Time to get strategic! Where do you turn for eradication methods and how do you determine what level of plant diversity is okay? Learn about organic methods of soil preparation like lasagna layering, how to be strategic with watering, and where to get information on preemergent and post emergent applications. Come away with some good Integrated Pest Management practices and tips to identifications for the pests we call weeds.
Questions & Answers:
Q—What’s the best way and time to remove perennial weeds like Lesser Celandine, Porcelain Vine, Wire Grass, Red Clover, Mile a Minute, Bind Weed, Trumpet Vine, Hydrocotyle, Bermuda Grass, mugwort, poison ivy, bittersweet, stickweed, and Tradescantia (spiderwort)?
A–Tradescantia is a native plant with a lovely flower, so consider keeping it in your garden palette.
Q–What types of groundcovers can be used to suppress weed growth? I’m thinking of things like ajuga and sedum — do they work?
A–Ajuga and sedum work very well to cover bare ground. You may still find a few weeds like wood sorrel (an annual) or chickweed, henbit, and dandelions. A few weed trees may also take root, especially things like elms, oaks, and Tree of Heaven. However, pull the few weeds you see regularly and your groundcovers will protect you from a weed invasion carpeting your garden.
Some native groundcovers for the Mid-Atlantic include:
|Latin Name||Common Name||Notes|
|Chrysogonum virginianum||Green and Gold||North or east side, part shade, yellow flowers.|
|Pachysandra procumbens||Allegheny Spurge||Part shade, shade, under trees, leaves are not shiny. Good substitute for Pachysandra terminalis, English ivy or liriope.|
|Spring bloomer, comes in white, periwinkle blue, pink, purple.
Part shade varieties are a good replacement for invasive Vinca minor.
|Iris cristata||Dwarfed crested iris||Full sun to part shade, short spring bloomer, white, blue and purple.|
|Packera aurea, often sold as Senecio aureus
|Golden ragwort||Part shade to sun, yellow flowers in April, fluffy seed heads follow. This native can spread aggressively.|
|Podophyllum peltatum||Mayapple||Shady woodland spring ephemeral. Use along with Aquilegia canadensis (columbine), Phlox divaricata, Trout lilies, Claytonia virginica (spring beauties), Sanguinaria canadensis (bloodroot),
Mertensia virginica (Virginia bluebells), Hepatica nobilis and Trilliums.
|Wild stonecrop||Can take light shade. Small white branched flowers provide good forage for bees.|
|Carex pensylvanica||Pennsylvania sedge||Part to full shade. Good replacement for invasive English ivy or liriope.|
Q–There’s a move to ‘cut, don’t pull.’ What are your thoughts on this?
A—When you pull weeds you disturb the soil surface. Seeds can germinate readily in disturbed soil, including any seeds existing in the soil’s seed bank. You may also just be looking for Cutting can also be a time-saving method. You can use string trimmers to knock the tops off annual weeds before they go to seed. (Perennial weeds need to be extracted roots and all.)
You can also use a scuffle hoe to “cut” the weeds just below the soil surface, pushing and dragging the hoe back and forth. It works rather quickly. But again, this method will disturb the soil, so you are providing partly–tilled ground for existing weed seeds. Scuffle hoeing needs to be repeated every couple of weeks during the growing season. This method works where you cannot mulch.
Q–What about hot water and vinegar water to kill weeds?
A–I have heard of using boiling water on the leaves and crown of a plant and around the soil. I have not tried it in the field, but if you can safely try it without harming yourself or your neighbors, it would make for a good experiment.
Household vinegar is only 5% acetic acid. It can burn new growth and leaves but may not kill plant roots. Making vinegar water would dilute it even further. Higher concentrations have been shown to be more effective, but at that point you are dealing with a strong acid, and both strong acids and strong bases damage skin and eyes. It is not labeled or registered as an herbicide by the EPA, so we cannot recommend it and neither can Cooperative Extension agents.
Q–Can you tell us about organic preemergent like corn gluten?
A–We briefly talked about corn gluten as an organic “weed and feed” preemergent solution. However, it is not very effective on perennial weeds or on crabgrass and spreading it in the spring can contribute to nitrogen runoff in our waterways. The University of Maryland Extension discourages the use of corn gluten in lawns, while also cautioning that traditional chemical preemergent herbicides should only be used as a last resort. Visit https://extension.umd.edu/hgic/topics/corn-gluten-crabgrass-control for more information.
Q—What’s the best way to minimize weeds in between bricks or stepping-stones in paths? Does allowing moss to grow in those spaces help or hurt the battle?
A–Embrace the moss as a charm of coastal living, unless it becomes a slick safety hazard. I’ve seen people agonize over bleaching their bricks, only to have the moss return again. If you have unsightly weeds with noticeable leaves or height, remove them by hand with a dandelion puller, invest in a weed torch, or consider
You can use a non-selective systemic herbicide as a last resort. Be sure to read and follow the label directions for safety and to protect yourself, children and pets.
Q–What type of clover are you talking about to mix with grasses?
A–White clover is the most common to mix fescues for a lawn. It doesn’t get as tall or as noticeable as red clover. Red clover makes an excellent cover crop for vegetable gardens, though.
Q–Japanese stilt grass? the only non-chemical option is to pull it?
