Three Sisters Molé Enchiladas with Blackberry Salsa

This is a special cooking edition of my blog.  Don't panic - cooking is nothing more than applied Biology (and Chemistry and Physics and Math and Art...), so there are some way interesting biological phenomena to write about here.  One of my cooking heroes, Isa Chandra Moskowitz, has issued a cooking challenge, and I took the bait.  Isa Chandra is the author of several vegan cookbooks, including Appetite for Reduction, the most useful and well-thought-out cookbook I've ever used.  The challenge is to cook a vegan entrée in under 40 minutes using black-eyed peas, bittersweet chocolate, mint and blackberries.  The recipe must be published on the internet, and there are prizes and fame/fortune involved.  I'll report back about the results of the contest!

Here is a picture of my invention, Three Sisters Molé Enchiladas with Blackberry Salsa: 

Three Sisters Molé Enchiladas with Blackberry Salsa

The contest ingredients got me thinking about agricultural centers of origin.  Agriculture started in many concentrated locations (centers of origin) throughout the world, and cuisines arose based on what crops were domesticated from the species growing in the area, with foreign ingredients added in to the extent that cultures traveled and mixed.  See if you can match the crops with the center of origin here based on what you know about modern international cuisines.  Answers are below the table.

Center of Origin	Crop
1. China	a. wheat, oats, fig, pomegranate
2. India	b. maize, beans, tomato, potato, pumpkins
3. Ethiopia	c. wheat, barley, sorghum, okra, coffee
4. South America	d. wheat, rape (canola), peas, lettuce, asparagus
5. Mediterranean	e. rice, chickpea, eggplant, tangerine
6. Middle East	f. millet, soybean, cabbage, radish, apple

Answers: 1f, 2e, 3c, 4b, 5d, 6a

Since two of the challenge ingredients (chocolate and blackberries) originate from the New World, and beans are mostly a New World crop (though black eyed peas originated in Africa), I decided to mine this vein and emphasize New World crops.  I ended up with enchiladas based on the agricultural icons, the three sisters: beans, corn and squash.  I also used tomatoes, peppers, onions, blackberries, chocolate and sunflower seeds in the recipe.  If only I could have figured out how to add cranberries, potatoes and sweet potatoes, I would have hit all the biggest crops to have originated from the New World.  Ingredients from elsewhere in the world include cumin, cilantro, oregano, wheat, cucumbers and lemon.  Isn't it strange that cumin, cilantro and cucumbers, ingredients that are indispensable to Tex-Mex cuisine, were imported to the Americas?  And that tomatoes were not an original part of Italian cuisine but were brought to Europe from Central America? 

So who are these sisters, and what are they doing in a vegan recipe?  The three sisters are staple crops grown together as companion plants by several groups of Native Americans.  The seeds of beans, corn and squash are planted in a group on a mound.  The corn grows tall and straight, providing both food for people and a pole for the beans to climb.  The squash stays low to the ground and keeps the soil cooler and moist by shading it around the base of all three plants.  It also has prickly leaves, deterring some potential pests.  The beans stalks climb the corn, lifting their fruits off the ground and taking advantage of light above the squash leaves.  At the same time, the bean roots enrich the soil with nitrogen for all three plants.  The three sisters produce more food when grown together than any of the plants grown separately.  In addition, the beans and corn provide complete protein from vegetarian sources for their human cultivators.

The recipe is below, if you'd like to make this meal-with-a-story.  It turned out to be delicious, and it's a low-calorie and low-fat meal as well.  The first time you make these, you'll need to be a Zen kitchen master to get everything done in less than 40 minutes: banish the pets and kids, turn off the radio, stand up straight, elbows in, breathe into your abdomen.  You can do it.  It's quick and easy the second time through, since all the components are actually very simple once you know how they go.  I've found this to be the case with any new recipe I make - there's always a learning curve.  You can also prepare the salsa a day ahead and let it sit in the fridge to develop its flavors, but that would violate the 40 minutes thing.

