Home » Gardening Information » Sustainable Techniques
Weeds—Design Tips for Minimizing Undesirable Plants
Plants & Gardens News Volume 10, Number 1 | Spring 1995
by Sheila Daar
The most effective way to prevent weeds is to design them out of your landscape. Easier said than done? Yes, but there are many ways to reduce or eliminate the kinds of conditions that support weeds. Landscape professionals, motivated for a variety of reasons to reduce their dependence on herbicides, have evolved design techniques to do just that. You can incorporate these techniques in your own garden, or they can be helpful to you in your dealings with landscape professionals.
First let's look at reducing weeds. The name of the game is not to eliminate all weeds from the landscape. Rather, the task is to keep weed populations low enough that they are not considered problems. Using this IPM (integrated pest management) approach to weed management, it is important to:
- Be realistic about how many weeds you're willing to tolerate and where,
- Minimize the conditions that produce more weeds than you are willing to tolerate, and
- Focus your efforts to reduce only those weed populations that threaten to exceed tolerance levels.
Given all the bad press weeds have received, many people find it difficult to develop realistic tolerance levels for weeds in the landscape. Although keeping weed populations to a minimum is definitely important to the growth and vigor of ornamental and edible garden plants as well as to your garden's overall landscape appearance, complete eradication of weeds now and forever is neither feasible nor desirable.
The oft-stated goal of a weed-free lawn, flower border or vegetable bed is fostered by a barrage of advertising geared to selling all kinds of hardware and chemical products that aid in "the war on weeds." This annihilate-the-weeds mindset is also nurtured by much of the popular and scientific garden literature, which tends to apply to the garden the same economic and aesthetic standards established for agricultural crops and putting greens. Such rigid standards are not appropriate to most landscapes-nor, many would argue, are they realistic for agriculture or golf courses, given the large amounts of pesticides, fertilizers, water and other resources required to maintain such standards.
The bottom line is to recognize that the presence of some weeds is not only inevitable, but actually good for the garden. For example, common weeds in the sunflower (Asteraceae), parsley (Apiaceae) and mustard (Cruciferae) families are nectar sources for beneficial insects that attack aphids, caterpillars and other insect pests of garden plants. Many of these "weedy" plants are also quite attractive. Sunflower family weeds include the English daisy (Bellis perennis), cornflower (Cetaurea cyanus), yarrow (Achillea millefolium) and dandelions (Taraxicum vulgarae). Parsley family weeds include the stately fennel (Foeniculum vulgare), cow parsnip (Heracleum lanatum) and wild carrot (Daucus carota). Weeds in the mustard family include common mustard (Brassica arvensis), hoary cress (Lepidium draba) and shepherd's purse (Capsella bursapastoris).
Designing Solutions
Designing to minimize weed growth can be as simple as installing permanent barriers over the soil or as complex as configuring a planting plan based on ecological relationships among plants to prevent weeds from dominating. Here are some suggestions:
- Installation of a brick, slate or concrete "mow strip" under fences, or over the soil separating a lawn area from a planting bed, is an excellent method for eliminating weed habitats. A mow strip is a paved surface wide enough to cover soil under and/or adjacent to a fence or wall, and to allow the wheel of a lawn mower to travel along it, keeping a neat, flush edge between lawn and fence.
- Paths and walkways can also serve as mow strips. One important design tip regarding mow strips is to use curves whenever feasible and avoid right angles. Curves allow mowers to navigate easily along mow strips, and saves both labor time and fuel costs.
- Roofing paper can also serve as a weed barrier. When installing paths, lay three layers of heavy grade roofing paper on the soil surface beneath gravel or "interlock" blocks, to prevent weeds from growing up through these porous surfacing materials.
- Unpaved garden paths, like those between raised beds in a vegetable garden, can be "weed-proofed" by covering them with a deep layer of sawdust. Dig pathways about six inches deep, then add the sawdust. Not only is it usually available free for the hauling, but microorganisms attempting to decompose the sawdust tie up soil nitrogen, rendering it unavailable for weed growth. In this sense, sawdust acts as an herbicide. Therefore, remember to use it only on paths, not as a mulch on planting beds.
- Another creative way to design weeds out of your yard is to use desirable plants to outcompete weeds. One of the simplest ways to employ this technique is to heavily overseed newly planted shrub beds or ground cover areas with fast-growing annual flowers. These include sweet alyssum (Lobularia maritima), farewell-to-spring (Clarkia amoena) and scarlet flax. (Linum grandiflorum var. rubrum). Such plants are colorful temporary fillers that germinate quickly and occupy the soil spaces between the slower-growing shrubs or ground covers, thus preventing weeds from growing. The annuals later give way as the more vigorous landscape plants fill in. The occasional weed that does appear can be hand pulled or cut down with a weed eater. Consult your favorite seed catalogs for quick-germinating annuals suited to your area.