A–Cynthia Brown, Moderator: If Japanese stiltgrass has invaded your garden beds, pulling is the best way to eliminate it. If stiltgrass is in your lawn, increase the health of the turf and it can outcompete the stiltgrass. If you want to start over, covering an area with clear plastic and leaving it in place for a full year will kill the stiltgrass, but not the seed bank.
Erin, Presenter: Because Japanese stiltgrass (Microstegium vimineum) is often found in wooded areas rather than cracks in pavement, I wouldn’t recommend a weed torch, unless you live in a very wet wooded area with no fire hazard and local statutes allow it. Remember that just the heat itself could harm tree bark and roots and desirable plants, fungi and soil life.
Partial prevention can be carried out by planting native groundcovers or using landscape fabrics with mulch or gravel on top. Simply mulching 1-3 inches in garden beds helps. You can accomplish full mitigation and eradication with early detection, identification and dedicated removal by hand or hoe.
You can also prevent and mitigate a Japanese stiltgrass infestation by changing the light, moisture and pH of the site.
Consider the following from Peter Nitzsche, an Agriculture and Natural Resources Agent with Rutgers University:
“Stiltgrass thrives in moderate-to-densely shaded areas subject to regular soil disturbances, such as flooding, mowing, tilling, and high foot traffic. Stiltgrass is commonly found along roadways and ditches, floodplains, moist woodlands, and power line corridors. Stiltgrass often thrives in soils that are moist, acidic to neutral, and fertile. Stiltgrass also threatens wooded areas where tree canopies are defoliated due to infestations of gypsy moths or other destructive events. In residential areas, stiltgrass can invade lawns, landscape beds, and vegetable gardens. Stiltgrass does not proliferate in full sunlight or in areas with standing water.”
Q–At our community garden we have an invasive weed commonly known as stickweed. It has an extensive underground root system. What’s the best way to eliminate it?
A–Stickweed is the common name for Verbesina occidentalis, also called Yellow Crownbeard. It is native to the Mid-West, Texas, the South and the Mid-Atlantic. It can grow quite tall. Community gardens are for growing food, so we don’t recommend chemical herbicides. Stickweed also flowers, so you’ll want to avoid the use of any herbicide that might be harmful to bees that forage on those flowers. Many herbicides say not to treat flowerbeds and ornamental plants as well as vegetables. Although it is unsightly and aggressive, Verbesina occidentalis does attract soldier beetles which eat insects that are harmful to other plants.
Herbicides are used on this plant commercially, but few are effective. At least one that is effective is only licensed for sale to or use by certified pesticide applicators and can contaminate drinking water and groundwater. The Virginia Cooperative Extension of Virginia Tech and Virginia State University has a publication on weeds like this and encourages taking two full years to eradicate broadleaf weeds: https://www.pubs.ext.vt.edu/427/427-002/427-002.html
You may need to curtail this weed with physical methods. Carve out a border for your community garden beds. Establish the bed area, a path barrier, and a mowing/lawn area on the borders of the garden.
- Bed area: Physically dig out the stickweed as best you can and smooth, level and tamp the soil. Spread and pin weed fabric and fit polyurethane tiles or stone pavers. Build and fill raised beds over the fabric.
- Path barrier: Dig down several inches of stickweed, install landscape fabric, and add clay-like soil. Tamp this soil down to make a paved path. (Get set up with materials and instructions at your local hardware store.) Pin down landscape fabric, place and fit polyurethane tiles tightly, add layers of sand and gravel as directed, place paving stones, pavers or stepping stones, fill the gaps and install edging. This path will be your barrier.
- Lawn area: Continuously mow any remaining stickweed in the grass to gradually remove energy from the mass. If construction or sidewalks come later, this is in your favor.
Q–What do you think of putting soil with possible weed seeds in an aluminum can that heats up all summer to kill the weed seeds (hopefully)?
A–Just like solarization using clear plastic in a sunny spot, heating soil in a bin would help sterilize the soil, killing many insects, nematodes, harmful fungi and weed seeds. This may be helpful for small amounts of soil you use for indoor plants, windowsill gardens or small raised planters.
If you haven’t noticed significant weeds in your wooded areas, let the soil’s existing biological relationships function. Many fungi and insects are beneficial. Keep an eye out for weeds and overseed or plant native groundcovers for in problem areas. Preserve the native soil and leaf litter to discourage new, unwanted weed seeds from germinating.
Q–Can I use herbicides in my mulch bins?
A–Are these areas where you store wood chips, shredded hardwood mulch, pine bark mulch, or leaf mold? You may not want to use herbicide on mulch you are going to use that season on desirable plants, in case rain or watering brings it in contact with leaves or stems. I would not spray any compost intended for edible or vegetable plant beds.
If you store large amounts in advance for non-edible plant beds another year, you may want to spray the weeds on top, but the natural decomposition process may help kill weed seeds in the middle of the pile. I wouldn’t worry about weeds too much because although your top layer may get weedy, remove that and you have compost that has been properly heated as it matures, making weed seed less viable.
Are you making your own compost with leaves and grass clippings? See our Smithsonian Gardens compost how-to to get the proper ratios of brown matter and green matter here.
Download these scenes from Smithsonian Gardens to use as your desktop background, or on your next Zoom meeting!