather an affirmation of what this garden has done so well for years. Few gardens are doing cold hardy, drought-tolerant plants at the level that Orton Botanical Garden is.

Many of the plants at Orton Botanical Garden are made available to the public for purchase through an annual plant sale in May, as well as through an online store. This is another great service because sourcing some of these plants is not easy, and this one of the few places they can be found for sale.

Wherever you live in the world, this is a garden that should be on your bucket list. Even at a mere 5 acres in size, one could easily spend hours exploring it, and each visit reveals something new. What follows is just a small sampling of the things you will find there.

Toroweap hedgehog (Echinocereus coccineus var. toroweapensis)

scarlet hedgehog (Echinocereus coccineus var. coccineus)

White Sands kingcup cactus (Echinocereus triglochidiatus var. triglochidiatus)

Orcutt’s foxtail cactus (Escobaria orcuttii var. koenigii)

a peak down a shallow gully flanked by cholla (Cylindropuntia spp.)

Colorado hookless cactus (Sclerocactus glaucus)

Fremont’s mahonia (Mahonia fremontii)

close up of Fremont’s mahonia (Mahonia fremontii)

spiny pillow (Ptilotrichum spinosum)

hairstreak on cliff fendlerbush (Fendlera rupicola)

Utah sweetvetch (Hedysarum boreale)

Several species of buckwheats were in bloom, including this Railroad Canyon buckwheat (Eriogonum soliceps).

There were also quite a few penstemon species blooming, like this sidebells penstemon (Penstemon secundiflorus).

More Awkward Botany Field Trips:

Drought Tolerant Plants: Ice Plants

Among the various strategies plants have for tolerating drought, succulence is easily one of the most common and most successful. A recent article in the new open source journal, Plants People Planet, explores the world of succulent plants, commenting on, among other things, their evolution and extent. At least 83 plant families contain succulent species, and as many as 3-5% of flowering plants are considered succulents.

Succulence involves the storage of water in the cells of one or more plant organs (i.e. roots, stems, or leaves) as a mechanism for surviving drought. One way that succulent species differ is the location and nature of this storage. Some succulents are all cell succulents, meaning that the cells involved in storing water are also involved in carrying out photosynthesis. Other succulents are storage succulents. They have specific cells called hydrenchyma designed for storing water. These cells are non-photosynthetic.

Plants in the family Aizoaceae are storage succulents. Commonly known as the ice plant or carpet weed family, this family consists of hundreds of species and is mainly distributed throughout a region of South Africa known as Succulent Karoo. Species in this family earn the name ice plant thanks to numerous bladder-like cells or hairs that cover their leaves and stems causing them to sparkle or glimmer in the light. Aizoaceae diversity is incredible, and while this post focuses mainly on a few select species, it’s worth browsing through the profiles listed on World of Succulents to appreciate the breadth of forms these plants can take.

common ice plant (Mesembryanthemum crystallinum)

Among many interesting features that plants in this family possess, one particularly fun thing to note is that their flowers, which are unapologetically showy, lack true petals. Instead, what appear as a series of flat, thin petals encircling the center of the flower are actually modified stamens. They act as petals – drawing in pollinators with their bright colors – so calling them petals is acceptable, just not entirely accurate. Another fun fact is that seed pods of plants in Aizoaceae are often hygrochastic – upon getting wet they burst open and expel their seeds.

The photosynthetic pathway in succulents is generally different compared to other plants. Instead of the common C3 pathway, succulents use a pathway called CAM, or Crassulacean Acid Metabolism. CAM photosynthesis is similar to C4 photosynthesis – another photosynthetic pathway common among drought tolerant plants – in that it uses PEP carboxylase instead of rubisco to fix carbon and then sends it to a separate cell to be converted into sugars. In C4 photosynthesis, this whole process happens during the day. CAM photosynthesis differs in that it fixes carbon during the night and then sends it to another cell to be converted into sugars during the day. Fixing carbon at night is a way to avoid the water loss that occurs when collecting carbon dioxide during the daytime.

In discussing Aizoaceae, this is an important consideration because, unlike many other succulents, plants in this family don’t rely solely on CAM photosynthesis, but can instead switch back and forth between C3 and CAM. The ability to do this is likely because they are storage succulents rather than all cell succulents, and because they can do this, they are very efficient carbon fixers.

flowers fading on purple ice plant (Delosperma cooperi)

I live in a region where winter temperatures can dip into the single digits (°F) and sometimes lower,  so my familiarity with ice plants is with cold hardy species and cultivars of the genus Delosperma. If you are familiar with this group of plants, it is most likely thanks to the Plant Select program based in Colorado, particularly the work of Mr. Delosperma himself, Panayoti Kelaidis. Several Delosperma species are cold hardy in the Intermountain West. Thanks to their promiscuous nature, numerous crosses have occurred between species and varieties, resulting in a wide array of flower colors. And speaking of their flowers, the glistening leaves of Delosperma have nothing on their shimmering flowers, some of which may have the ability to temporarily blind you if you’re not careful. Sun is essential though, as they usually close up when shaded.

