Key points about weeds and weed control
- “Weeds” are defined as plants growing where they are not wanted. They can range from harmless plants that show up to fill an empty space, or invasive plants that are regulated by federal and state authorities because they can cause harm to economic, environmental, or human health.
- Chemical herbicides, as with any pesticides, have risks associated with exposure by way of drift, runoff, or improper use. If one desires to manage health concerns, protect water quality, and/or reduce harm to non-target organisms in the environment, physical and cultural methods can be used to kill weeds without chemical herbicides.
- Weed prevention and management require regular monitoring, learning, and persistence! A variety of non-chemical methods will be needed based on your site conditions, the types of weeds you have, and your garden or landscape goals, preferences, and budget. There is not a one-size-fits-all solution.
Understanding weed biology
If you are looking for ways to kill weeds without chemicals, it is helpful first to know a few definitions about the lifecycle of weeds. This is important in determining the most effective methods for managing them.
- Annuals complete their life cycle (seed germination to seed dispersal) in one year or less. Annuals are categorized into two basic groups:
- Winter annuals germinate from seeds in late summer to early fall, survive the winter, regrow and flower in the spring, and die when the temperature rises in the summer (e.g., henbit, chickweed). Winter annual seeds also can germinate in the spring but die in the summer.
- Summer annuals germinate from seeds in the spring, grow in the summer, and die in the fall (e.g. Japanese stiltgrass, crabgrass).
- Limiting seed germination and seed dispersal are priorities for managing annual weeds.
- Biennials complete their growth in two years. They produce leaves and store food in their first year. In the second year, they flower, produce seeds, and die. These are easiest to control as seedlings in their first year of growth.
- Perennials live for two or more years. They reproduce by vegetative structures (roots, rhizomes, stolons, tubers) and seeds. They can be herbaceous or woody. The top growth of herbaceous weeds typically dies down in the winter, but underground storage structures enable the plants to survive and grow again the following year. These are the most difficult weeds to control, since they will regenerate from any underground parts left alive in the soil.
Garden soils have a seed bank of viable seeds that have accumulated in the soil over time. Some weed seeds can stay dormant in the ground for many years, ready to germinate when environmental conditions (light, moisture, temperature, scarification) are optimal for growth. Taking steps to minimize soil disturbance and churning up seeds from the deeper soil profile is an important step in weed management.
Weeds & Climate Change
In our changing climate, invasive weeds with greater adaptability to higher temperatures can emerge earlier in the spring, flower earlier, and outcompete slower growing native plants that provide important functions, e.g. pollinator host plants. (Zettlemoyer, et. al. 2019). Climate-friendly gardening practices make a difference.
Before you plant, kill the weeds
When establishing or renovating a garden bed, take time to remove weeds that are present in your soil before you plant. Your site might have annual, biennial, or perennial weeds, or a combination of them, in addition to weed seeds in the soil. Focus your site preparation efforts on controlling existing vegetation and minimizing soil disturbance. Any of the options below, or a combination thereof, may be used:
- Carefully dig out or hand-pull weeds that are visible, making sure to remove the underground root structures as much as you can. This is easier to do when the soil is slightly moist. Remove weeds prior to seed development, if possible, to minimize dispersal of seeds onto your soil.
- Cover the planting site to kill weeds (use “no-till” approaches). Tilling or disturbing soil by digging and overturning it can increase weed problems by exposing seeds to light and cutting and spreading pieces of perennial weeds that can re-grow. Canada thistle, bermudagrass, and mugwort, for example, can worsen after tilling when vegetative pieces are cut up and left in the soil. Each piece can start a new plant.
- Occultation (e.g. “tarping”, “smothering”) blocks sunlight from stimulating the growth of vegetation. This can be done by using an opaque covering (e.g. a reusable tarp, thick black plastic, commercial weed block material, or cardboard) laid over the intended planting area temporarily. Hold down the edges of the material securely with rocks or boards. Four weeks is a minimum amount of time to leave your soil covered, but the success of this approach will vary depending on the type of weeds at the site and soil conditions.
