What is a weed and what harm do weeds do?
For the gardener’s purposes, a weed is frequently defined as “a plant out of place” or “a plant whose virtues have not yet been discovered,” to quote Ralph Waldo Emerson.
Admittedly, some weedy plants are edible or provide shelter for wildlife. Some are beneficial as sources of pollen and nectar. So, why can’t we just “live and let live?”
Weeds are considered “bullies” because they tend to grow fast and compete with desirable plants for light, nutrients, water, and space. They can impede airflow, creating a favorable environment for plant diseases. Weeds can also provide habitat for insect and disease pests that can spread to desirable plants and may even pose a health hazard to humans (e.g., poison ivy, ragweed). A vegetable garden full of weeds will produce fewer vegetables. Weeds also reduce the aesthetics and functionality of our landscaped areas.
In our changing climate, many invasive weeds have a greater ability to adapt to higher temperatures and emerge earlier in the spring, flower earlier, and outcompete many slower growing native plants that provide food and shelter for our birds and pollinators. This article does not address the invasive weed problem other than to say that climate-friendly gardening practices make a difference. Read more on combatting invasive plants here. Do NOT introduce any non-native ornamental plants that we know to be invasive but are still being sold at garden centers.
How do weeds get into your gardens?
Most certainly, all soils in Virginia contain weed seeds, and weeds will always be a part of your garden ecosystem. Weeds are spread by many sources, including the wind, water, bird deposits, and last year’s crops and weeds. Seeds can be brought to your yard in a load of compost or in potting soil of transplants, or they can be stuck to the sole of a shoe or the hoof of an animal. They can also come in from adjacent lawns, fields, or woods. Underground stems of creeping grasses or Canada thistle can travel as much as a foot through the soil before emerging in your garden. Believe it or not, every square inch of your garden soil contains weed seeds, but only those in the top inch or two will get enough light to germinate.
Identify your weeds.
Learning to recognize problem weeds before they are established makes it easier to keep them from overtaking your garden. There are several ways to identify your weeds.
- Use a dichotomous key, a tool that relies on a step-by-step comparison of vegetative characteristics to identify a plant.When you see a weedy plant you cannot identify, dig it out, including the roots, and lay it on a paper towel. Then turn to the dichotomous key in a practical guide such as Weeds of the Northeast, which covers a region from Virginia to southeastern Canada. Do the leaves come out of the base of the plant or on the stem? Are the leaves opposite each other or alternating on the stem? Is the leaf simple or compound, having several leaflets? Are the roots short and numerous or long and running? Is there a long taproot? Look at the flower or fruit structure. The species descriptions in this guide provide a lot of information and photos of the plant at the various stages of growth.
- Use a mobile plant ID app, such as PictureThis. To get the best possible results, take multiple pictures of the same plant from different angles and distances, and make sure results are consistent. Don’t use those results to make a positive ID immediately but as a starting place to confirm that ID through other sources. Also, make sure you are using a plant ID app rated highly for reliability and accuracy. Click HERE to see Michigan State University Extension’s recent evaluation of some popular plant ID apps.
- Consult Virginia Cooperative Extension. Send a photo via email to the Albemarle VCE Helpdesk at albemarlevcehelpdesk@gmail.com, or bring a sample to our Helpdesk at 460 Stagecoach Road, Charlottesville. Or, submit a photo or a sample directly to the Virginia Tech Weed ID clinic.
Understand weed biology.
When you identify a weedy plant, learn about its growth habit. The lifecycle of weeds falls into three categories: annual, biennial (seed), or perennial (root and seeds). Knowing the difference can help you determine the most effective method for managing a weed problem.
Annual weeds grow predominantly by seed and 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-early fall, survive the winter, regrow, flower and set seed in the spring, and die when summer heat arrives. They can also germinate in the spring but die in the summer. Examples: henbit, chickweed.
- Summer annuals germinate from seeds in the spring, grow in the summer, and die in the fall. Examples: Japanese stiltgrass, crabgrass.
Biennials require two growing seasons to produce seed and complete their life cycle.
Perennials live for two or more years. They reproduce not only by seed but also by spreading vegetatively through roots, rhizomes, stolons, or tubers. They can be herbaceous (without a woody stem) or woody. Typically, they die back in the fall and send up new growth in the spring. These are the most difficult weeds to control, since they will regenerate from any parts left alive in the soil. They are much harder to pull, and when they do come out, the root often breaks, leaving pieces in the soil that will often propagate again. Cutting these weeds at the soil level repeatedly will eventually deplete food reserves and kill the plant.
Practices and strategies for weed management
Prevention is the most important strategy. Keep in mind that weed seeds germinate in open and disturbed soils.
Leave no surface bare. Nature abhors a vacuum. Close spacing of garden plants can reduce weed growth by shading the ground, thus limiting seed germination and allowing your plants to outcompete undesirables. Eventually, tight planting will eliminate the need for repeat mulching.
In the vegetable garden, shade the weeds by:
- using organic mulch, such as straw or leaves, at the time of planting,
- interplanting with cover crops such as buckwheat after vegetables are established,
- planting crops intensively so that leaves of one plant will barely touch the next plant, thus covering and shading the bed when plants mature.
Minimize soil disturbance. Tilling or overworking the soil can increase weed problems by exposing seeds to light or by cutting and spreading pieces of perennial weeds that can regrow.
- When planting, minimize the size of hole you dig to limit soil disturbance, and then apply the appropriate mulch for your bed. (Vegetable gardens have different mulching needs from perennial beds).
- Remove young weeds by hand, or use shallow cultivation with superficial hoeing with a stirrup or loop hoe, which will cut weeds off at or just below the soil line with minimal soil disturbance. Remember, even the act of pulling a weed creates a soil disturbance and can encourage weed seed germination.
- Use the right tool (e.g., dandelion or hori hori knife) to dig up roots and underground parts of perennial weeds without overly disturbing the soil. (For best results, dig when plant is young and soil has some moisture but is not overly saturated.)
- Cut the weed at the soil level repeatedly (on a weekly schedule) until the leaves cannot provide enough food to sustain the plant.
Limit seed germination and seed dispersal, whether the plant is an annual, biennial, or perennial weed. There is an old proverb that says, “one year’s seeding, seven years weeding.” 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 and germinate when they receive the right environmental conditions (light, temperature, moisture, scarification) to grow. Preventing weeds from producing seeds will result in fewer seeds in the soil in the following years. The rule of weeding is: if you see a weed in bloom, pull it immediately before it sets seeds!
Keep the creeping, perennial weeds with strong, extensive root systems and weed seeds out of your compost pile.
What about herbicides?
In general, in an integrated weed management program, herbicides should be used only as a last resort for those special, difficult-to-control weed problems. Because of the variety of plants and situations in the home landscape, it is impossible to find one herbicide suitable to manage all weeds in all situations. Always read the product labels carefully, and follow instructions on proper use, storage, and disposal.
Do weed patrol weekly during the growing season and be persistent! Young weeds are easier to kill than established ones. Happy weeding!
References and further reading:
Combating Invasive Plants, Piedmont Master Gardeners
Keep It Covered: The Best Organic Mulches for Your Vegetable Garden, Piedmont Master Gardeners
4thesoil, Four Core Principle
Weeds, NC State Extension
Weed ID Clinic, Virginia Tech
Weed Identification, University of Maryland Extension
Weeds in the Home Vegetable Garden, VCE PUB 426-364
Manage Weeds on Your Farm, Ch 4. Mechanical and Other Physical Weed Management, SARE
Featured image: William Clifford, CC BY 2.0 (cropped)
