Weed Management Post-Webinar Q&A

Extension Specialist Emma Erler Answers Real Questions About Managing Weeds in Established Gardens
weeds

In a recent webinar from UNH Extension and UMaine Cooperative Extension, UNH Extension field specialist Emma Erler delved into managing weeds in the garden, yard and landscape. 

We didn't get to all the questions viewers had for us, so we are sharing them here as a written Q&A. Enjoy!

How do you manage horse tail?

Horsetail is very hard to eliminate once it has become fully established in an area. Small populations could potentially be managed with repeated pulling or digging. Larger patches of horsetail may be easier to control with smothering. In early summer mow horse tail close to the ground and cover it with black plastic, non-woven landscape fabric, or heavy layers of cardboard. Put mulch on top of the smothering material to improve its appearance and leave it in place for at least a year.

A final option is to treat horse tail with a non-selective, systemic herbicide that contains glyphosate. Herbicides can be applied any time goutweed is actively growing but will likely be most effective in late summer or fall. If you use an herbicide, make sure to read and follow all instructions on the product label. 

How can I get rid of ground ivy?

As in dealing with many other turfgrass problems, the first thing you should do is evaluate your site conditions. The conditions that favor the growth of ground ivy are quite the opposite of those that are best for turf. Improving drainage, increasing fertility, allowing additional light to reach the lawn, and raising the mower height to three inches will go a long way toward giving turf grass a competitive advantage. Consider getting your soil tested to determine the fertilizer needs of your lawn, and when reseeding, choose a turf grass mix that is shade tolerant. 

If you have improved your site conditions and ground ivy is still an issue, the next step is to eliminate the weed itself. Hand-pulling is an option for those with endless time and patience but be careful to remove all the many roots along the stem. If hand-removal sounds too tedious, some broadleaf herbicides can be effective if they are applied at the right time. The most effective time to apply herbicides is during bloom in the spring and in the late fall. Ask UNH Extension for specific recommendations and always read and follow the label before applying any pesticide product.

Can you address using corn gluten as a pre emergent weed control?

Some gardeners swear using corn gluten as an organic pre-emergent herbicide, but university research to support its effectiveness is inconclusive. Research data show mixed results, and some studies have found that corn gluten does not work especially well against crabgrass. Corn gluten typically contains about ten percent nitrogen, and this fertilizer may help green up lawns and result in dense growth that out-competes weeds. Yet, it may contribute to ground and surface water contamination depending on the application rate and any other fertilizer products used. For weed control, the established recommended rate is 20 pounds per 1,000 square feet. At this rate, about 2 pounds of actual nitrogen are added to the lawn. However, New Hampshire’s turf fertilizer law allows no more than 0.9 pounds of actual nitrogen to be applied per 1,000 square feet per application. Thus, applying corn gluten at a rate that may suppress weeds puts the applicator on the other side of the law and may cause environmental harm. Corn gluten can certainly be used as an organic nitrogen fertilizer if no more than 0.9 pounds nitrogen per 1,000 square feet are applied at one time, not exceeding 3.25 pounds of nitrogen in a year. 

Late last summer, I scraped peastone off the paths among perennial beds, laid down landscape cloth, and re-raked the peastone back onto the cloth. This did not work to block the weeds, I can see that already. Now what? Weeding the peastone is a monumental pain.

Hand-weeding will probably always be part of maintaining a peastone path. When weeds are very young and small you could try treating them with horticultural vinegar, being very careful not to spray any neighboring plants. 

Is handweeding sorrel a good option? How deep are the rhizomes?

Hand-weeding red sorrel is a viable option, though it requires diligence. You are unlikely to remove all the rhizomes in a single effort and will need to continually revisit the same area to pull up resprouts. Most of the rhizomes will be in the upper 1-3 inches of soil. Another thing to consider is that red sorrel tends to grow in low fertility, acidic, dry, or sandy soils. Changing the soil environment where the red sorrel is an issue will help desirable plants be more competitive, whether in a lawn or garden situation. 

We have a grass that spreads underground and is a recurring issue in our tilled flower garden. Can you guess what it might be, and recommend how to manage it?

It sounds like you might have quackgrass (Elymus repens) in your garden. Quackgrass is a cool-season perennial grass that spreads by rhizomes. It can be challenging to control because soil disturbance can promote propagation. Tilling chops up the rhizomes and can end up exacerbating the problem. Carefully digging up quackgrass and removing as many rhizomes as possible is the best non-chemical approach, outside of smothering the grass for a few months. Hand weeding will need to be repeated frequently to completely eradicate quackgrass. A fast option is to use a non-selective systemic herbicide to control quackgrass. Spot treatment with glyphosate is effective, though two or three applications may be necessary to achieve full control. 

How can I control hairy galinsoga (also known as shaggy soldier weed)?

Hairy galisoga is a weed that grows well in high fertility soils, often placing it alongside vegetable crops. Thorough hand-weeding followed by a liberal mulch application is often enough to control galinsoga. A 3–4-inch layer of leaf mulch, grass clippings or straw, or 2 inches of wood mulch should be plenty to suppress weed germination. 

How can I control hawkweed and plantain. I pull by hand which leaves large bare areas, with roots still under the surface.

Hawkweed and plantain are typically lawn weeds that grow well when soil conditions are not ideal for grass growth. Hawkweed prefers dry, acid soils that have low fertility, and plantain does well in poor, compacted soils. Hand weeding these species out of lawn areas is a good way to control the existing weed populations, but you will likely need to amend the lawn soil if you want grass to have a competitive advantage. Establishing a dense and healthy lawn is the best way to keep these weeds at bay. Start by getting your soil tested, then adjust the soil pH, add appropriate nutrients and fix drainage issues. After amending the soil, overseed the bare areas with new grass seed. The best time to do this is in the early fall. 