Some very tough perennial weeds with strong, extensive root systems (e.g. Canada thistle, Japanese knotweed) would need to be covered for a longer period of time (up to a year or possibly longer) to deplete the roots of carbohydrate reserves that can support re-growth. In the case of plants that have an extensive creeping root system, if any part of the plant is left uncovered and reaches above ground to receive sunlight, it will continue to generate energy to support continued growth. Some plants can have roots that will remain viable in deep soil for up to a year.
No-till gardening (also sheet composting or sheet mulching) is a method of smothering unwanted vegetation and limiting soil disturbance. This approach can be used not only to control weeds but also to remove turfgrass or to prepare a food or native plant garden.
- Solarization involves covering the site with a clear plastic sheet for a minimum of 2-3 weeks, up to 6 weeks. This method is only effective if used during the longest, hottest days of the summer (July and August). The use of a transparent plastic cover allows radiant heat to reach the soil (as opposed to being absorbed by black plastic) and heats the soil better than tarping. Conditions under the plastic kill weed seeds, seedlings, and some pathogens.
Remove visible vegetation first, water the area thoroughly, then place a clear plastic (3 to 6 mil) sheet down and bury or pin down the edges to hold it in place. This technique can be used in the summer to prepare a site for fall planting. Do not till the area after you solarize, as this will bring up seeds from deeper in the soil. Do not use this technique under the dripline of a tree; it can damage or kill fine tree roots.
- Stale seedbed technique (also called “stale bedding” or “stale seedbed”) involves stimulating dormant seeds to germinate and then killing the seedlings rapidly. This is an alternative weed management technique for commercial vegetable production and also is a recommended approach to prepare the ground for establishing meadows. This involves repeated shallow digging or tilling over an area in the springtime. Dig or till the upper surface of the soil (2”), allow weeds to germinate, then kill them by tarping the area and smothering the weeds.
When you plant, cover the soil to prevent weeds
Weeds will find open spaces in a garden. There will always be weed seeds arriving by wind, rain, and animals (e.g. bird droppings), seeking to fill an empty niche. This is the way of plants and a natural ecological process called plant succession.
An approach to weed management is to encourage rapid plant establishment (with soil testing, proper planting technique, watering), keep your soil covered in all seasons, and allow plants to fill in so that light exposure to the ground is minimized. Options for covering your soil are as follows:
- Groundcovers are relatively low-growing plants (2’ or lower) which, once established and grown in, can cover bare ground and out-compete weeds. There are advantages to using groundcover plants instead of common hardwood mulches or wood chips. Having living plant roots in the ground year-round improves soil quality and water retention, minimizes soil erosion, and stores carbon. Refer to our groundcovers page and groundcover list.
- Close spacing of garden plants can reduce weed growth by shading the ground, limiting seed germination, and outcompeting undesired plants. Annual flowers and vegetables can be spaced so that their leaves slightly overlap when mature. Native perennial gardens, rain gardens, and foundation plantings can be designed so that plants eventually grow in and touch one another and reduce or eliminate the need for repeat mulching.
- Research the recommended mature size, habit, and a disease-susceptibility of your desired plants and space them accordingly. Some plants prone to fungal disease like powdery mildew, for example, are better off with more breathing room and good air circulation.
“The alternative to mulch is green mulch; that is plants themselves. By planting additional species to occupy the open areas, we create a lush, year-round ground cover that reduces weed invasion.”Thomas Rainer and Claudia West Planting in a Post-Wild World: Designing Plant Communities for Resilient Landscapes
- Cover crops (or “green manures”) are plants such as oats, wheat, rye, buckwheat, daikon radish, and crimson clover, which are grown for a temporary period of time to cover the ground, control weeds, and protect and improve soil quality. After growing for several months, a cover crop is cut down, smothered, or harvested to make way for planting a desired new crop. This method is most often used for annual vegetable gardens to cover the soil during the winter months or in between periods of active crop production. Some of these cover crops can suppress weeds in the following season. Refer to our cover crops page.
