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By Patricia F. Neyman, May 1998
This paper contains everything you will ever need to know in order to conquer weeds forever! That may seem like an exaggerated claim for such a short paper, but it is essentially true. Books for identifying weeds would be useful, but your own observation of weed habits will suffice.
After you read this, you’ll see why I can’t explain all about what I do in a 5-minute conversation. This represents a compilation of information from diverse sources most people don’t have readily available, plus my own 18 years of intense weed-battling experience in an amazing assortment of gardens.
What is weed control? : it is not the same as weeding!
The two fundamental rules : two things you absolutely must do be successful.
Little-known facts about weed biology : if you know them, it will all make sense.
Pat’s weed-control bible : following the 4 rules will solve many of your problems.
Weed prevention strategies: an ounce of prevention ……..will make gardening a lot more fun.
Before you get rid of all your weeds : something to think about.
Weed control is not the same as weeding. The objective in weed control is to decrease or keep weed populations to some desired level. Weeding – the removal (or attempted removal) of weeds – may or may not accomplish this objective, depending on how and when it is done, and what weed you are dealing with. It is, in fact, possible to actually increase weed numbers through weeding.
Time weeding efforts not by how large the weeds are getting, but by observing their development. No matter how small a weed may be, if you are seeing flowers, your time is running out. Seeds are already starting to form in those flowers.
The flowers of weeds
Flowers? Yes, practically all of our weeds form flowers, though the flowers (as for example in the grasses) may not “look like” our ordinary concept of a flower. That’s because they lack petals and other similar structures—but these actually play no part in reproduction itself. Basically only the male parts (stamens) and/or female parts are present (ovary and pistol). Once you start looking for them, you can find them. Often they are quite small. Even in grasses, the fruiting heads are obvious. When the flowers are exchanging pollen, the parts of the head open up, and you can often notice the enlarged ends of the stamens (anthers) hanging out. Reduced flowers are characteristic of wind-pollinated flowers, while those with petals and other associated structures are generally insect-pollinated. In fact, the geological record indicates that insects using flowers evolved aroung the same time, so biologists suggest that their development was mutually interdependent.
The annual weed population, i.e. those weeds which live only one year and die back, root and all, after producing seed, will gradually decrease if their seed production is prevented. If you don’t have time to do a thorough job of removing weeds and you notice they are starting to flower, then just get the ones with flowers- or even just pick off the flowers, and be very careful how you dispose of them.
If you live in a place like California where summers are dry, you should be aware that as the rains taper off, many weeds flower and set seed earlier in their growth cycle. A weed that may grow to 2 feet in early spring before flowering may in late spring be found flowering while quite small – even only an inch or two is not uncommon. Of course such a tiny weed will form very few seeds, but even 5 seeds still represents a 5-fold increase in your weed population the next year!
Be aware of weed growth flushes in your area. In California there are two main ones, fall and spring. If you let the fall weeds go, the resulting dropped seed will increase the number of weeds to deal with in spring.
When to spray (if you must)
Weed-control chemicals such as Roundup should be applied before weeds have begun to set seed – preferably before flowers have formed. If applied after seeds have formed, it is a useless exercise. If applied when seed formation is in progress, the dying plants will often still manage to mature seed. If you doubt this, just watch some weeds with flowers on them very carefully after you spray them!
Rule number 2: know which weeds can reproduce vegetatively.
The no-seed-set applies to all weeds, even those, which can re-grow an entire plant from just a piece, left in the soil- for instance, Bermuda grass. If you are trying to eradicate such a weed, you should time your eradication to occur before it sets seed, or you will be doing a lot of work for nothing.
Cultivation can increase numbers of weeds
Even more important for these weeds, however, is to be aware that their numbers can be increased through practices, such as rototilling, which some people use to try to eliminate weeds. Field bindweed (wild morning-glory) is another perfect example. If you have a weed that is increasing despite your control efforts, and you are certain you never see it with flowers or seeds, perhaps you are increasing it through your tilling practices.
With such plants, early eradication is your only hope. The plants referred to here are examples of perennial weeds, as opposed to annual weeds. They live for many years instead of dying at the end of summer after maturing their seed. There may be a winter dormancy (as with Bermuda and Kikuyu grass), or the top may die back (Oxalis and othe clovers), but re-growth occurs in spring from underground storage organs.
How about poison on these plants?
Even a systemic poison such as Roundup is practically ineffective on creep-and-root plants, once they get established. Algerian or Canary Island Ivy, for instance, cannot be eradicated by a systemic that is applied to a section. A stand of such a plant that creeps through the soil, rooting as it goes, is essentially all one plant, so a systemic poison, in theory, should kill it. Nonetheless I find Roundup does not work that way on ivy, blackberry, bamboo. Perhaps roots are actually killed in the sprayed area, but there is no or little die-back. Probably other roots farther away are drawn on to provide nutrients for the sprayed section. Baby Tears is another creeper which is resistant to Roundup, perhaps for the same reason.(By the way, Baby Tears can regenerate itself from a single leaf left on the ground, so be careful how you handle it if you want to get rid of it.)
