How to Identify Puncture Vine (a.k.a. the Goathead Monster)

This post originally appeared on Idaho Botanical Garden’s blog. With the first annual Boise Goathead Fest fast approaching, the purpose of this post is to help people in the Treasure Valley identify goatheads so that they can collect them for drink tokens to use at the event. I’m reposting it here in hopes that people around the globe who are tormented by goatheads might benefit from it. All photos in this post were taken by Anna Lindquist.

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If you have spent much time on a bicycle in Boise, chances are you have been the victim of a goathead-induced flat tire. You probably even got a good look at the spiky nutlet as you went to remove it from your tire. But where did the culprit come from? No doubt, it came from a plant. But which one?

This is particularly useful to know right now because the first annual Boise Goathead Fest is coming up, and if you manage to fill a garbage bag full of these noxious weeds before the end of July, you will earn yourself a drink token. Fortunately, this plant is fairly easy to identify; however, there are a few look-a-likes, so it is important to familiarize yourself with the plant in question so you can be sure you are pulling the right one.

puncture vine (Tribulus terrestris)

Puncture vine, also known as goathead or Tribulus terrestris, is a warm season annual that is native to the Mediterranean region of southern Europe. It was introduced to North America unintentionally by early European settlers when the plant’s blasted burs snuck their way across the ocean in sheep wool. Since then, puncture vine has spread across the continent prolifically thanks to the hitchhiking prowess of its seeds.

Behold, the infamous Goathead Monster.

Puncture vine has a prostrate habit, meaning that its branches lie flat on the ground, spreading outward from a central location. It grows upward only when it is being shaded or crowded out. Its leaves are divided into several tiny leaflets, and its flowers are small and bright yellow with five petals. It is an otherwise pretty plant were it not for the threatening, jagged fruits that follow the flowers. As these fruits dry, they dislodge from the plant, split into five pieces, and lay in wait to puncture your tire, work their way into the bottom of your shoe or the foot of an animal, or latch onto some errant fur.

puncture vine (Tribulus terrestris)

Depending on the conditions, puncture vine either remains fairly small or spreads as much as six feet wide. Fruits start forming shortly after flowering, and seeds ripen soon after that, so if the plant isn’t removed quickly – nutlets and all – future populations are guaranteed. Luckily the plants are fairly easy to remove. Unless the ground is particularly compact, they pull up easily, and if they break off at the root, they generally don’t sprout back.

Virtually any plant that has a prostrate growth habit and is actively growing in the summer could, at first glance, be mistaken for puncture vine. Closer inspection will help confirm the plant’s true identity. Two plants that might confuse you are purslane and spotted spurge. Both of these species can be found growing in full sun in disturbed or neglected sites in close company with puncture vine.

Purslane has tiny, yellow, five-petaled flowers similar to puncture vine; however, its leaves are glossy and succulent-like and its stems and leaves often have a red to purple hue to them. Purslane seeds are miniscule, and while the plant can be a nuisance in a garden bed, it poses no threat to bicycles or wildlife.

purslane (Portulaca oleracea)

Spotted spurge, also known as prostrate spurge, can be quickly distinguished by the milky sap that oozes from its broken stems. Its leaves are generally reddish purple on the undersides with a purple spot on top. Its flowers are minute and its seeds even smaller. Because its sap contains latex and other chemicals, it can irritate the skin and poison creatures that dare eat it.

spotted spurge (Euphorbia maculata)

Both of these plants are introduced, weedy species, so even if they won’t count towards your drink token, it still doesn’t hurt to pull them. Puncture vine, however, is included on Idaho’s noxious weed list, which means it is particularly problematic. So take this opportunity to pull as many as you can, and hopefully we can put a sizeable dent in the population of a plant that has tormented Boise bicyclists for far too long.

See Also: Plant vs. Bike

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Eating Weeds: Burdock

If we agree that weeds can be famous while simultaneously being infamous, a list of famous weeds must include burdock. Its fame largely comes from being an inspiration for the hook-and-loop fastener, Velcro. The idea for this revolutionary product came when Swiss inventor, George de Mestral, was removing burs – the dried inflorescences of burdock – from his dog in the early 1940’s. Most of us have experienced this, pulling out burs from animal hair or our own clothing, but few have felt inspired to develop a new product. Infamy reigns supreme.

