Sunday, July 06, 2014

Weeds: What Are They Good For?

It just doesn't seem fair.  When you clean a surface, it ought to stay clean, unless you put something on it that dirties it.  Right?!  So why is it that, after you've worked hard to weed a flower bed or vegetable garden and you have a beautifully pristine expanse of bare soil in between each plant, the weeds show back up?  Seemingly like magic!  Sometimes they seem to spring up overnight.

These weedy interlopers are unwanted and they crowd out the plants that you want to grow!  Isn't this a design flaw in nature?!

Well, actually, no, there's a very good reason for this to happen in nature.  Weeds are nature's way of "preserving its capital", to use economic terms.  So what do I mean by "capital"?

In an ecosystem, soil is one of the most important resources available and one of the hardest resources to manufacture.  Originally soil formed from bedrock.  It takes thousands of years for soil to form:  soil formation involves the slow action of water and weather (especially the freeze/thaw cycle), acids from lichens, and eventually, organic debris and the action of plant roots.  Soil is NOT a substance that it's wise to waste, because its replacement takes thousands of years.

Enter most "weeds".  What we consider weeds are usually fast growing plants that produce thousands and thousands of seeds which transport readily, in a variety of ways, and which require sunlight and open soil to germinate and grow.  Once established in a good spot, these plants will reproduce rapidly and repeat the cycle of producing thousands and thousands of easily transportable seeds.  What's the point of this process?

Well, these same plants' seeds and seedlings cannot compete in a crowded, shaded location.  In a setting like that - a more established setting - they get outcompeted by other plants that don't have to start from scratch every year.  Plants like perennials, with well established roots that can send up tall stalks in a short period of time and dominate the sunlight, will outcompete them.  Plants like shrubs and trees whose roots are not only established, but that can leaf out and block the sunlight seemingly overnight each spring, will outcompete them.

In fact, if you stop to think about it, if humans do nothing to a piece of land, the vegetation changes through the years in a fairly predictable pattern.  First, the open ground gets covered with "weeds", often a mix of annual grasses and annual, broad-leafed, flowering plants.  Pretty soon, a few perennials start to show up and establish themselves and, before long, there are some seedlings from woody plants beginning to grow.  As the perennials and woody seedlings grow up, the "weeds" that need full sunlight and open soil, so that they can start from scratch every year, start disappearing.  Eventually, the vegetation continues to change until only plants that can germinate and grow in shady, crowded conditions dominate.  This entire process is called ecological succession; the final types of vegetation that will grow in an area are known as the "climax vegetation."

So what happens if the climax vegetation gets removed?  Succession begins again - with the "weeds" whose seeds are easily transportable showing up.  Their job, if you will, is to hold the soil in place until more permanent vegetation can take over and maintain the capital, a.k.a. the soil, that has built up over thousands of years.

The more the soil is disturbed, the more "weeds" will appear.

What we are doing when we garden or farm is to continually remove the perennial vegetation and take the soil back to its most vulnerable state - bare and open.  Nature wants to stabilize that soil as soon as possible, so that the soil doesn't wash or blow away.  In come the weeds.

The gardener - or farmer - cusses the weeds and clears the soil again, so that ONLY the plant he/she wants to grow will be there.  But rarely is the desired plant able to fill the cleared space rapidly enough and completely enough to truly stabilize the soil.  So in come the weeds again.  And the cycle continues.

Along the roadside, this mix of plains coreopsis, cheat, wheat stubble and field weeds is composed of essentially all annual plants.  The wheat was desired; the rest of the mix is considered "weeds", at least to the farmer.  Why is it all growing here?  Because the soil has been exposed recently and nature is trying to stabilize it.

If you widen the view to take in the entire ditch, you see that where the soil hasn't been plowed or opened by herbicide action recently, a mix of perennials is growing:  butterfly milkweed and various prairie grasses in this case.  It's far from perfect, since frequent low mowing often opens up soil, encouraging other, early successional, "weedy", plants to move in.