Three Sisters Molé Enchiladas with Blackberry Salsa

Three Sisters Molé Enchiladas with Blackberry Salsa
* Ingredients with asterisks evolved and were domesticated in the New World
Serves 6, at approximately 450 calories per serving, depending on your corn tortillas

Molé Enchilada Sauce Ingredients
1 T sunflower oil*
1 T whole wheat flour
2 T chili powder*
1 t cumin
2 c vegetable broth
1 small can tomato paste*
2 t fresh oregano, minced (1 t dried)
2 oz vegan bittersweet chocolate*
1/2 t salt


Enchilada Ingredients
1 t sunflower oil*
1 large onion, chopped (reserve 2 T for salsa)*
1 medium zucchini, chopped*
2 cloves garlic, minced
1 jalepeno, diced*
1 t cumin
1 15 oz. can black eyed peas, drained and rinsed (*most beans originate from South America, but not black eyed peas)
1 15 oz can hominy, drained and rinsed* (you could substitute sweet corn if you prefer, but add it with the zucchini while cooking)
12 soft corn tortillas
2 T roasted, salted, shelled sunflower seeds*

Salsa Ingredients
2 T chopped onion (from above)*
Juice of one lemon
1 t olive oil (ok, you could use sunflower, but olive oil is yummy in salsa)
2 cucumbers, chopped (peel if you like)
1/4 c chopped fresh cilantro
1/4 c chopped fresh mint
1/2 t cumin
1/4 t cayenne*
1/2 t salt
1/2 cup blackberries*

1. Preheat oven to 400 degrees Fahrenheit.

2. Make the enchilada sauce:  In a medium saucepan heat the oil on medium, add the flour and stir until toasty (30 seconds), stir in chili powder and cumin for 30 seconds, stir in broth, add all other ingredients.  Bring to a boil, reduce temperature and simmer 5 minutes, stirring often.

3. Make the enchiladas:

• In a large sauté pan on medium heat, sauté the onions in the oil until they start to soften, add the zucchini, garlic, jalepeno and cumin and continue to sauté until the zucchini starts to soften.  Stir in the black eyed peas and hominy and allow to heat up. 
• Dunk the tortillas in the enchilada sauce and wrap a large spoonful of filling in each tortilla.  Careful - everything should be hot at this point - don't burn yourself.  Line up the tortillas in a large glass baking dish.  Pour the enchilada sauce over the enchiladas and sprinkle sunflower seeds on the top.  
• Bake for 10-15 minutes uncovered, until the edges of the tortillas start to brown and the sauce is bubbling.

4. Make the salsa while the enchiladas bake: Combine all ingredients except blackberries in a dish and stir well.  Gently stir in the blackberries.

5. Serve enchiladas topped with a spoonful of salsa.



I owe a big thanks to my friend, Jenny, a vegan food aficionado, for the idea to make enchiladas.  Thanks also to my friends for being guinea pigs: Bruce, Coke and Linda, your bravery and enthusiasm are inspiring!

Rethinking the Canada Goose

Don't say Canadian - it's Canada.  The Canada goose (Branta canadensis) is a common sight in Lincoln Park, and I got to know their behaviors a little better as I watched them raise their young this spring.  Despite what some people say, they're really quite enjoyable neighbors.

Canada geese pair and young in Lincoln Park.

The geese started hatching in early May this year, and within a day or two, they were out of the nest walking and swimming and finding food.  Adult geese mate for life, with the average goose living around 24 years of age in the wild.  They tend and defend their young, but they do not feed them.  The goslings feed themselves as soon as they are out of the nest. 

Dabbling geese with a duck observing them, probably finding their upended tails completely hilarious.

Geese eat muck from the bottom of ponds, as you can see them doing in the photo above.  The tails-up bobbing for scum is called dabbling, and it never fails to crack me up.  Geese also eat grass, grains, berries and other plants, with the occasional bug thrown in.  Herbivores in general must eat a lot more volume of food to survive, and geese seem to eat constantly.  Such a large volume of food results in respectable quantities of goose poop, which people sometimes find annoying.  Admittedly, there are some beach areas in the US with hundreds of Canada geese where I would not want to walk barefoot or swim, but these are not the norm.  A little bit of goose poop is certainly better than a little bit of dog poop, since carnivore poop usually contains more harmful disease bacteria than herbivore poop.