But burdock’s fame isn’t tied to Velcro. Its tenacious, sticky burs, which house the seeds, have been attaching themselves to humans and other animals for centuries, frustrating those who have to remove them but finding new places to grow in the process. And what better way to pay tribute to this phenomenon than to dress oneself in hundreds of burs and parade around town calling yourself the Burry Man? Lest you think I’m crazy, just such a thing has been part of an annual celebration for over 300 years in a town outside of Edinburgh, Scotland.

burs of common burdock (Arctium minus)

Of course, burdock is more than its burs. Other, perhaps less celebrated features, are its edible roots and shoots. Its leaves are also edible, but most people find them too bitter to bother. Green Deane suggests wrapping the leaves around food to cook on a campfire. Both the roots and shoots can be eaten raw or cooked, and the fermented roots along with dandelion roots are traditional ingredients in the British beverage, dandelion and burdock. The roots, shoots, and leaves of burdock have also had a wide variety of medicinal uses.

Two species of burdock have become naturalized in North America – Arctium minus and Arctium lappa. Both species are biennials or short-lived perennials. They start out as rosettes of large leaves with woolly undersides. The leaves grow to a foot or more long and wide. At this stage burdock is similar in appearance to rhubarb. Burdock has a large taproot, which can extend down to three feet in the ground. The taproot continues to grow as the rosette expands. When the plant has reached a certain size it begins to put up a branching flower stalk. It is in the rosette stage, before the plant bolts, that the taproot should be harvested.

As the flower stalk grows, the plant takes on a pyramidal shape, with the leaves along the stalk getting increasingly smaller with height. The plant can reach several feet tall, with one source describing them as towering up to ten feet. The stalks should be harvested before the plants start flowering. Multiple flower heads are produced at the ends of the branching stalk. The inflorescences are composed of purple, tubular, disc florets that are encased and encircled in a series of hooked bracts. The flower heads resemble thistle flowers, but the plant is easy to distinguish from thistles due to its large, soft leaves. Speaking of the leaves, one photographer found them alluring enough to compile a series of photos of them.

Common burdock (Arctium minus): the woolly undersides of the leaves and the tops of the taproots

While burdock can be nuisance plant, it is not particularly noxious. In The Book of Field and Roadside, John Eastman writes, “Burdock cannot be labeled a truly invasive weed, for it rarely intrudes into cultivated fields. Tilling usually controls and eradicates burdock populations. Its favored havens are the disturbed soils of roadsides, railroads, fence rows, vacant lots, and around sheds and old buildings.” In Wild Urban Plants of the Northeast, Peter Del Tredici also comments on burdock’s preference for minimally maintained locations including “vacant lots and rubble dump sites; the edges of emergent woodlands; the sunny borders of freshwater wetlands, ponds, and streams; and on unmowed highway banks and median strips with frequent salt applications.”

I harvested my burdock roots along an unmaintained fence line surrounding a series of raised flower beds. I chose a simple recipe for making burdock chips that involved peeling the roots, cutting them into thin slices, dressing them with olive oil and salt, and baking them in the oven. Since the author of this recipe mentioned buying burdock from a store, they were probably using Arctium lappa, or greater burdock, which is commonly cultivated, especially in Asian countries. Both species can be prepared in similar ways.

burdock roots

The burdock chips had a pleasant nutty flavor, but they were also a little stringy and tough to chew. If I were to do it again, I would probably use a recipe like this one that involves parboiling and then frying. Sierra suggested grating the roots and frying them in bacon grease, which would probably do the trick. There are also recipes for pickled burdock roots, which would be fun to try.

Because the plants I harvested were still in their rosette stage and there weren’t any other plants in the area that were bolting, I didn’t try the shoots. But I’ll keep my eye out, and when I find some I may have to write a part two.

Eating Weeds: Pineapple Weed

When I wrote about pineapple weed (Matricaria discoidea) last year during the Summer of Weeds, I knew that it was edible but I didn’t bother trying it. Pineapple weed is one of my favorite native weeds (yes, it happens to be a native of northwestern North America). I enjoy its sweet fragrance, its frilly leaves, its “petal”-less flowers, and its diminutive size. I also appreciate its tough nature. Now that I have tried pineapple weed tea, I have another thing to add to this list of pros.

pineapple weed (Matricaria discoidea)

One thing about pineapple weed that always impresses me is its ability to grow in the most compacted soils. It actually seems to prefer them. It is consistently found in abundance in highly trafficked areas, like driveways, parking lots, and pathways, seemingly unfazed by regular trampling. Referring to pineapple weed in one of his books about wildflowers, botanist John Hutchinson wrote, “the more it is trodden on the better it seems to thrive.” This is not something you can say about too many other plants.