Grasses, such as Bermuda grass and Kikuyu grass, are quite susceptible to Roundup, but only the portions sprayed will die- and there can be recovery. Weeds reproducing through bulbs, like Oxalis and wild onion, are also not killed by Roundup. They may appear to be dying but will come back. Yellow nutgrass is another weed with underground storage organs to which the same remarks apply. And there are many others.
Pampas grass can be killed by cutting it off at the base and immediately spraying or painting the cut ends. I also sometimes use full strength Roundup (in the 41% formulation) to paint on the ends of unwanted shrubs just after cutting them, with some success. (You can try this on large ivy stumps too.)
1) Many weeds set astounding numbers of seed. See Weeds (Muenscher, 1955). If you want to be amazed, try counting the number of seeds in a dandelion head. Only one plant going to seed could populate your whole garden. Only a few of those seeds have to germinate in order for the weed population to be larger next spring.
2) Weed seeds need light to germinate. Light is probably involved in removing chemicals naturally present which prevent a seed from germinating unless it is in a situation favorable to the seedling plant’s survival. This is why you find weeds out in the sunny open areas much more than under shrubs and in the shade. It’s also the reason that covering the ground with mulch can reduce or eliminate those weeds which would come from the seed already on the ground..
(3) Weed seeds can remain viable in the ground for years. This is why tilling the soil can reverse weed-control efforts, especially where the land was covered by weeds for a long period and there is a large weed-seed component. This subject has been very well studied in the context of crop science. Some of the data can be found summarized in the attached page referred to in (1).
(4) Seeds dispersed by a particular plant vary with respect to germination requirements. This ensures that germination is spaced-out and that one unfavorable event – such as destruction wrought by an energetic weeder - will not wipe out all of a plant’s progeny.
(5) Weeds are not mainly native plants. Most of the plants we call weeds in the United States are actually of European origin. Only 12 out of 110 of northern California’s most common weeds which I recently studied for a class on weeds were native to California. California natives comprise thirty seven percent of the 703 weeds cited in Weeds of California (Robbins, Bellvue, and Ball, 1951 ed.). Many weeds have a long association with man. They apparently evolved as exploiters of disturbed (i.e. bare) soil, which occurs naturally in connection with events such as landslides, volcanic events, floods, and fire. Ever since we began clearing away natural vegetation from land –at first to engage in agriculture, then for many other reasons – mankind has consistently provided lots of bare soil for weeds to thrive in.
(6) Many plants we presently see no use for actually have been used and cultivated in the past for medicines and food for ourselves or our pets or livestock; or used for fiber, dyes, flavors, and so on. People purposely or accidentally moved them from place to place. A large percentage of the vegetation that covers the California hills now consists of plants that are not native to California. This was the result of a combination of factors: heavy grazing pressure which the native grasses could not withstand for various reasons, together with purposeful and accidental introduction of new plants which could withstand the grazing and did very well in California’s climate.
(7) Weeds can rejuvenate barren land. (After all, that is the niche they evolved to fill). Some have been used as “nurse plants” for cultivated crops. Their roots "refiberize" the soil, thereby adding organic matter and providing making it possible for our cultivated, coddled, plants to grow, since their roots can now follow where weeds have penetrated. They also bring up minerals from lower layers of soil. Joseph Cocannouer (1976 Weeds, Guardians of the Soil. Devin-Adair Co., Old Greenwich, Conn.) recounts experiences in various parts of the world where he observed gardeners/farmers purposely growing certain "weeds" (or noncrop plants) among their crop plants because they found it improved crop production. (An article on this subject is Rodale, R. 1982. Healing Land With Weeds, Organic Gardening, Nov. 1982: 24-28.)
Muenscher, W.C. 1955. Weeds, Second Edition. Cornell University Press, Ithaca, NY.
In addition to the two primary rules discussed above:
(A) Provide conditions inhospitable to weed seed germination:
(B) Handle weeds sensibly
(C) Be thorough
(D) Get the underground parts
An Ounce of prevention…..
Don’t you hate the way you find yourself repeating the annoying truths that your parents used to repeat? But, it’s true! Weed prevention is easier and, though it can cost money, probably cheaper in the long run than eradication.
This is especially so if you are doing the work and you value your time, bodily resources, and energy. (Weeding is hard on the body, and does take its toll!) Less time doing weeding certainly is more fun, since it frees one up for other cultivating plants we do want.
Prevention strategy #1: Mulch, mulch, mulch !
Mulch is a lifesaver for large areas where there are mainly weeds, which perpetuate themselves through seed. There are three caveats, however. One, the mulch should be thick enough – at least an inch. Two, it will have to be renewed periodically, depending on how fast it gets broken down. And three, don’t think you can sit back and do nothing just because you’ve mulched!
The biggest mistake people make when they mulch (other than not applying it thickly enough) is to think that weeds are beaten permanently and they need no longer be vigilant about weeds. Quite the contrary! After the mulch is applied is exactly when vigilance can most pay off. Get those few seemingly insignificant weeds that manage to blow in or come in on someone’s feet, and you can keep an area like this essentially weed-free. Let those “insignificant” few go to seed, and the situation can quickly revert back to what it was before mulch was applied. And, of course, all the rules regarding weed-handling apply when you are working with a mulched area.