The cold hardy ice plants of the Delosperma genus are all groundcovers, maintaining a low and creeping profile. Some creep further than others. They are generally not fond of heavy clay soils, and instead prefer soil with good drainage. During the hot, dry days of summer, they appreciate a little water now and then, but watering should be cut off at the end of summer so that they aren’t sitting in saturated soils as winter approaches. They love the sun and will generally flower from late spring throughout the summer. Of course, thanks to their interesting foliage, they catch the eye and provide interest in the garden even when they aren’t flowering.

Fire Spinner® ice plant (Delosperma ‘P001S’)

Within Aizoaceae there are several species that go by the name ice plant that are not so cold hardy. Some are grown as house plants, while others are common in gardens. Still others, like Carpobrotus edulis, were once employed by land managers in California to help control erosion. However, like a number of species introduced for this purpose, C. edulis (commonly known as highway ice plant or hottentot fig) has made itself at home in areas where it wasn’t invited. It has become particularly problematic in coastal ecosystems, spreading quickly across sandy soils and outcompeting native plants. Despite being brought in to control erosion, it actually causes erosion in steep, sandy areas when its carpet-like growth becomes heavy with water and begins sliding down the hill.

highway ice plant (Carpbrotus edulis) carpeting a slope near San Diego – photo credit: Sierra Laverty

Introducing plants to our gardens that come to us from the other side of the globe should be done with caution and care. We don’t want to be responsible for the next invasive species. Since ice plant species have become problematic in California, should we be concerned about cold hardy delospermas? In trialing their plants, invasive qualities are among those that the Plant Select program watches out for, and delospermas seem pretty safe. However, as Kelaidis observes in a blog post from 2014, we should remain vigilant.

Select Resources:

Investigating the Soil Seed Bank

Near the top of the world, deep inside a snow-covered mountain located on a Norwegian island, a vault houses nearly a million packets of seeds sent in from around the world. The purpose of the Svalbard Global Seed Vault is to maintain collections of crop seeds to ensure that these important species and varieties are not lost to neglect or catastrophe. In this way, our food supply is made more secure, buffered against the unpredictability of the future. Seed banks like this can be found around the world and are essential resources for plant conservation. While some, like Svalbard, are in the business of preserving crop species, others, like the Millennium Seed Bank, are focused on preserving seeds of plants found in the wild.

Svalbard Global Seed Vault via wikimedida commons

Outside of human-built seed banks, many plants maintain their own seed banks in the soil where they grow. This is the soil seed bank, a term that refers to either a collection of seeds from numerous plant species or, simply, the seeds of a single species. All seed bearing plants pass through a period as a seed waiting for the chance to germinate. Some do this quickly, as soon as the opportunity arises, while others wait, sometimes for many years, before germinating. Plants whose seeds germinate quickly, generally do not maintain a seed bank. However, seeds that don’t germinate right away and become incorporated in the soil make up what is known as a persistent soil seed bank.

A seed is a tiny plant encased in a protective layer. Germination is not the birth of a plant; rather, the plant was born when the seed was formed. The dispersal of seeds is both a spatial and temporal phenomenon. First the seed gets to where its going via wind, water, gravity, animal assistance, or some other means. Then it waits for a good opportunity to sprout. A seed lying in wait in the soil seed bank is an example of dispersal through time. Years can pass before the seed germinates, and when it does, the plant joins the above ground plant community.

Because seeds are living plants, seeds found in the soil seed bank are members of a plant community, even though they are virtually invisible and hard to account for. Often, the above ground plant community does not represent the population of seeds found in the soil below. Conversely, seeds in a seed bank may not be representative of the plants growing above them. This is because, as mentioned earlier, not all plant species maintain soil seed banks, and those that do have differences in how long their seeds remain viable. Depending on which stage of ecological succession the plant community is in, the collection of seeds below and the plants growing above can look quite different.

Soil seed banks are difficult to study. The only way to know what is truly there is to dig up the soil and either extract all the seeds or encourage them to germinate. Thanks to ecologists like Ken Thompson, who have studied seed banks extensively for many years, there is still a lot we can say about them. First, for the seeds of a plant to persist in the soil, they must become incorporated. Few seeds can bury themselves, so those with traits that make it easy for them to slip down through the soil will have a greater chance of being buried. Thompson’s studies have shown that “persistent seeds tend to be small and compact, while short-lived seeds are normally larger and either flattened or elongate.” Persistent seeds generally weigh less than 3 milligrams and tend to lack appendages like awns that can prevent them from working their way into the soil.

The seeds of moth mullein (Verbascum blattaria) are tiny and compact and known to persist in the soil for decades as revealed in Dr. Beal’s seed viability experiment. (photo credit: wikimedia commons)

Slipping into cracks in the soil is a major way seeds move through the soil profile, but it isn’t the only way. In a study published in New Phytologist, Thompson suggests that “the association between small seeds and possession of a seed bank owes much to the activities of earthworms,” who ingest seeds at the surface and deposit them underground. Later, they may even bring them back up the same way. Ants also play a role in seed burial, as well as humans and their various activities. Some seeds, like those of Avena fatua and Erodium spp., have specialized appendages that actually help work the seeds into the soil.