- Living mulches are cover crops planted underneath actively growing vegetable crops to suppress weeds. A downside to this approach is that living mulches compete with the main crop for nutrients and water and reduce yield, so they require active management. For experienced vegetable gardeners who want to experiment with this approach, Dutch white clover and New Zealand white clover would be two options to try sowing under your main crop. A mini-tiller, hoe, or weed-whacker could be used to weaken or remove encroaching clover plants.
- Organic mulches are natural materials such as arborist wood chips, bark mulch, straw, pine needles, grass clippings, and shredded leaves that will gradually decompose over time and add nutrients to your soil. Layered on top of soil, they will limit weed growth. They also are good for maintaining even soil moisture and creating habitat for beneficial organisms like ground beetles (natural pest control). Refer to our page about types of mulch.
- Biodegradable landscape paper comes in rolls and can be laid out on the surface of a straight garden row. Cut holes into the paper and transplant into the holes. The edges of the paper need to be held down to keep the paper from blowing up and away. This is a good option for a temporary cover to reduce weed growth, such as for an annual vegetable garden. The paper degrades gradually and can be turned into the soil or composted.
- Inorganic mulches are materials that do not decay and do not provide any benefits to the living soil underneath. Landscape fabrics, plastic sheeting, “rubber mulch”, rocks and stones are examples. These might seem to be a good long-term solution to weed control, but they have limitations in a landscape setting. Over time, weeds will germinate in the soil or organic matter laying on top of landscape fabrics, sending their roots through the fabric and into the soil below making it difficult to work around or remove. As leaf debris and soil particles accumulate between rocks and stones, weed seeds will have a substrate in which to germinate and grow. Some of these inorganic mulches -- landscape fabric and black plastic -- are best used to temporarily cover soil in an annual vegetable garden. They can be rolled up and reused for multiple seasons.
Monitor, identify, and decide: Is this a weed or not?
- Monitor your garden regularly; young weeds are easier to kill than established ones. Learn how to identify new plants you find. There are a variety of mobile apps, online references, and field guides available to help with plant identification. For starters, take a look at our photo gallery of the most common weeds in Maryland.
Weed Identification Photos
- If you need further help with plant identification, Ask Extension or a Master Gardener. Sometimes flowers are needed in order to confirm the identification of a plant, since many plant seedlings look similar.
- Cultivate curiosity and tolerance. “Weeds” that show up unintentionally in your garden might not be “bad” plants at all. You might get some native plant volunteers or tree seedlings and decide you want to keep them and cultivate them on your property (free plants!). It is a challenge to manage a lawn completely free of weeds. You can decide to accept other plants, like native wild violets or non-native white clover, which commonly show up in lawns and have the benefit of providing food sources for wildlife. Consider reducing the size of your lawn and replacing some with lawn alternatives that will add biodiversity and beauty in your landscape. Appropriate soil fertility and turfgrass density are excellent cultural methods to suppress some weeds.
- When you identify a weedy plant, learn about its growth habit -- is it an annual, biennial, or perennial? Does it reproduce primarily by seed dispersal? Does it have perennial root structures like rhizomes or tubers? This knowledge will help you understand how best to manage the weeds.
How to prevent weeds
As you manage your garden and property, be mindful about ways to minimize chances of bringing in new weeds.
- White-tailed deer are effective carriers of small weed seeds (e.g. Japanese stiltgrass, wavyleaf basketgrass) on their coats and feet. Manage white-tailed deer with fencing or other methods, if the deer population is high in your area.
- Ask landscaping companies to clean their equipment prior to coming on to your property.
- Be careful about using composts, mulches, and/or manure, which can be sources of weeds. Inquire about how products were produced and stored to minimize weed contamination.
- Avoid putting weeds with seed heads into home composting systems.
- Remove weeds from purchased or shared plants prior to planting.
