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A Weed by Any Other Name

May 17, 2013

Most gardeners have heard the saying or perhaps dictionary definition, “A weed is just a plant growing where it isn’t wanted.” Except that isn’t quite true. It’s certainly true in the common usage of the term, but it’s false in the suggestion that a plant is only a weed because of our perception. This saying implies that these plants would stop being weeds if only we embraced their potential. But the fact is, there are a number of common characteristics that make weeds both successful at invading our gardens and essential to their ecological niches.

Many plants we think of as weeds are “pioneer plants.” These are the plants that are first on the scene after algae and moss. They make their claim on rocks, sand, extremely poor soil, and areas that have been recently disrupted. Whether after a fire, flood, or landslide, they are essential to starting the process of building new soil. Their seeds can often survive the harshest conditions, waiting until it’s just right to sprout. Once they do, they physically break down the soil, their roots shooting through the rock in search of water or nutrients. If their roots host nitrogen-fixing bacteria, they can chemically access nutrients that would be otherwise unavailable. They basically transform a formidable landscape into one where other plants can start to grow there as part of secondary succession. To meet these evolutionary needs, pioneer plants grow fast, have either very deep or shallow but wide root systems, and are capable of pulling nutrients efficiently and quickly from even the poorest conditions.

While not pioneer plants, other “weeds” are from very competitive ecosystems that have a premium on light or nutrients. As a result of a fast-paced evolutionary race, they are lean and mean, the sprinters of the plant world. Oddly enough, from my observation, this is actually the case in the Amazonian rainforest. Unlike what most people think – including myself before I visited, despite my ecology background – the rainforest actually has very nutrient-poor soil. In most forests, topsoil forms over many years through the decomposition of organic matter, like leaves, branches, and other dead things. However, the rainforest never has the chance to build up nutrients, because the rainy season washes it away annually. This is one reason why “slash and burn” methods of turning the rainforest into agricultural land are particularly destructive – the soil is so poor that the farmers can only use it for a year or two before they have to cut down more rainforest. As a result, the rainforest plants are constantly battling back and forth to claim the few nutrients left. This is in part why there is so much diversity in the rainforest – every plant tries to establish its own ecological niche that others can’t fill and once established, defends it to its last.

Considering the biological characteristics of these plants, it’s no wonder that many of them adapt so well to human civilization! The dandelion poking up through the concrete sidewalk is just fulfilling its evolutionary role of breaking up impenetrable surfaces. Tilling to plant seeds disrupts and exposes soil, allowing these weeds to drop their own hearty, fast sprouting seeds and move in right away. Using easily absorbed chemical fertilizers or nutrient-dense organic ones (including compost) allows weeds to quickly draw up a maximum amount of nutrients and choke or shade out less competitive plants. Basically, everything we do under normal gardening routines actively encourages weeds.

So how can we create an environment that’s far less weed-friendly? One word – permaculture. As I’ve mentioned before, permaculture is all about applying ecological principles to gardening (or more broadly, to living in general). It combines cultivating the land in a way that encourages the environments the plants we want will thrive in with choosing plants that are suited for the environments we already have. Based on these principles, I have three long-term weed busting methods. While weeds do invade my garden some, I spend far less time weeding than most gardeners. I usually weed a little bit every few days or if I let things get away from me (like last summer), a couple times during the summer for a few hours each.

1) Build good soil.

Weeds are ecologically adapted to grow in nutrient-poor, compacted soil. Unfortunately, this is pretty similar to my yard’s base soil, which is mostly clay. In contrast, weeds aren’t suited for soil that is rich and requires a slow uptake of nutrients. I build soil by using lasagna gardening, a technique that mimics the natural pile-up of organic matter in a forest or field in an accelerated fashion. You can also get similar results by planting a cover crop in the late fall, like clover or rye, and then cutting the tops off of the plants in the spring. Because the organic matter takes all season to break down, weeds have difficulty drawing up the nutrients quickly. In contrast, slower growing plants do well and have the chance to establish themselves before the weeds move in.

2) Minimize exposed soil.
Because I lasagna garden, I haven’t tilled the soil since I first broke ground for the garden. Even when I plant seeds or seedlings, I plant them in small clumps of LeafGro surrounded by leaves rather than doing a lot of digging. Because there is little exposed soil, weed seeds don’t have anywhere to plant themselves. If you don’t lasagna garden, one way to carry this out is to mulch your garden after you plant. Rather than using wood mulch, I recommend using something that will break down and contribute to building your soil, like leaves or straw. Plus, I find wood mulch creates too many air pockets where crafty weeds can send roots and runners to establish themselves in new places.

Tomato seedling mulched with leaves

A tomato seedling surrounded by leaves, which serve as a mulch.

3) Fight fire with fire.
Have a particularly weedy area? Plant evolutionarily-adapted plants you want that can legitimately compete with the weeds rather than being smothered. For example, one side of our yard always gets covered with weeds. The first year, we tried and failed to keep them back with wood mulch. Last year, we planted two perennials that we knew could hold their own – mint and strawberries. While the strawberries haven’t yet fulfilled their potential, due to being root-bound when I first planted them, the mint has given the weeds a serious run for their money. Besides mint, other useful plants that are highly competitive include horseradish, blackberries, and bamboo. I wouldn’t plant these in a garden where they could take over other sections, but they can work great if you have a separate area where you can allow them to thrive.

Mint plant

This mint started from one little plant last year…

With a little ecological knowledge, knowing weeds’ environment and avoiding it as much as possible can be more powerful in a sustainable way than many herbicides.

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2 Comments leave one →
  1. Loquat permalink
    May 19, 2013 10:09 am

    It’s funny – the two most troublesome weeds in my garden are edible plants that are actually cultivated elsewhere. Yellow Nutsedge is almost impossible to get rid of completely, due the stems’ propensity to break off and leave the tubers in the ground to sprout again, but the tubers are edible and apparently even taste good. Ground-ivy, aka Creeping Charlie, is easier to control, but it’s also a tasty salad green rich in vitamin C and was used as a medicinal herb for centuries.

    I’m not sure what I can do about the nutsedge – its initial shoots are thin and pointed to work their way through almost any obstacle, and it grows maybe 18-24 inches tall if left to itself. I may have to try cover cropping this fall and see if it helps.

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  1. This week in the Slacktiverse, May 20th, 2013 | The Slacktiverse

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