The "Ideal" Weed

My major professor, Herbert G. Baker, caused a stir in academic botany in 1965 by publishing a list of the characteristics of an "ideal" weed. Since weeds are not popular, an ideal one is a troubling idea.

What Baker did was to look at the characteristics of successful weeds and reduce them to a list, characters of plants highly adapted to reproducing rapidly under any conditions. No real plant has all those characteristics, but some have several. 

The paper made me look analytically at troublesome, weedy plants...and reluctantly admire their resilience and productivity. Fifty-seven years later, these are still good ideas. 

Here are the characteristics:

1. Has no special environmental requirements for germination. Put the seeds in the ground and they will germinate. That is common in cultivated plants--farmers have selected for rapid uniform germination--but natives often require a period of dormancy or particular temperatures. Horseweed (aka marestail, Conyza canadensis), a major crop weed, will germinate in fall or spring, and sometimes germinates in summer. (Cornell Weeds link)

2. Has discontiuous germination (self-controlled) and great longevity of seed. The seeds don't all come up at once, so not all are killed by a late frost or a drought. Seeds of curly dock (Rumex crispus) survive in the soil for more than 10 years, Canada thistle (Cirsium arvense) more than 20 years, field bindweed (Convolvulus arvense) for more than 50 years (Oregon State Extension link) 

field bindweed, Convolvulus sepium (white flowers, dark green leaves)

3. Shows rapid seedling growth. and

4. Spends only a short period of time in the vegetative condition before beginning to flower. A great weed moves quickly from being a seed to reproducing. Palmer amaranth (Amaranthus palmeri), one of the worst crop weeds, grows 2-3 inches per day and can flower four weeks after germinating. 

5. Maintains a continuous seed production for as long as growing conditions permit. The ideal weed doesn't stop flowering when it has a few mature seeds, doesn't have a defined two week flowering period after which it is done, doesn't have determinate flowering (the number of flowers was set the previous year). Weeds create new buds, flowers, and seeds as long as they live.

6. Is self-compatible, but not obligatorily self-pollinated or apomictic. This says "can create seeds if when there is only one plant all by itself but crosses with others when they're available." Self-pollination and apomixis (bypassing sexual reproduction altogether) limit variation in the plant, which reduces its ability to adapt to conditions across entire continents, as great weeds do.

7. When cross-pollinated, this can be achieved by a nonspecialized flower visitor or wind. For example, musk thistle, also called nodding thistle, Carduus nutans, has big red purple flowers that attract different butterflies, bees, and flies in different regions, but all of them can pollinate it. 

Flowers of musk thistle, Carduus nutans

8. Has very high seed output in favorable environmental circumstances. The great weeds can turn out 100,000 seeds from a single plant in a single year. For example: one plant of common lambsquarters, aka goosefoot, Chenopodium album, produced more than 500,000 seeds.

Most weeds can grow very large and
 make thousands of seeds.
This is lambsquarters, Chenopodium album.

9. Can produce some seed in a very wide range of environmental circumstances. Has high tolerance (and often plasticity in face of) climatic and edaphic variation. While common purslane (Portulaca oleracea) can produce several thousand seeds on one plant, a tiny purslane in a crack in the sidewalk will manage one flower and a couple seeds.

Purslane, Portulaca oleracea, blue spruce cone for scale

10. Has special adaptations for both long-distance and short-distance dispersal. For example, the winged dandelion (Taraxacum officinale) seeds. They could fall near the parent, but they can also fly across the lawn. 

11. If a perennial, has vigorous vegetative reproduction. Canada thistle sends out long rhizomes that come up all over a garden. Or it can spread to make a dense patch.  

Canada thistle, Cirsium arvense

12. If a perennial, has brittleness at the lower nodes or of the rhizomes or rootstock. Dandelions are hard to pull up by the roots because the tops break off in your hand.

13. If a perennial, shows an ability to regenerate from several portions of the rootstock. Dandelions you dug out often reappear, regenerating from some deep piece of root. Purple nutsedge (Cyperus rotundus) forms small tubers that break off very easily so pulling them up will just spread the plant. 

14. Has ability to compete by special means: rosette formation, choking growth, exocrine production (but not fouling of soil for itself), etc. For example, the English ivy (Hedera helix) below, climbing over neighboring plants, cutting them off from sunlight. 

English ivy, Hedera helix, enveloping a fence and
the other plants along it

To these, today we would probably add "resists herbicides."

I tried for diverse examples, but many of our worst weeds do very well on several of these points. 

This list of characters works best for garden and crop weeds (for example Agweb). What we call a weed is a plant we don't want; a plant that interferes with a human activity. If the activity is growing walnut trees, the weeds will be different from the weeds of a cow pasture or golf course. For other kinds of crops, other plant characteristics may be important, but you can see the approach. 

I have used these ideas in a number of contexts since I first learned them in the 1970s. I like thinking about the traits a weed needs to succeed. When, as a gardener, the seeds I carefully planted do not germinate, or another plant aborted its one bud, the contrast to weeds is striking. 

Plants adapt to their environment. Natives evolved complex conditions for germination that work over long years of climate variation. Natives competed for resources and since some years were quite bad, conserving resources and growing slowly as a result, was adaptive. Common, really successful, weeds skipped all that. They evolved to hop from disturbance to disturbance. A plant produces thousands of seeds so that, by luck, one or two land in favorable sites, to grow really fast, flower a lot, and scatter another generation of seeds, of which a few land in favorable sites, and so on. At one time weedy plants may have hung out on eroding hillsides or quickly colonized after spring floods. In our modern world, with continuous, repeated disturbances--large areas cleared for agriculture, roadsides that are mowed and driven on, and trampled yards and parks--the weed lifestyle works very well indeed. 

Can you appreciate how good bad weeds are at what they do?

All the white seed heads and all the yellow flowers beside
them are yellow salsify, Tragopogon, which took over
this neglected field.

Comments and corrections welcome.

Final note: I'm working on a blog post about common purslane, Portulaca oleracea, which is very widespread. It has many of the characteristics Baker's ideal weed. Check it out next week. 

Sources: 

Agweb. Worst Weeds, Ranked. link (Accessed 8/4/22)
Baker, H.G. Characteristics and modes of origin of weeds. pp. 147-172 in H.G. Baker and G.L. Stebbins (reds). The Genetics of Colonizing Species. !st Int. Union bol. sci. Asilomar, CA. link (Accessed 8/4/22)
Cornell Weeds. Horseweed. link (Accessed 8/4/22)
Minnesota Department of Agriculture. Palmer amaranth link (Accessed 8/4/22)

Kathy Keeler, A Wandering Botanist
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