Managing Perennial Weeds in the Garden

September 8, 2022

By Caleb Goossen, MOFGA’s Crop Specialist

One of the events that I look forward to at the Common Ground Country Fair every year is the “Answers to Your Organic Vegetable Questions” session that I put on each morning in the MOFGA Tent with Eric Sideman, MOFGA’s crop specialist emeritus, and Mark Hutton, University of Maine Cooperative Extension vegetable specialist. Sideman started offering the sessions over a decade ago and luckily for all of us is still willing to join in. Attendees range from new or aspiring gardeners to very experienced growers who sometimes share their own valuable insights and strategies with the group.

Questions often come up about managing weeds, particularly perennial weeds. (While I know that many “weedy” species can be valued by some folks, depending upon the context, I’m defining “weeds” here as any plant species that is growing where it is not wanted.) In my Q&A events I try to emphasize how rarely there are “right” or “wrong” ways to garden but instead aim to share background information and provide a frame of reference to help folks figure out what strategy may best fit into their own systems and situations.

As with any pest or disease question, the preferred first step is to identify the species of concern. That can tell you whether you are dealing with a weed that’s an annual (completes its life cycle — i.e., sets seeds — and then dies), biennial (grows vegetatively the first year, then flowers and sets seeds the second year, and then dies) or perennial (keeps coming back from its crown or roots, regardless of whether it has set seeds or not). Knowing what species you are dealing with can also help you learn about plant’s growth habits and reproductive structures and strategies.

Though the mechanisms involved with managing annual and perennial weed life cycles vary, I often find myself describing the approach to both as a “war of attrition,” reminding people that the most effective organic weed management is accomplished over a long time span and that patience and diligence are repaid with less effort needed in the future. In both cases, we are trying to draw down the reserve of weed plant propagules in a growing area, whether they be weed seeds or the dormant growth points of perennial weeds.

Understanding the management of annual garden weeds can help set the scene for how we think about managing weeds generally. Apart from some nuances (such as knowing an annual weed species’ favored growth conditions), the annual weed management discussion typically boils down to strategies to manage — and minimize — the weed seedbank in your soil. In other words, the objective is to try to draw down the number of viable weed seeds already present in the soil, while preventing the creation of new ones.

Drawing down the number of propagules is primarily accomplished by removing plants before they can set viable seed (stopping the addition of new propagules) and ideally by also accelerating the removal of seeds already in the soil (the weed seedbank). By intentionally stimulating the germination of weed seeds prior to planting in a prepared garden area, and then killing the resultant seedlings, we are drawing down the overall seedbank population. Importantly, if that can be achieved without significant mixing of the soil, the weed seeds that were exhausted will have been the ones that were best positioned to interfere with the crop. If things get ahead of us later in the season and some weeds end up dropping their seeds, it is best to allow those seeds to remain on the soil surface over winter where they are more likely to die or be eaten than if they are incorporated into the soil.

Although it is true that some weed seeds can remain viable in the soil for very long periods of time — even decades in some cases — the vast majority of annual weed seeds in the top few inches of your garden soil are likely to remain viable for only a few years. If you can stop the addition of new weed seeds to the soil, a few years of diligence can pay you back with dramatically reduced weed pressure in the future.

The management of perennial weed species also entails propagule management as described above for annual weeds but with the added complication that a weedy perennial plant will regrow the following year if not killed or removed. Additionally, perennial weed species often propagate and spread vegetatively in ways that can be much more difficult to notice than annual weeds going to flower and setting seed. Common vegetative means of reproduction and spreading include stolons — above-ground creeping stems from which a “daughter plant” will sprout and take root (think of white clover or strawberry runners), and rhizomes — below-ground spreading stems (think of quackgrass spreading to neighboring spots in your garden). Many plants in the mint family have a well-earned reputation for not remaining where they were planted, and they frequently spread by both stolons and rhizomes. Some plants also re-grow and spread by root fragments left behind after our initial efforts to remove them — comfrey and horseradish are well-known for their weedy potential because of their impressive ability to sprout from even small root fragments left in the soil.

Crops 2
Strawberry runners (stolons). Pearson Scott Foresman illustration, courtesy of Wikimedia Commons

The regrowth and spread of perennial weeds is often a product of their energy reserves and quantity of dormant growth points. For this reason, the first management step is to physically remove as much of a weedy plant as possible. The quandary of course is that perennial weeds are typically found to be most problematic in places where they are not physically disturbed very often — but it will help to remove as much of the plant as possible and to take full advantage of any opportunity to do so (such as when dividing perennials or swapping a section of lawn for garden in a long-term rotation).

Even if you are only cutting a plant down to ground level, you’re still removing the plant’s leaves, which are the source of all of its energy, and effectively ceasing its “energy deposits” into its energy reserves. I often make the analogy that a plant’s leaves are akin to solar panels which produce energy; new growth (including fruiting and seed production) is like an electrical appliance drawing down the plant’s energy budget, and the plant’s roots and crown are like batteries where the plant stores energy. When weeding, it has to be assumed that some plant parts will be left behind and begin to re-grow or sprout anew. That initial growth is an energy expenditure of the plant, and the key to drawing down its energy reserves is to prevent new leaves from reaching an energy balance stage of growth (which is to say, living long enough to make as much, or more, energy for the plant than it had used to make the leaves).

This is the crux of the war of attrition: the gardener can hopefully draw down the weedy plants’ energy reserves enough to kill them, or at least to create conditions favorable enough that our desired garden plants can out-compete the weeds (though care must be taken not to become complacent). When conditions allow, I prefer to use a lawnmower to continue “knocking back” tall-growing weed species like quackgrass or Japanese knotweed before they can reach that critical energy balance stage of growth. Lawn grasses can tolerate repeated mowings, but the perennial weeds will eventually give in, and I find it easier to convert lawn to a garden than to attempt to make a garden in a quackgrass or knotweed infestation! An additional benefit of drawing down a perennial weed’s energy reserve is that it will force the plant to slough off some of its roots, making it easier to physically remove.

This article was originally published in the fall 2022 issue of The Maine Organic Farmer & Gardener.

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