So, after you mulch, get in the habit of spending a few minutes walking around about once a month or so and pulling out the weeds that manage to get through before they get more than a few inches high. You will find it is vastly less work than you used to spend before.
How to apply mulch :
If applied over the bulb or creep-and-root type of weed, mulch needs to be at least 6 inches thick or have cardboard or a thick layer of paper laid directly over the weeds under the mulch. (The paper or cardboard disappears in an amazingly short time, thanks to the soil organisms.) Even so, you must watch for those few which manage to come in around the edges or through the mulch, and quickly eradicate them. Creep-and-root weeds can actually be encouraged by mulch, for it provides a perfect, easily penetrated medium to grow in.
You can put weed-barrier cloth or plastic under a thin layer of mulch, but I don’t recommend it. The mulch breaks down and a new layer of what is essentially newly formed soil accumulates over the cloth or plastic. I prefer a thicker layer of mulch, which allows the mulch-breakdown products to move downward away from the surface. However, if assiduously maintained, mulch and plastic can work well for many years.
Planting into mulch :
If you want to mulch a large area and then plant into it, be aware of this: The soil under the mulch is teeming with weed seeds! If you bring it to the surface and scatter it about on top of your mulch, you’ve just planted weeds into your new mulch. I suggest pulling back the mulch before digging, and then replacing it; or to plant first, then mulch.
It’s a good idea to plant high, so that when the mulch is in place it just barely covers the root crown (point where top meets roots). Woody plants may suffer from rot in the crown area if it is buried under a lot of mulch. If the mulch is thick, this means that you will have to mound the soil around the plant – preferably using clean (weed-seed-free) soil.
Other advantages of mulch
Mulch has four other pluses besides stopping already-existing weeds and seeds. Weeds which do manage to grow in it from seed lodged on the surface are inherently weak because they have to grow more to reach water. They are easier to pull. Mulch improves the soil below, by fostering and protecting the soil microorganisms, and it adds organic matter. And mulch retards the loss of water from the soil below.
Prevention Strategy #2: Control sources of newly introduced weed seeds
Nearby weed stands can be substantial sources of weed seeds, especially those which are wind-dispersed (for instance, the dandelion-type of seed). Don’t overlook those seemingly insignificant weeds in the path, along the curb. Seeds can be borne on feet, on tools, by water (e.g. drainage from a neighbor’s property during the rains). Compost has already been mentioned as a possible source of seed.
In some cases it can be well worth your while to spend a little time weed-whacking the roadside weeds or the neighbor’s weeds (before they are setting seed, of course!). And, by the way, weed-whacking also spews seeds everywhere.
This article was mostly written before I went to live in Nevada, where I came to realize that strong wind can be a significant source of weed seeds. Other than put in wind breaks you haven't much defense against them.
Prevention Strategy #3: Encourage weeds that you like
I have known gardeners who did this. Both of them had large areas to manage, far too large for planting, mulching, or even weeding in the way we normally think of it. So, they did selective management. They encouraged weeds that thrived and that they liked (maybe they had attractive foliage or flowers, or stayed low instead of growing high and creating a fire hazard, for instance), and discouraged the others, until after a period of time the ones that they liked mainly excluded the others.
I can’t end this without adding something about butterflies. It isn’t going to help you win your weed battle, in the usual sense; but it might give you something to think about. I’ve already alluded to the uses human beings have had in the past for those plants that we currently call weeds. Well, many of them are useful to some of our wildlife, especially insects, ….. and butterflies are a prime example.
Did you realize that quite a few of your favorite butterflies lay their eggs on weeds? And that their larvae can’t make it on other kinds of plants? Many people don’t know this. If you love butterflies and want them around you, and you have a space on your land that you wouldn’t mind dedicating to weeds, I encourage you to investigate which local weed species support which butterflies.
Even a home gardener can do this on a small scale. For example, I met a woman in Berkeley, California, who allows Jerusalem Pellitory to grow thickly in a bed along her house where she has been unsuccessful in growing anything else. She did it to encourage the Red Admiral butterfly. The Red Admiral only lays eggs on plants of the family Urticaceae (nettle).
I’ve been thinking for a long time of planting Fennel along the side of my house where the fence is only a few feet from the house. That’s because I love seeing the Anise Swallowtails, which utilize only plants of the carrot family for their larval stage. Fennel is quite a pernicious weed, but at the same time it has some redeeming qualities, such as a wonderful smell. It would keep other weeds from growing there, and at the same time encourage the butterflies. I’ve also found aphids on it, and I like this because it provides a food supply for natural enemies and encourages their presence in my garden (and the aphids don’t, as is true of most aphids, move to other plant species). The Anise Swallowtail used to use natives of the carrot family, but nowadays Fennel, an introduced species, is one of its mainstays.
So, think about it. Maybe you can manage your weeds and make a difference for wildlife at the same time.
Well, that’s it! Thanks for visiting .