Both the leaves and flowers of pineapple weed are edible. The flowers seem to be the more common of the two to consume, generally in tea form. In his book Wild Edible and Useful Plants of Idaho, Ray Vizgirdas writes, “A delicious tea can be made from the dried flowers of the plant. The leaves are edible, but bitter. The medicinal uses of pineapple weed are identical to that of chamomile (Anthemis). Used as a tea it is a carminative, antispasmodic, and mild sedative.” In Wild Urban Plants of the Northeast, Peter Del Tredici writes, “A tea made from the leaves has been used in traditional medicine for stomachaches and colds.”

I harvested my pineapple weed at the end of a dirt parking lot and in an adjacent driveway/pathway. I noted how the pineapple weed’s presence waned as I reached the edges of the parking lot and pathway where, presumably, the ground was less compact. Maybe it has more to compete with there – other weeds – and so it shows up less, or maybe its roots simply “prefer” compact soils. Perhaps a little of both. Once I got my harvest home, I rinsed it off and left it to dry. Later, I snipped off the flower heads and made a tea.

I probably used more water than I needed to, so it was a bit diluted, but it was still delicious. It smelled and tasted a lot like chamomile. Sierra agreed. With a little honey added, it was especially nice. Sierra agreed again. The flowers of pineapple weed can be used fresh or dried. They can also be mixed with other ingredients to make a more interesting tea, like the recipe found here.

If you are hesitant to take the leap into eating weeds, a tea may be the simplest thing you can try. Pineapple weed tea is a great way to ease yourself into it. Apart from maybe having to harvest it from strange places, it probably isn’t much different from other teas you have tried, and, from my experience, it’s delightful.

Spring Weeds in the Mustard Family

Is there a plant family that consists of more weedy species than the mustard family? Asteraceae and Poaceae, for sure. Fabaceae or Lamiaceae, perhaps. Regardless, Brassicaceae is replete with dozens of species – mostly annual – that are skilled at taking advantage of the disturbed environments that humans are in the habit of creating.

It helps that the mustard family is so large: 372 genera and over 4,000 species distributed across the globe. Around 55 genera are said to occur in North America. Most of the plants in this family are herbaceous; few are shrubs. Foliage is aromatic, especially when crushed. Flowers are particularly distinctive. Each flower has four petals – in some species petals are divided, giving the impression that there are more than four – arranged in the shape of a cross or “X.” Flowers are often small, have 4 tall stamens and 2 short stamens, and commonly come in white, yellow, pink, or purple. They are arranged on a raceme, which is typically either tall and straight or compact and flat-topped.

Fruits in the mustard family are capsules with two compartments separated by a clear membrane. The capsules may be at least three times longer than they are wide, in which case they are referred to as a silique; or they may be less than three times longer than they are wide and referred to as a silicle. This is a curious distinction, and it doesn’t tell you all that much. It’s more important to understand that the capsules of mustards can come in various sizes and shapes, and that some can be long and narrow while others are short and either round or angular.

mustard seeds via wikimedia commons

Despite the size or shape of the capsule, enclosed are numerous seeds – sometimes dozens. Surely one of the reasons why plants in the mustard family are so successful at proliferating is their ability to produce thousands, even tens of thousands, of seeds per plant. The seeds are typically tiny; and while they may not make it very far from the parent plant, they are numerous. Depending on the species, they can also remain viable for years, affording them the opportunity to sprout whenever conditions are right. You may have heard the biblical verse about faith the size of a mustard seed giving one the ability to move mountains. Size seems irrelevant here, so how about faith as tough, resilient, opportunistic, and resourceful as a mustard seed? If a mountain can be moved, mustards might be the one to do it.

While it isn’t the scope of this post, it’s worth mentioning the chemical compounds present in mustards that give them the flavors and health benefits we enjoy as well as the toxicity that can harm us and any other organisms that dare consume them. Glucosinolates, which are present in various concentrations depending on the species, are a defining characteristic of plants in the mustard family. They contribute to the spicy-ness of things like horseradish, radish, and condiment mustard while also acting as a natural insecticide, deterring herbivory.

And now on to the cast of characters:

Whitetop (Lepidium spp.)

Garlic mustard (Alliaria petiolata) – a noxious weed in many parts of North America – is fortunately not an issue in southwestern Idaho, otherwise it would be first on the list. Instead, we deal with whitetop – a noxious weed in Idaho and several other states. As the common name suggests, individual plants – up to two feet tall – are topped with a dense cluster of tiny, white flowers. Seed production in this group isn’t as abundant as other mustards; instead, the tour de force are their rhizomes. Whitetop is a perennial plant that spreads aggressively via underground stems as well as root fragments and can easily form expansive, dense patches, outcompeting other plants in the area.