Many of the geese in Lincoln Park probably flew north to Chicago earlier in the spring from the southern US or Mexico.  More Canada geese are migratory than non-migratory, though some members of the species have begun to live in the same location year-round.  The newer, non-migratory geese are an evolutionary response to the recent changes in their habitat: in the past 100 years, permanent open grassy areas with maintained, predator-free ponds have appeared everywhere.  Indeed, golf courses, airports, office complexes and neighborhoods provide goose heaven, and Canada geese have increased their numbers in response.  Once a fairly rare species, the Canada goose has become common enough to annoy annoyable people, sometimes even being called a pest.

In Lincoln Park, people mostly seem to enjoy the geese.  They watch them, photograph them and feed them, despite signs forbidding the feeding of wildlife.  The goslings provide food for the black-crowned night herons, and the geese eat excess pond vegetation.  Canada goose behaviors are fairly simple and easy to observe, so lots of kids and adults learn about bird biology by observing geese.  Unfortunately, geese seem to be such a common sight that some people have lost their respect for the geese's size and strength.  Parents let their toddlers chase the geese and approach the young, even with the parent geese hissing and ready to bite (yes, they lunge, flap, bite and cause an unnerving ruckus in self-defense).

A wood duck (left) hanging out with Canada gees (right) at the Alfred Caldwell Lily Pond in Lincoln Park.

Where Canada geese are a considered a nuisance, wildlife management personnel have responded with extended hunting seasons, attack dogs, culling of adult birds and donation to food charities, collection of eggs for human consumption (I hear they are delicious) and addling of goose eggs.  Addling is a technical term that means preventing an egg from developing without destroying the egg, by shaking or coating with oil to prevent oxygen from diffusing into the egg.  Goose eggs are addled instead of being simply taken because the geese can lay a second clutch of eggs.  Canada geese, like most animals, are regulated.  Geese are regulated by the Migratory Bird Act, so before someone dabbles in addling, they must apply for a permit.  An easier humane solution to overpopulation of geese would be to modify habitat to make it less inviting.  Geese will not nest in an area without good sight-lines for detecting predators, so smaller patches of lawn interspersed with shrubs or potential predator hiding places will deter them.  If we take a little time to appreciate and get to know Canada geese, they seem much less like nuisances and much more like fascinating clowns of the bird variety.

Trilobites at the Chicago Field Museum

Museums can make great places for social outings, but I also love going to them alone.  I can skip the uninteresting parts and obsess as much as I want over the good parts.  On a recent trip with friends to the Field Museum a few weeks ago, the miles of enticing, juicy exhibits drove me mad with unsatisfied curiosity.  So I went back by myself and spent an entire day!

Who knew trilobites had such amazing appendages?

There was so much to see, but my absolute favorite was their collection of exquisitely articulated trilobite fossils.  Trilobites were among the earliest Arthropods, which are the jointed-appendaged animals like insects, spiders and roly-poly bugs.  They were commonly-fossilized animals from about 525 million years ago through 250 million years ago, which is an astonishingly long period of time in evolutionary history.

Trilobites are so much cuter with antennae.

I'm not sure how these fossils were separated from their surrounding rock, but their details are incredible.  Looking at these fossils, I could imagine these little critters skittering across the ocean floor and using those appendages to sense water movement and scent of prey items.

Trilobite diversity.

The exoskeleton of trilobites was thick and tough, which made it easier for fossils to form.  Scientists have been able to study the evolution of eye structures, other sense organs and delicate feeding apparatuses over time using trilobites.  There was considerable diversity of trilobite species.  It is not understood why trilobites went extinct.  Their closest living species relative alive today is probably the horseshoe crab.

A trilobite called Walliserops with what has to be a smell/taste organ on its front end.

Parthenogenesis and the Dandelion

If you've been following along, you know that flowers are the sex organs of plants, specifically adapted to combine half of the DNA of a male plant (in pollen) with half the DNA of a female plant (in ovules) to produce a new plant, packaged in layers of a seed and wrapped in a fruit.  Sexual reproduction allows for each parent to pass on some of his/her genes to make an offspring that is combined from those genes.  It's why we all can see a little of each of our parents when we look in the mirror, and plants would too, if they used mirrors.  Unless they are dandelions.