How to kill weeds by physical & mechanical removal
Following are several options you can use to kill weeds without chemicals:
- Hand-pulling, hand-digging, shallow cultivation. Weeds can be pulled or dug out more easily by hand when they are young and small and the soil is slightly moist. Annual weeds have finer roots than perennials and are easier to pull out. Shallow cultivation (no more than 2” deep) with a hoe or other hand-tool can kill small weeds before they get established. Do not cultivate too deeply, or you will bring up weed seeds from deeper in the soil. If you are working around ornamental plants with shallow roots, cultivation can damage the fine roots. Hand-pulling or mulching in such cases would be better options.
For tougher perennial weeds, make sure to remove as much of the root structure as possible by grabbing at the base of the plant and pulling up carefully or digging out the roots. Simply breaking off the top growth will not kill the plant. Some perennial weeds have a deep taproot or extensive creeping rhizomes that are difficult to remove. Perennial root structures can be spread by cultivation. Carefully pull or dig what you can, but recognize that this may need to be repeated, in some cases multiple times, until the plants are depleted.
A variety of garden tools - hand shovel, trowel, hoe, hori-hori knife - can be used to make these tasks easier.
- Mowing and trimming. Mowing or cutting down with a string trimmer is useful for preventing seed dispersal and suppressing weeds. This method is more effective on broadleaf weeds rather than grasses because the growing point of grasses is at ground level where new growth will regenerate. Perennials with well established root systems will not be killed by mowing or trimming, but these techniques can be used repeatedly to keep them under control and make them less noticeable. In some cases, you may be able to “starve” perennial weeds by continuously cutting off new growth at ground level. The success of this approach will depend upon the frequency of mowing/cutting and the biology of the plant(s) you are dealing with. Research has demonstrated that mugwort, for example, a tough perennial weed with rhizomes, was able to keep growing after two years of repeated mowing.
- Mulching. All plants need sunlight to generate energy to grow. Excluding sunlight with mulches can limit seed germination and starve plants of their energy. A variety of mulches (as described above) can be used in different landscape settings. A mulch layer of 1” around herbaceous perennials or 2”-3” inches around woody plants is sufficient for weed suppression. More is not better. Avoid excess mulch.
A few limitations of mulching: (1) Perennial weeds can regrow and push through mulch if the root structures are not controlled first, and (2) Weed seeds can land on the surface (by wind, bird droppings) and germinate. Mulch applications will need to be repeated.
- Boiling water. Some annual weeds can be killed if boiling water is poured directly on top of them. This works best when weeds are young, tender, and small. This method can help to suppress perennial weeds by killing off the top growth. It is not effective for permanently killing perennial root structures. This method can be used to control weeds in sidewalk or driveway cracks and crevices or other hardscapes, not in lawns or native or ornamental plant beds. Use caution when using this method for personal safety.
- Flame weeder. If you want to kill weeds in a location where no flammable materials are present (e.g. sidewalks, stone driveways) a hand-held propane torch can be used to burn the vegetation. This is most effective on annual weeds and broadleaf weeds. Heat will kill the top growth of perennials but will not kill the underground root system. It is not necessary to burn the weed. The flame makes the moisture in the plant become steam and damages the cells. Read your owner's manual for instructions on how to best use your flammer. Do not use this method near burnable materials such as mulch, wood, dry leaves, ornamental grasses, etc.
References & resources
Comparing Solarization & Occultation | University of Maine
Living Mulches Suppress Weeds and Yields in Organic Vegetable Plots | University of Wisconsin
Using the Sun to Kill Weeds and Prepare Garden Plots | University of Minnesota
Weed Management in Landscapes | University of California
Zettlemoyer, M.A., Schultheis, E.H. and Lau, J.A. 2019. Phenology in a warming world: differences between native and non-native plant species. Ecol Lett, 22: 1253-1263. https://doi.org/10.1111/ele.13290
Author: Christa Carignan, Certified Professional Horticulturist & Coordinator, University of Maryland Extension Home & Garden Information Center (HGIC). Reviewers: Chuck Schuster, Extension Education (retired); Jon Traunfeld, Director, HGIC; Debra Ricigliano, Lead Horticulturist, HGIC. 2022
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