Another common name for this group is hoary cress on account of their gray-green, fuzzy foliage. They are further distinguished by the shape of their seed pods: lens-podded hoary cress (L. chalepense), heart-podded hoary cress (L. draba), and globe-podded hoary cress (L. applelianum).

white top (Lepidium sp.)

white top (Lepidium sp.)

Tansymustard (Descurainia spp.)

There are two species of tansymustard (also known as flixweed) that occur in my part of the world, one is native and the other is introduced from Europe. They are indistinguishable to my untrained eye. If I have seen them side by side, I wouldn’t have known it. They are both annuals and can be as short as a few inches to over two feet tall. They have highly dissected, fern-like leaves and tiny, pale yellow or green-yellow flowers. The seed pods are very skinny and around an inch long. Each pod can hold 40 seeds, and a large plant can produce over 75,000 seeds. They are quick to take advantage of disturbed soil and come up in abundance after a fire. I’m not sure what it is about this year, but they have been particularly prolific this spring.

tansymustard (Descurainia sp.)

tansymustard (Descurainia sp.)

Blue Mustard (Chorispora tenella)

Also known as musk mustard or crossflower, this sticky, stinky, annual plant apparently makes cow’s milk taste funny; however some people still enjoy eating it. It can get to about a foot and a half tall, and is adorned with pretty, little, blue-purple flowers. The pointy seed pods split crosswise rather than lengthwise, an uncommon trait in mustards.

blue mustard (Chorispora tenella)

blue mustard (Chorispora tenella)

Desert Madwort (Alyssum desertorum)

Like tansymustard, this species is very similar in appearance to another closely related species, Alyssum alyssoides (commonly known as pale madwort or yellow alyssum). Both are annuals under a foot tall, covered in tiny hairs, with minuscule yellow flowers, and numerous round seed pods. They are adapted to dry, neglected sites.

yellow alyssum (Alyssum dessertorum)

Annual Honesty (Lunaria annua)

If you don’t recognize this plant when it’s flowering, you will when its seed pods ripen. They are thin, round discs up to three inches across. Eventually, the outer layers of the seed pods fall away, and translucent membranes remain, sometimes with seeds still attached. This trait has earned this species common names like money plant and silver dollar. The plants are attractive, reach up to three feet tall, and produce showy, purple flowers, which explains why they are popular ornamentals. However, like other mustards, the proficiency with which they reproduce in abundance via seeds, means they also easily migrate into natural areas and neglected sites.

annual honesty (Lunaria annua)

Hairy Bittercress (Cardamine hirsuta)

This little, quick-growing, fast-spreading annual is a common nuisance in greenhouses and nurseries. On stalks above compact rosettes are borne clusters of white flowers that, as Ken Thompson writes in The Book of Weeds, “are so tiny they are almost invisible.” The slender seed pods burst open at maturity, sending minuscule seeds flying. Brush your hand over a patch of mature hairy bittercress and you will be bombarded.

hairy bittercress (Cardamine hirsuta)

And the list goes on…

I’ve observed several other weedy species in this family recently, but to keep the length of this post reasonable I will just list them here: shepherd’s purse (Capsella bursa-pastoris), clasping pepperweed (Lepidium perfoliatum), spring draba (Draba verna), tumble mustard (Sisymbrium altissimum), and pennycress (Thlaspi arvense). This list only scracthes the surface; there are many other weeds in the mustard family. All deserve to be mentioned, so perhaps another time.

See Also: In Defense of Plants – One Mustard, Many Flavors

Awkward Botanical Sketches #1

At the beginning of the year I unveiled my plan to share some of my sketches with you as I learn how to draw. This is to make up for not writing quite as many posts so that I can spend time working on some other projects. It also serves as a great motivator to actually draw, which isn’t something I do very often. Turns out that if you want to get better at something, you actually have to do it.

To help me in my quest, I collected a few books. Some are instructional and others simply feature inspirational artwork. I’ve included links to a few of these books with my drawings below. If you have any books you would like to recommend, particularly a book that has helped you learn to draw, please let me know in the comment section below.