Not remaining on the soil surface keeps seeds from either germinating, being eating, or being transported away to another site. Avoiding these things, they become part of the soil seed bank. But burial is only part of the story. In an article published in Functional Ecology, Thompson et al. state that burial is “an essential prelude to persistence,” but other factors like “germination requirements, dormancy mechanisms, and resistance to pathogens also contribute to persistence.” If a buried seed rots away or germinates too early, its days as a member of the soil seed bank are cut short.

The seeds of redstem filare (Erodium circutarium) have long awns that start out straight, then coil up, straighten out, and coil up again with changes in humidity. This action helps drill the seeds into the soil. (photo credit: wikimedia commons)

Soil seed banks can be found wherever plants are found – from natural areas to agricultural fields, and even in our own backyards. Thompson and others carried out a study of the soil seed banks of backyard gardens in Sheffield, UK. They collected 6 soil cores each (down to 10 centimeters deep) from 56 different gardens, and grew out the seeds found in each core to identify them. Most of the seeds recovered were from species known to have persistent seed banks, and to no surprise, the seed banks were dominated by short-lived, weedy species. The seeds were also found to be fairly evenly distributed throughout the soil cores. On this note, Thompson et al. remarked that due to “the highly disturbed nature of most gardens, regular cultivation probably ensures that seeds rapidly become distributed throughout the top 10 centimeters of soil.”

Like the seed banks we build to preserve plant species for the future, soil seed banks are an essential long-term survival strategy for many plant species. They are also an important consideration when it comes to managing weeds, which is something we will get into in a future post.

Dr. Beal’s Seed Viability Experiment

In 1879, Dr. William J. Beal buried 20 jars full of sand and seeds on the grounds of Michigan State University. He was hoping to answer questions about seed dormancy and long-term seed viability. Farmers and gardeners have often wondered: “How many years would one have to spend weeding until there are no more weeds left to pull?” Seeds only remain viable for so long, so if weeds were removed before having a chance to make more seeds, the seed bank could, theoretically, be depleted over time. This ignores, of course, the consistent and persistent introduction of weed seeds from elsewhere, but that’s beside the point. The question is still worth asking, and the study still worth doing.

When Dr. Beal set up the experiment, he expected it would last about 100 years, as one jar would be tested every 5 years. However, things changed, and Dr. Beal’s study is now in its 140th year, making it the longest-running scientific experiment to date. If things go as planned, the study will continue until at least 2100. That’s because 40 years into the study, a jar had to be extracted in the spring instead of the fall, as had been done previously, and at that point it was decided to test the remaining jars at 10 year intervals. In 1990, things changed again when the period was extended to 20 years between jars. The 15th jar was tested in 2000, which means the next test will occur in the spring of next year.

In preparing the study, Dr. Beal filled each of the 20 narrow-necked pint jars with a mixture of moist sand and 50 seeds each of 21 plant species. All but one of the species (Thuja occidentalis) were common weeds. He buried the jars upside down – “so that water would not accumulate about the seeds” – about 20 inches below ground. Near each bottle he also buried seeds of red oak and black walnut, but they all rotted away early in the study.

After the retrieval of each bottle, the sand and seed mixture is dumped into trays and exposed to conditions suitable for germination. The number of germinates are then counted and recorded. Over the years, the majority of the seeds have lost their viability. In 2000, only three species germinated  – Verbascum blattaria, a Verbascum hybrid, and Malva rotundifolia. There were only two individuals of the Verbascum hybrid, and only one Malva rotundifolia. The seeds of Verbascum blattaria, however, produced 23 individuals, suggesting that even after 120 years, the seeds of this species could potentially remain viable long into the future.

moth mullein (Verbascum blattaria)

In the 2000 test, the single seedling of Malva rotundifolia germinated after a cold treatment. Had the cold treatment not been tried, germination may not have occurred, which begs the question, how many seeds in previous studies would have germinated if subjected to additional treatments? Dr. Beal himself had wondered this, expressing that the results he had seen were “indefinite and far from satisfactory.” He admitted that he had “never felt certain that [he] had induced all sound seeds to germinate.”

There are also some questions about the seeds themselves. For example, the authors of the 2000 report speculate that poor germination seen in Malva rotundifolia over most of the study period could be “the result of poor seed set rather than loss of long-term viability.” The presence of a Verbascum hybrid also calls into question the original source of those particular seeds. A report published in 1922 questions whether or not the seeds of Thuja occidentalis were ever actually added to the jars, and also expresses uncertainty about the identify of a couple other species in the study.

Despite these minor issues, Dr. Beal’s study has shed a great deal of light on questions of seed dormancy and long-term seed viability and has inspired numerous related studies. While questions about weeds were the inspiration for the study, the things we have been able to learn about seed banks has implications beyond agriculture. Seed bank dynamics are particularly important in conservation and restoration. If plants that have disappeared due to human activity have maintained a seed bank in the soil, there is potential for the original population to be restored.

In future posts we will dive deeper into seed banks, seed dormancy, and germination. In the meantime, you can read more about Dr. Beal’s seed viability study by visiting the following links: