Weeds have adapted to many kinds of selective forces created by our crop management systems. Below take a look at just some of the examples of this weedy adaptation.
Weeds have developed specialized structures on their seeds to ensure the seed is dispersed far from their parent plants. Below are two examples of this. Barbed hooks on common cocklebur (left) and burdock (right) do the job.
Climbing and creeping can be a real advantage to a weed in its relentless struggle for survival. Wild buckwheat (left; center) has the ability to wrap itself around its neighbors and get above all the other nearby plants to capture sunlight. On the left an obliging timothy plant is helping out, in the center wild carrot is helping. On the right a handy tree is providing this poison ivy plant a chance to get at the right place to wipe its irritating oils on some unsuspecting person.
Germination of weed seed needs to occur at just the right time; to early an it gets killed by seedbed preparation tillage (a good weed management tool; sometimes called the stale seedbed technique). Too late and it gets overwhelmed by the crop. Crops have been bred by plant breeders to emerge uniformly, a crop adaptation to artificial selection (below).
Human selection of weeds isn't always to create better and better (worse and worse) weeds. Most of our crops were at one time a wild plant that some early farmer decided to select for bigger grain, uniform shattering (seed drop at the same time) or other crop qualities. A crop of more recent origin was selected from among yellow foxtail variants. This yellow foxtail crop is grown in India, I took the pictures below in a nursery in Japan from a scientist that was studying this interesting crop. I wonder if they sell Yellow Foxtail Crop futures on the Chicago exchange?
Stress adaptation is one of the important ways weeds adapt to agroecosystems, as well as other stressful situations. Below (left) is a small common lambsquarters plant that seems to have adapted to the Czech railroad car sitting idle in the Northern Bohemia rail yard of Usti ad Labem. The little green foxtail plants (center) have adapted to the salty seashore soil in southern Japan next to the Sea of Japan. Below right is a plaque I saw in Japan next to an volcano. I enjoyed the English translation, I guess adaptation to volcanic eruptions is quite an accomplishment for any form of biology.
Sunlight is one of the most important resources plants need for growth. They have adapted many interesting traits to allow them to both capture light, as well as avoid light when it is a stress. In the section above we saw weedy adaptations by weeds to move both horizontally and vertically to capture light. Velvetleaf leaves, as well as sunflower seedheads (below, left) have the ability to move during the day so they are always facing the sun to capture the maximum amount of light available: solar tracking. Sunflower (as well as velvetleaf) also has the ability to grow differentially depending on the amount of sunlight available: plastic growth. Below (right) is a row of sunflower plants of different heights, ranging along a gradient of increasing sunlight. The sunlight gradient was formed by the shadow created by my old garage in Fergus, Ontario, Canada.
Sometimes plant adaptations scientist observe are hard to explain. Can you figure out what the interesting adaptive interaction is between the two freshly excavated carrots below?