How can I get rid of goutweed?

Goutweed can be tricky to control because it has a robust underground rhizome system. Small populations may be easiest to remove by hand digging. Use a shovel to extract plants and the surrounding soil, filling in the excavated area with new topsoil. Larger patches of goutweed may be easier to control with smothering. Smothering is most successful in early summer once the plants have put on a significant amount of growth and have exhausted some of the reserves in their rhizomes. In the beginning of June, mow goutweed close to the ground and cover them with black plastic, non-woven landscape fabric, or heavy layers of cardboard. Put mulch on top of the smothering material to improve its appearance and leave it in place for at least six months.

A final option is to treat goutweed with a non-selective, systemic herbicide that contains glyphosate. Herbicides can be applied any time goutweed is actively growing but will likely be most effective in late summer or fall. If you use an herbicide, make sure to read and follow all instructions on the product label. 

How can I eradicate Japanese knotweed? We have a big patch despite smothering part of it for a couple years.

For those who do not want to use chemicals, a few techniques have been proven effective. Small, new populations are easiest to remove by hand pulling or digging. Use a shovel or digging fork at any point during the growing season to loosen the soil around the rhizome and extract the plant. Larger, well-established patches of Japanese knotweed are easier to control by smothering. Smothering is most successful in early summer once the plants have put on a significant amount of growth and have exhausted some of the reserves in their rhizomes. In the beginning of June, cut stems close to the ground and cover them with heavy 7-mil black plastic or non-woven landscape fabric. Put mulch on top of the smothering material to improve its appearance and leave it in place for five years.

Japanese knotweed can also be effectively controlled with the herbicide glyphosate, but timing is key. The flow of nutrients in Japanese knotweed is in one direction. In the spring and throughout the growing season, sugars and nutrients move upward from the roots to the shoots. In the later summer and fall, the flow reverses, and sugars and nutrients are returned to the plants’ rhizomes in preparation for winter. As a result, research has found that chemical applications work best after flowering, and up until the first killing frost. If you are thinking about controlling Japanese knotweed this fall, it’s a good idea to wait until after the plants are done flowering to limit impacts to pollinators. Japanese knotweed is a favorite late season food source for foraging bees. 

How can I manage dandelions? The seeds are everywhere!

Generally, dandelions can be controlled by hand weeding or with a broadleaf herbicide. There are many products that are labelled for dandelion control that can be used in lawn areas. If only a few dandelions are present spot treatment is the best method of control. Spot treat broadleaf herbicides that contain 2,4-D, dicamba, MCPP, sulfentrazone, or quinclorac. These herbicides are selective and will not harm grass. In landscape beds, digging up individual plants by hand is usually the best approach. Herbicides can be used carefully, just be aware that they can also damage trees, shrubs, perennials, and annuals. In vegetable gardens dandelions should only be removed by hand weeding. 

How can we control weeds in a gravel driveway?

If you are sick of hand-weeding but want to avoid using chemicals, you could try using a flame weeder (propane or butane torch) to burn and kill weeds. Herbicide treatment is another option. Horticultural vinegar will control annual weeds and very young perennial weeds. For mature perennial weeds, a systemic herbicide will be a better option. Before purchasing an herbicide product be sure to accurately identify the weed species in need of control. 

How can I manage invasive buckthorn?

A primary goal in controlling buckthorn is preventing seed production and distribution. Buckthorn is best controlled with an Integrated Pest Management (IPM) for Woody Plants, which means combining mechanical and chemical control methods. Young plants can be pulled by hand or dug up, and mowing will prevent seedlings from establishing. Cutting down mature buckthorn without removing the root system will not eliminate it, but rather encourage extensive root sprouts to develop, making the plant even harder to control. 

If dealing with mature plants, chemical control may be necessary. Cutting stems and treating them with an herbicide with the active ingredients glyphosate or triclopyr can limit resprouting. Make sure to read and follow the product label instructions for cut stump application. Apply herbicides carefully to avoid harming non-target plants.

If you're trying to replace your lawn with something that isn't grass and wouldn't require a lot of mowing, what would you recommend? You mentioned Ajuga, Is Ajuga the best choice? Would clover also work? I'm just wondering what some of the better options are?

There are few, if any, plants that can serve as an exact replacement to turf grass, but there are some that can fill a similar role. Think about using creeping thyme (Thymus sp.) or bugleweed (Ajuga reptans) in spots that will receive occasional foot traffic. When mowed infrequently, these perennials produce flowers that are enjoyed by many different pollinators. For a similar look to grass without the maintenance, consider planting a native sedge such as Pennsylvania sedge (Carex pensylvanica). Pennsylvania sedge has a grassy look, suppresses weeds, is drought tolerant, and shade loving. A downside is that these perennials cannot be directly sown from seed into the lawn area. They must be planted as transplants. 

White clover can also be a great substitution or addition to turf. Not only do the flowers provide pollen and nectar for pollinators, but the plants can produce their own nitrogen, reducing the need to apply chemical fertilizers. If you want to make sure the clover does not overtake your lawn, simply raise your mower deck to a height of three inches or more. The only downsides to clover are that it is slower to establish than grass and tends to grow in clumps, potentially leaving bare spots in the lawn that could be filled with weeds. 


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