Dandelions and Lake Michigan

Common dandelions, belonging to the species Taraxacum officinale, mostly do not reproduce sexually, though a few dandelions in Southern and Central Europe do.  The dandelions in North America are all clones of a few original European dandelions.  The method of cloning, or asexual reproduction, used by dandelions is called parthenogenesis.  In parthenogenesis, offspring are produced that look like normal offspring - starting out as embryos and growing to adults (versus asexual reproduction by fragmentation where the adult parent breaks in two, and viola, you have two new organisms).  For plants, parthenogenesis usually means producing ovules that have a complete copy of the mom plant's DNA, called apomixis.  The apomictically-produced ovules develop into seeds that are genetically identical to the mom plant, and there is no pollination involved.  That's female liberation (though there is a plant that does male apomixis!).

Dandelions decorating a lawn.

It takes a while to get used to the idea that plants have sex, but once you think about it, it makes perfect sense.  Sexual reproduction allows for continuity of traits being passed on from generation to generation along with genetic variation to survive in diverse habitats.  After you're used to 'normal' plant reproduction, parthenogenesis seems positively bizarre. Why would a plant want to give up on genetic diversity? Actually, lots of plants reproduce asexually by parthenogenesis (blackberries, onions, grasses and more), and there are some specific advantages.

The main advantage to asexual reproduction is the ability to capitalize on a successful genotype.  If humans could do this, we might have hundreds of little Steve Jobs growing up to make our future world a better place.  Instead, he left us a few offspring, but they each only have half his genes - and they may or may not have gotten the good ones.  For dandelions, their successful growth strategy can be reproduced in perpetuity because they are identical copies, with only random mutations providing genetic diversity.  Dandelions may have less need for genetic diversity, since the DNA they have allows them to grow differently depending on their growth conditions.  This is what makes them such successful and widespread weeds.

One last issue for today:  dandelions grow without being pollinated, but most dandelions do produce pollen (Did your childhood friends used to rub dandelions on your face?  Remember the yellow dust they left behind?).  Botanists scratch their heads a bit on this issue.  Dandelions that don't produce pollen can make more seeds, since they are not wasting their energy.  It may just be that because pollen production is genetically determined and dandelions don't evolve very quickly, the pollen production may just be an evolutionary leftover.  Also, there are some sexually-reproducing dandelions in the world, so if dandelions maintain the ability to pollinate potential dandelion mates, they could just happen upon a better combination of genes than the ones they already have.  I don't know that dandelions really need to become more successful at growing...though I love them, we have enough already!

Gorilla Feet

I promise every post in Chicago won't be about the apes at the zoo, but there will probably be a few more after this one too!

This is a gorilla foot:

Gorilla Foot

I was able to get such a great picture because the gorilla was sleeping with his foot up on the glass.  I touched the glass after I took this pictures, and it appeared to be about 1.5 inches thick.  After seeing the adult male gorilla kick the door in his enclosure a few days ago, I'm hoping that glass is thick enough!  That gorilla is massive and extremely strong. 

Gorillas are usually fairly mellow, and they seem to have less intense social interactions than chimpanzees do.  They do engage in grooming behavior, aggression and play, but their interactions are much less constant than the chimpanzees'.  They are vegetarian, mostly forest floor dwelling and less active in general than chimps.

Gorilla's forelimbs are massive and long.  Their arms are about six times stronger than ours.  Their legs are much smaller, though still strong.  Even though they spend lots of time on the forest floor, they are still very good climbers, and they use their arms to amble around the branches and vines with ease. 

Gorilla's feet have opposable thumbs.  They are good at grasping things - branches or food, but they are not good at walking upright.  Our feet have all toes pointing forward, which is great for bipedal locomotion, but have you ever tried to pick up anything with your toes?  Not easy.

Beyond shape, there are many similarities in our feet and gorillas'.  First, scroll back up to the picture and notice the prints.  I can't call them fingerprints, because they are on the sole of the foot, but we have these too.  We have prints all over the bottom sides of our fingers, palms, toes and soles, and so do gorillas.  These prints are due to ridges in the subskin (dermis) where it attaches to the upper skin (epidermis).  The ridges and valleys allow for more contact between the two layers, which reduces separation of the layers from friction.  What that means in everyday English is that it reduces the incidence of blisters.   

Next, notice the nails.  These guys have fingernails and toenails, not claws.  Nails are good for manipulation of plants and small structures, and they protect the fingers from stomping or insect bites, but they're not good for ripping flesh.  Since gorillas are herbivores and big and strong, they don't need claws for food or defense, so they have nails.  We are not entirely herbivores, but we use tools to catch our animal prey, so we don't really need claws either.  We do, however, need to manipulate small items, and nails are useful for that.

Gorilla and human feet are homologous structures, meaning they have the same evolutionary origin, and have developed from the same bone and muscle pattern.  Since gorillas and humans separated on the evolutionary tree a long time ago (well, in evolutionary time, not so long ago) and have adapted to different environments, our feet have evolved in slightly different directions too.  Scientists think the evolutionary ancestral foot to humans and gorillas was more like the gorilla one, though probably smaller and more hand-like.  Primate ancestors were even less bipedal and more tree-dwelling. 

Decisions, Decisions

As I was walking on my new daily exercise route in Chicago (up Lake Michigan's shore and down through Lincoln Park and the zoo - tough, I know), I almost stepped on these two mating cicadas.  They made me ponder the behavioral decision-making processes in animals. 

Two-headed cicada!

I'll start with the assumption that behaviors exhibited by animals are generally the result of a long evolutionary history, with a little improvisation and chance thrown in here and there.  This means that behaviors exhibited by animals are ones that caused their ancestors to have more offspring and pass on the traits for exhibiting those behaviors to their offspring (thanks, Charles Darwin).

Clearly the mating behavior I observed in the cicadas is necessary for the production of offspring and passing on genes to the next generation.  However, the fact that the cicadas were mating in the middle of a well-traveled concrete pathway would seem to be counterproductive in the Darwinian struggle for progeny.  Cicadas as a species are probably in the middle of a subtle evolutionary shift wherein those cicadas that mate on tree branches and in grass survive more often than those that mate on concrete.  How genes could be involved in that particular behavior, I don't know.  But I do know that fairly complex behaviors have genetic components.  For example, type of tree branch used for nest site choice in birds can be genetic. 

Obviously the cicadas were incapable of the complex thought process needed to see that mating on a sidewalk would get them both squished and that they should just move 6 inches to the left into the grass.  Animals are not thought to behave rationally.  That means they don't weigh the pluses and minuses of behavioral choices - they just act.  This always brings me to the following questions:  If animals don't actually decide what they are going to do, why do they do anything at all?  And how do they know what they should be doing?

For behaviors to evolve, there has to be a way for animals to 'know' that they are choosing a good behavior versus a bad behavior.  There must be something similar to pleasure and pain to reinforce good and bad choices.  In fact, the hormone that causes pleasure and contentment in humans (oxytocin) has been found in animals.  Animals also have adrenaline rushes, which elicit fear, panic and aggression in humans. 

Unfortunately, there is no way to know what an animal actually feels, so we must come to conclusions by analogy.  If oxytocin is released, the animal must feel something positive - perhaps calming or pleasurable, because that's how it works in people; if adrenaline is released, it must be the equivalent of scared, etc.  An animal that feels pleasure when it swallows food, drinks water, mates and finds safe locations to rest will probably survive.  An animal that feels fear when a larger, unfamiliar animal is near will respond by running away or fighting, both of which can extend life span. 

My favorite example of how this might work involves a study of my least favorite organism: roaches.  In a study to mimic roach behavior, scientists programmed tiny, roach-shaped robots to be still when they are in dark places and near other roaches and to run fast when they were in the light.  With this simple behavioral program, the roaches ended up behaving almost identically to real roaches.  I imagine that in real roaches' ganglia (little clusters of nerves throughout their bodies that act like little brains), they feel roach-y contentment in dark, tight spaces with buddies around, and they feel panic when the lights come on.  Oddly enough, in a test to see if roaches have a stronger preference for darkness or being around their roach pals, they chose the social environment over the safe environment.  Who knew roaches were such followers? More. 