- "Barrier plants" can also be used to outcompete weeds. For example, on the island of Mauritius, bermudagrass (Cynodon dactylon) is kept out of sugarcane fields by planting a dense row of turf lilies (Liriope spp.) at the borders of the fields. The lily plant is apparently aggressive enough to keep encroaching bermudagrass out of the cane. This strategy might also be employed at lawn edges to keep grass from creeping into adjoining flower beds. Liriope species are sold in nurseries throughout the United States.
- One of the most imaginative uses of the competitive planting concept is
that developed by David Bigham, a landscape designer in Berkeley, California.
Working almost exclusively with California native plants suited for use in
ornamental plantings, Bigham has developed a unique planting procedure that
mimics the natural vegetation succession that occurs on "unmanaged" land.
Basically, Bigham installs an entire plant succession at once. He selects plant
species that represent the major stages of a vegetative succession leading up
to the final, or climax, planting designed for the site—usually a mixture
of ground covers, shrubs and trees. This approach takes into account what
nature will do to a freshly cultivated planting bed-colonize it with
weeds—and essentially outwits nature by including in the design
aesthetically pleasing wildflowers or grasses to colonize the typical weed
habitat.
For example, if the ultimate goal is a mature oak grove, Bigham plants the oaks and all phases of the understory at once. Such a planting might include a selection of ornamental grasses and wildflowers, semi-woody ground covers, taller shrubs and the oak trees. The plants interact with each other much as they would if they were growing under natural conditions. Thus, in the first year or two, colorful grasses, wildflowers and ground covers visually dominate. As the seasons pass and the shrubs and trees become well-established, they eventually overtop and shade out the lower-growing plants. Finally, the mature oaks largely shade out the understory trees.
Bigham's planting and maintenance techniques work for small- and large-scale landscapes. Since the planting design itself keeps the need for weed control to a minimum, his company is able to maintain plantings in a cost-effective manner even on large private estates, despite the fact that no herbicides are used. Although Bigham developed this method in California, the concept should work anywhere in the country so long as the plant species used mimic the local vegetation succession patterns.
- Another ingenious concept based on competitive planting comes from
Wisconsin ecologist and landscape designer Neil Diboll. Easily adapted to
perennial borders and other nonwoody plantings as well as wildflower meadows,
his plan uses the natural prairie and grassland as a model for landscaping. By
combining tap-rooted flowers and those from corms with surface-rooted grasses
and fibrous-rooted flowers, he is able to control "the rooting zone in the
surface soil." With the surface soil locked up, there is no habitat left for
weeds.
Recommended grasses include little bluestem (Schizachyrium scoparium), prairie dropseed (Sporobolus hederolepis) and India grass (Sorphastrum avenaceum); among the recommended fibrous-rooted flowers are: goldenrods (Solidago spp.), smooth aster (Aster laevis), yellow coneflower (Ratibida pinnata) and common purple coneflower (Echinacea purpurea). Some flowers that work well planted in conjunction with the surfaced-rooted grasses and flowers are the tap-rooted butterfly weed (Asclepias tuberosa), pale purple coneflower (Echinacea pallida) and wild indigos (Baptisia spp.); and Liatrus spp. from corms.
Weeding Out
When weeds appear in your garden, as they will, ask yourself how much weed presence you will tolerate. Your tolerance will depend on such variables as the weed's growth cycle (annual or perennial), habit (tall, short, spreading), where it is located (front or rear of your garden), its positive or negative role (nectar source for beneficial insects vs. sand burs in the lawn), etc.
If you have a sand-laid brick path, you will inevitably find the spaces between the bricks being colonized by weeds. If the microclimate is at all moist, moss will be a component of these weeds. Many gardeners find moss attractive. If you do, try selectively removing the grasses and other undesirable weeds and allow moss to remain. In this way, you will encourage this lovely green perennial to colonize and prevent the return of unsightly weeds. Some people colonize the spaces with a spreading plant, such as thyme.
Scarlet pimpernel, an annual weed that appears in spring gardens, has a very prostrate habit, a short growing season and is a poor competitor with other plants. Furthermore it produces tiny red flowers. Because of these characteristics, many gardeners enjoy the flowers and let the plant die out on its own and mulch the soil.
When weeds do exceed your tolerance, remove them, but tolerate those that aren't worth your time and effort to remove.
Sheila Daar is Executive Director of the Bio-Integral Resource Center and an IPM specialist. BIRC is a non-profit organization that provides practical information on least-toxic methods for solving pest problems. For a copy of BIRC's publication catalog, write BIRC, POB7414, Berkeley, CA 94707; fax 510-524-1758