And now on to my dumb drawings…

My first drawing in Drawing Nature by Jill Bliss

Drawing of a hibiscus flower with help from Illustration School: Let’s Draw Plants and Small Creatures by Sachiko Umoto

A sketch inspired by Carcassonne: Over Hill and Dale

Sketch of an old tree inspired by a drawing in Clare Walker Leslie’s book, Drawn to Nature

Sketch of agave in bloom inspired by an image on the back of some guy’s shirt at Treefort Music Fest

Sketch of a tiny tuft of grass I was trying to identify. It’s still a bit of a mystery.

A Few Fun Facts About Pollen

Sexual reproduction in vascular plants requires producing and transporting pollen grains – the male gametophytes or sperm cells of a plant. These reproductive cells must make their way to the egg cells in or order to form seeds – plants in embryo. The movement of pollen is something we can all observe. It’s happening all around us on a regular basis. Any time a seed-bearing plant (also known as a spermatophyte) develops mature cones or flowers, pollen is on the move. Pollen is a ubiquitous and enduring substance and a fascinating subject of study. In case you don’t believe me, here are a few fun facts.

Bee covered in pollen – photo credit: wikimedia commons

Pollen is as diverse as the species that produce it. Pollen grains are measured in micrometers and are so tiny that the only reason we can see them with the naked eye is because they are often found en masse. Yet they are incredibly diverse in size, shape, and texture, and each plant species produces its own unique looking pollen. With the help of a good microscope, plants can even be identified simply by looking at their pollen. See images of the pollen grains of dozens of plant species here and here.

Pollen helps us answer questions about the past. Because pollen grains are so characteristic and because their outer coating (known as exine) is so durable and long-lasting, studying pollen found in sediments and sedimentary rocks helps us discover all sorts of things about deep time. The study of pollen and other particulates is called palynology. Numerous disciplines look to palynology to help them answer questions and solve mysteries. Its even used in forensics to help solve crimes. Criminals should be aware that brushing up against a plant in bloom may provide damning evidence.

Pollen oddities. While all pollen is different, some plants produce particularly unique pollen. The pollen grains of plants in the orchid and milkweed families, for example, are formed into united masses called pollinia. Each pollinium is picked up by pollinators and transferred to the stigmas of flowers as a single unit. A number of other species produce other types of compound pollen grains. The pollen grains of pines and other conifers are winged, and the pollen grains of seagrass species, like Zostera spp., are filamentous and said to hold the record for longest pollen grains.

The pollinia of milkweed (Asclepias spp.) look like the helicopter-esque fruits of maple trees. photo credit: wikimedia commons

Pollen tube oddities. In flowering plants, when pollen grains reach the stigma of a compatible flower, a vegetative cell within the grain forms a tube in order to transport the regenerative cells into the ovule. This tube varies in length depending on the length of the flower’s style. Because corn flowers produce such long styles (also know as corn silk), corn pollen grains hold the record for longest pollen tube, which can measure 12 inches or more. Species found in the mallow, gourd, and bellflower families produce multiple pollen tubes per pollen grain. Hence, their pollen is said to be polysiphonous.

Pollen is transported in myriad ways. Plants have diverse ways of getting their pollen grains where they need to be. Anemophilous plants rely on wind and gravity. They produce large quantities of light-weight pollen grains that are easily dislodged. Most of this pollen won’t make it, but enough of it will to make this strategy worth it. Hydrophilous plants use water and, like wind pollinated plants, may produce lots of pollen due to the unpredictably of this method. Some hydrophilous plants transport their pollen on the surface of the water, while others are completely submerged during pollination.

Employing animals to move pollen is a familiar strategy. Entomophily (insect pollination) is the most common, but there is also ornithophily (bird pollination) and chiropterophily (bat pollination), among others. Plants that rely on animals for pollination generally produce pollen grains that are sticky and nutritious. They attract animals using showy flowers, fragrance, and nectar. The bodies of pollinating insects have modifications that allow them to collect and transport pollen. Certain bees, like honey bees and bumblebees, have pollen baskets on their hind legs, while other bees have modified hairs called scopae on certain parts of their bodies.

Pollen is edible. Some animals – both pollinating and non-pollinating – use pollen as a food source. Animals that eat pollen are palynivores. Bees, of course, eat pollen, but lots of other insects do, too. Even some spiders, which are generally thought of as carnivores, have been observed eating pollen that gets trapped in their webs.