There must be at least a few cicadas out there with a mutation that makes them fear stepping on concrete and therefore move away quickly when they find themselves on it.  The concrete-fear gene should confer an advantage to those cicadas, as they would be less likely to be scraped out of the treads of a tennis shoe one day.  Assuming humans keep building and using concrete sidewalks, some day all the genes for sidewalk-mating will have been eliminated, and only the concrete-fearing cicadas will have survived.  Cicadas as a species will be smarter, all without a single cicada having to think an actual thought.

Of course, humans always make their decisions based on cold rationality, because we have the ability to think and separate our emotions from our thoughts.  We have evolved brains capable of weighing our options and choosing the best path, so we don't need instinctual behaviors.  ...Not so fast!  We have plenty of positively- and negatively- reinforced behaviors: pleasure for eating, playing, making a home and mating, and pain from loss, fighting and overwork.  Humans have repeatedly been shown to act irrationally in less obviously evolutionary ways too.  Some of us take unnecessary risks, get pregnant too young or fall in love with the wrong person.  We have even learned that the average of millions of human decisions is not based on rational principles, as it is impossible to explain or predict the stock markets. 

We must be careful not to fall into the trap of assuming humans are so different from other animals.  There are evolutionary payoffs to many irrational behaviors.  Risk-taking can result in big rewards in status and resources, and perhaps more mates to pass on risk-taking behaviors.  Teen pregnancies tend to pass on genes early and often (not that I'm advocating teen pregnancy).  Romantic decisions are made at least sometimes based on evolved chemical signals (pheromones and antibodies) that might signal compatibility and more healthy offspring.  Human social tendencies can lead to careless financial decisions just as roach social tendencies can draw roaches into unsafe locations. 

So how much of our behavior is instinct and how much is logic? 

Japanese Beetles Put their Thumbs on the Scales

The stability of any ecosystem depends on having a multitude of species playing different roles in that ecosystem.  Some are producers, some are decomposers, some are predators, and no one gets out of hand.  The predators don't take over because there are bigger predators, disease organisms, and competitors for food and other limiting factors. 

Invasive species are the newcomer species that upset the fine balance, altering ecosystems and even causing extinctions.  For example, entire food chains have collapsed in the Great Lakes due to the invasion and overgrowth of the zebra mussel.  The zebra mussel eats all available algae, and other species starve.

Invasive species arrive in many ways - sometimes by expanding their range but usually because they have been carried from one place to another by humans.  Our species of the day, the Japanese beetle (Popilla japonica) (picture), was accidentally brought into the United States from Japan as grubs in soil of ornamental plants.  The new species was detected in New Jersey in 1917 and quickly traced to a plant nursery in the area.

Because invasive species are new to an ecosystem, they are neighbors with other species to which they have not evolved.  That means their neighbors have not had a chance to adapt to eating the invasive species, or competing with it or infecting it.  That's how you end up with entire mountains covered in kudzu.  Or the entire eastern United States covered in Japanese beetles (range map), despite massive federal, state and private efforts to destroy this aggressive farm pest.

But even invasive species are fascinating.  Japanese beetles are beautiful, metallic beetles.  Also, they might just be the only insect capable of growing one body part at a time, according to Dr. Hans-Willi Honegger, an entomologist who also works on the farm.  Dr. H thinks they might be able to grow their jaws, which would make sense since they are such voracious feeders.  Japanese beetles' life cycles are also very interesting.  They emerge in late spring, feed like crazy on leaves of their favorite plants, mate and lay their eggs in the soil.  Most of the adults die off by late August, but the larvae are just starting to get busy.  They burrow in the upper layers of soil throughout the fall, eating roots and organic matter in the soil.  When the temperatures start to drop, the beetles burrow deeper and deeper to stay warm only to come back up when the spring thaw comes again.