The moral of my tale?  Understand what nature is trying to accomplish with "weeds" and plan accordingly.  Learn to work WITH nature, rather than fighting against it.  Mulching between plants, for example, helps tremendously because it covers (and helps stabilize) the soil.  Mowing high (or not mowing at all) is likely to leave much less soil exposed and thus make an area less amenable to "weed" growth.

Why do I keep putting the word, "weed," in quotes?  Because a weed is simply a plant in the wrong place.  To nature, bare soil is a form of an emergency and what we humans call "weeds" are nature's emergency response team.  Therefore, to nature, annuals are important...and aren't weeds at all.


Gardener on Sherlock Street said...

Everything has a purpose. Great explanation of why we have 'weeds.'
The plains coreopsis is really beautiful this year. My mom and I were driving past several fields with it blooming on the edges Saturday. She shared with me a story of one year her dad's wheat field was filled with it because it had been so wet. That would have been in the 40s.

Gaia Gardener: said...

I talked to one person who remembered avoiding patches of plains coreopsis as they harvested because it signalled a wet spot. I'm sure enjoying how beautiful it is this year!

ProfessorRoush said...

I'm with you Gaia....but why does Nature have to use Warty Spurge and Eastern-toothed Spurge to try to fill in? I'd be happy with Black-eyed Susans, or butterfly milkweed. But Spurge???

Gaia Gardener: said...

Ah, but Prof, Black-eyed Susans are short-lived perennials and butterfly milkweed is a long-lived perennial. I would take plains coreopsis, though, which hasn't decided to colonize my garden beds! I get crab grass, prostate spurge, and (this year) poison ivy.

rebecca said...

I enjoyed this explanation and the "moral of the story".....Just last week at the Extension Garden, we had a brief conversation with a Real Gardener (as I call you Master Gardeners) about several unfamiliar weeds that have recently sprung up in our flower beds.

Karin / Southern Meadows said...

Really great explanation. I understand the purpose of weeds better now.

Gaia Gardener: said...

Thanks, guys. I especially get frustrated with the county road crews who keep disturbing/spraying/mowing the ditches and roadsides to "get rid of weeds before they infest the fields". All they are doing is encouraging the growth of more weedy annuals and getting rid of perennials. The perennials would be much less likely to create a problem in the fields, since they take so much longer to establish.

Indie said...

Great explanation! How useful and wise it seems now for Nature to have all those number of easily grown weeds around - though I'm not sure how appreciative I'll be of this my next weeding go-round :) What I want to know though is, why do all those weeds grow even in my mulch?!

Gaia Gardener: said...

Indie, thanks for stopping by!

To answer your question, mulch basically works by covering up the weed seeds so they don't get enough light to germinate. Otherwise, it's just the beginning of new soil. :) Any new seeds that fall onto/blow into it will germinate as they are preprogrammed to do.

Adrian Ayres Fisher said...

Hi Gaia,
Ah, weeds, a subject dear to my heart. I, myself, distinguish between invasives such as phragmites and buckthorn and more neutral or even more-or-less beneficial ones such as lady's thumb that have settled in less obtrusively.

Here's a prairie blog I like (not your state, but well done):

Gaia Gardener: said...

Hi Adrian,

Thanks for stopping by! I totally agree with you about different "classes" of weeds. The exotic invasives are unwelcome no matter when or where they pop up, in my garden too.

In that regard, I was thinking about the more "standard" weeds - especially native species, which may be aggressive in habit because of their function in stabilizing open soil.

Thanks for the heads up about Chris Helzer's blog, The Prairie Ecologist. I was lucky enough to hear him speak at a prairie conference in 2010 and have followed his blog ever since then. It's interesting to see what a professional ecologist is up to, as well as following along with some of what The Nature Conservancy is doing, vis a vis prairies as well.