Pollen is thought to be highly nutritious for humans as well, and so, along with being taken as a supplement, it is used in all sorts of food products. To collect pollen, beekeepers install pollen traps on their beehives that strip incoming worker bees of their booty. Pollen from various wind pollinated plants, like cattails and pine trees, are also collected for human consumption. For example, a Korean dessert called dasik is made using pine pollen.

pine pollen – photo credit: wikimedia commons

Pollen makes many people sick. Hay fever is a pretty common condition and is caused by an allergy to wind-borne pollen. This condition is also known as pollinosis or allergic rhinitis. Not all flowering plants are to blame though, so here is a list of some of the main culprits. Because so many people suffer from hay fever, pollen counts are often included in weather reports. Learn more about what those counts mean here.

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Lettuce Gone Wild, part two

The lettuce we eat is a close relative to the lettuce we weed out of our gardens. Last week we discussed the potential that wild relatives may have for improving cultivated lettuce. But if wild lettuce can be crossed with cultivated lettuce to create new cultivars, can cultivated lettuce cross with wild lettuce to make it more weedy?

Because so many of our crops are closely related to some of the weeds found along with them or the plants growing in nearby natural areas, the creation of crop-wild hybrids has long been a concern. This concern is heightened in the age of transgenic crops (also known as GMOs), for fear that hybrids between weeds and such crops could create super weeds – fast spreading or highly adapted weeds resistant to traditional control methods such as certain herbicides. To reduce this risk, extensive research is necessary before such crops are released for commercial use.

flowers of prickly lettuce (Lactuca serriola)

There are no commercially available, genetically modified varieties of cultivated lettuce, so this is not a concern when it comes to crop-wild hybrids; however, due to how prevalent weedy species like prickly lettuce (Lactuca serriola) are, hybridization with cultivated lettuce is still a concern. So, it is important to understand what the consequences might be when hybridization occurs.

In a paper published in Journal of Applied Ecology in 2005, Hooftman et al. examined a group of second-generation hybrids (L. sativa x L. serriola), and found that the hybrids behaved and appeared very similarly to non-hybrid prickly lettuce. They also found that the seeds produced by the hybrids had a significantly higher germination rate than non-hybrid plants. This is an example of hybrid vigor. Thus, “if hybridization does occur, this could lead to better performing and thus potentially more invasive (hybrid) genotypes.” However, the authors cautioned that “better performing genotypes do not automatically result in higher invasiveness,” and that much depends on the conditions they are found in, the level of human disturbance, etc.

Another thing to consider is that hybrids are not stable. In an article published in Nature Reviews Genetics in 2003, Stewart et al. adress the “misunderstanding that can arise through the confusion of hybridization and … introgression.” It is wrong to assume that hybrids between crops and wild relatives will automatically lead to super weeds. For this to occur, repeated crosses with parental lines (also known as backcrossing) must occur, and “backcross generations to the wild relative must progress to the point at which the transgene [or other gene(s) in question] is incorporated into the genome of the wild relative.” That is what is meant by “introgression.” This may happen quickly or over many generations or it may never happen at all. Each case is different.

prickly leaf of prickly lettuce (Lactuca serriola)

In a paper published in Journal of Applied Ecology in 2007, Hooftman et al. observe the breakdown of crop-wild lettuce hybrids. They note that “fitness surplus through [hybrid vigor] will often be reduced over few generations,” which is what was seen in the hybrids they observed. One possible reason why this occurs is that lettuce is predominantly a self-crossing species; outcrossing is rare, occurring 1 – 5% of the time thanks to pollinating insects. But that doesn’t mean that a stable, aggressive genotype could never develop. Again, much depends on environmental conditions, as well as rates of outcrossing and other factors relating to population dynamics.

A significant expansion of prickly lettuce across parts of Europe led some to hypothesize that crop-wild hybrids were partly to blame. In a paper published in Molecular Ecology in 2012 Uwimana et al. ran population genetic analyses on extensive data sets to determine the role that hybridization had in the expansion. They concluded that, at a level of only 7% in wild habitats, crop-wild hybrids were not having a significant impact. They observed greater fitness in the hybrids, as has been observed in other studies (including the one above), but they acknowledged the instability of hybrids, especially in self-pollinating annuals like lettuce.

seed head of prickly lettuce (Lactuca serriola)

It is more likely that the expansion of prickly lettuce in Europe is due to “the expansion of favorable habitat as a result of climate warming and anthropogenic habitat disturbance and to seed dispersal because of transportation of goods.” Uwimana et al. did warn, however, that “the occurrence of 7% crop-wild hybrids among natural L. serriola populations is relatively high [for a predominantly self-pollinating species] and reveals a potential [for] transgene movement from crop to wild relatives [in] self-pollinating crops.”