Organic Herbicides and Methods

Wellfield staff and volunteers are often lost in the weeds this time of year, as late spring rains and early summer warmth pushes spring annual weeds to complete their life cycles and summer annual weeds to fast growth and flowering. The overlap of seasons means lots of decisions about what gets to stay and play nicely and what is consigned to the compost heap (if it does not have any flower or seed heads).

Recently, someone asked about the use of our propane torch, and whether the Gardens use any kind of “environmentally friendly” weedkiller. This question is full of all kinds of teachable moments, so this Main Street gardener could not resist a blog entry on the subject.

Let me start by defining a few terms and categories (so you can sip your coffee a little longer and not go pull weeds yourself. You are learning after all!). Herbicides, whether organic or conventional, are categorized in all sorts of ways (systemic or contact, pre- or post-emergent, etc.). When it comes to the efficacy of “environmentally friendly” (defined usually as plant-based ingredients) versus “factory” synthetic herbicides, one must start dispelling a bit of a myth surrounding equating organic with environmentally friendly. The two do not always match up. Plants and other biological life forms produce many super nasty chemicals that, if released by humans, could just be as harmful to humans, animals and ecosystems as the synthetic.

One should always make wise, science-based, decisions in the landscape. A more ecologically principled approach turns to chemicals of any sort (including fertilizers) as little as possible (a major goal of any organically preferred operation). The more one impacts the land, the more one impacts ecosystems. We are a part of the natural system, so impact is unavoidable, but we strive for positive impact – a net gain whenever possible. Wellfield staff and volunteers utilize several other strategies before reaching for the spray gun. Each strategy mentioned has a decreasing order of efficacy. The most important tool is covering the soil with mulches or plant cover, and doing as little else as possible. The more one disturbs the soil, the more weed pressure (as covered in other articles). Next, we remove as many weeds mechanically as possible, turning as little soil over as possible. Next, we reach for the propane torch to save time and sore muscles. The torch works really well on seedlings and younger plants which lack an established root system. It does not work well at all on annual and perennial weed grass species. Grasses survive due to the fact that their growing points (points of regeneration) are low and close to the ground. The only thing burned is a few days of effort. Rarely a big loss. The torch need only pass over the leaf surface for a few seconds, removing the waxy cuticle layer protecting the leaf against water loss. I move on as soon as I see the plant begin to wilt. I know it does not have long for this world. 

There are some areas or plants where the only option left is to deal with the offending species via a chemical bath, but what to choose? A quick web search will bring up lots of ideas from homemade brews to commercially prepared options. I myself have used commercially produced citric acid/clove oil combo as well as fatty-acid based sprays. Some suggest vinegar dilutions, vinegar with salt, boiling water and so forth. But, like any tool, there are some drawbacks of which one should be aware. All organic herbicide options are post-emergent contact only, meaning they only kill plant tissues in direct contact with the chemical. If it is a young seedling, there is a greater chance of seeing weeds disappear. Non-selective, contact herbicides are often called “burn down” herbicides due to the burned look to the foliage or stem damaged by the chemicals. It may be burned, but like the propane burner and grasses, not dead, readily regenerating from undamaged, well established plant tissue. You may have burned the tops, but left a vigorous wide spread root system intact. There is NO systemic organic option currently available which will be absorbed into the plant, spreading top to bottom killing the entire root/shoot system. Where and when this is the only option left I, like many a farmer, will mix an organic burn down with a synthetic systemic (say that three times fast), so the top dies quickly and roots soon thereafter.

There is a growing awareness of the negative impact some systemic herbicides are beginning to have within the environment, so the need is there to find sound ecologically-based strategies to deal with the threat some weeds pose in the landscape. The more one uses a multi-tiered approach, not overly relying on one approach, the more positive the impact upon the landscape will be for generations to come.

Josh Steffen, Horticulture and Facilities Manager


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