On the farm, the beetles are just starting to emerge.  According to the farm owner, the beetles have gotten a little worse over the past few years as their range has expanded.  Now that they are here, they are probably already laying eggs here for next year, and their numbers may increase.  Once beetles establish themselves, they are really impossible to get rid of, but several strategies can help.  A soil drought during early larval stages can kill many larvae; parasitic wasps, worms or bacteria can be released into the area to help kill beetles; traps do attract beetles, but they don't seem to help the overall problem; and insecticides are of course used on non-organic farms.  A surefire way to control Japanese beetles would be to simply wait around long enough for evolution to take its course.  All those Japanese beetles are a great food source for any animal that could evolve to eat them.  Given the rate of evolution, we might only have to wait about 20,000 years for Japanese beetles to become a part of our balanced ecosystem.

Weedy Strategies

The onions have weeds. LOTS of weeds. As we weeded for a couple of hours, I contemplated the varying successful strategies of the primary weeds we pulled. Weeds are any organism humans have deemed annoying, and we are annoyed by lots of organisms! Usually weeds compete with humans for food. For example, Monks and Schultheis (1) demonstrated that for every week crabgrass was allowed to grow alongside watermelons, the watermelon yield per hectare (2) was reduced by 716 watermelons (that's almost 4000 kg, or 8800 pounds, equivalent 2 average American cars)! So weeds GREATLY decrease productivity.

Why do weeds decrease productivity? Well, they are the greedy kids that grab the most M&M's at snack time. They are better at using available resources quickly. Resources for plants include sunlight, water and soil nutrients. Weeds out-compete crop plants for some or all of those resources. Crop plants are usually somewhat wimpy competitors. Plant breeders have selected for tender, juicy fruits or pretty flours or large seeds, which often come at the expense of defensive or offensive plant growth strategies.

Our hours of weeding enabled us to become intimately familiar with three weeds: crab grass (3), yellow wood sorrel (4) and spiny amaranth (5). Oddly enough, each of these weeds has a different strategy for success as a weed.

Most of us know crab grass, even if we don't realize it. Crab grass is a vicious, ubiquitous, nonnative weed found throughout most of North America. It is a weed of agriculture, lawns, sidewalks, and vacant lots. Crab grass's strategy is to dominate territory throughout the summer. It puts as much energy into roots as it does leaves as it develops its death grip on the soil. It is an annual that leaves holes in lawns when it dies back in the Fall after it has deprived all neighboring plants from all necessary resources. Removal of crabgrass requires tough hands and strong muscles and also a strong tolerance of soil disturbance by neighboring plants as the crabgrass clings to the soil even as it leaves the earth.

Wood sorrel is ironically the sweetest of the three weeds to pull. While it would taste sour due to its oxalic acid content, it comes out of the ground as smooth as honey - a great relief to a weeder who is fed up with crab grass. Wood sorrel grows quickly, and it puts little energy into maintaining its turf. It just needs a little patch of soil to get in, grow some seeds and get out. It is done with its life cycle early in the summer.

Spiny amaranth blends strategies of the other two plants and adds its own twist to the mix. It grows quickly but also holds on tightly to the soil. For a little flourish, it grows sharp spines all along it's stems discouraging predators and pullers.

From an evolutionary perspective, each of these plants has found a successful strategy. Their growth patterns yielded plants that could use available resources to produce offspring with similar characteristics to the parent plants. Each survives in a similar habitat with a different answer the the ultimate biological question, "How do I survive and pass on my DNA?". After yesterday, at least a few of those weeds lost the evolutionary battle to their wimpier onion competitors. Onions, though, have their own secret weapon. They have managed to be useful and appealing enough to humans that humans are willing to fight off their competitors for them!



(1)

David W. Monks and Jonathan R. Schultheis, 1998. Critical Weed-Free Period for Large Crabgrass (Digitaria sanguinalis) in Transplanted Watermelon (Citrullus lanatus)

Weed Science. Vol. 46, No. 5 (Sep. - Oct., 1998), pp. 530-532
(2) A hectare is about 2.5 acres, which would take about 5 hours to mow with a push mower. No breaks.
(3) http://plants.usda.gov/java/profile?symbol=DISA
(4) http://plants.usda.gov/java/profile?symbol=OXST
(5) http://plants.usda.gov/java/profile?symbol=AMSP