Why Do Weeds Continue to Invade the Garden Every Year

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Quick facts

All soils in Minnesota contain weed seeds.

• Weeds spread by many sources, including the wind, bird deposits and last year's crops and weeds.
• Do not let weeds flower and set seeds.
• Unless all perennial plants are absent, they will continue to emerge in your new vegetable garden.
• There are few chemical herbicides suitable for use in the vegetable garden.
• If you can recognize problem weeds before they establish, you may have an easier time keeping weeds from overtaking your garden.

How do weeds get into your garden?

Dandelion flower

Every gardener needs to manage weeds every year. Insect pests may or may not show up in the garden. Diseases may be devastating during some weather conditions and absent during others. Weeds will always be part of your garden ecosystem.

All soils in Minnesota contain weed seeds. Weeds spread by many sources, including the wind, bird deposits and last year's crops and weeds.

They may enter the garden in a load of compost, they could be stuck to the sole of a shoe and tracked in, or they could be in the potting mix of transplants.

Weeds also come into the garden from adjacent lawns, fields or woods. Vining and creeping weeds can grow a small shoot that enters the garden, sends down roots, and flourishes.

Underground stems of creeping grasses and Canada thistle can travel as much as a foot through the soil before emerging in your garden and growing vigorously.

Weeds can invade a very controlled garden, even ones with raised beds, patio containers or areas covered by plastic mulch.

What harm do weeds do?

• Competition from weeds can reduce yields of the crops you grow in your garden.
• Weeds make it difficult for your garden plants to get enough water, nutrients and sunlight.
• They can harbor insect pests and impede airflow, creating a favorable environment for plant diseases.

Strategies and tactics

• Make weeding a part of every interaction you have with your garden. Always look for a new flush of weed seedlings or an invasion of plants from other parts of your yard.

• Do not let weeds flower and set seeds. Prevent the number of weeds from increasing by eliminating weeds before they flower.

• Every gardener needs a hoe of some kind. It may be a small hand hoe, a short-handled Asian-style hoe, a tall solid-blade hoe or a stirrup hoe.

• With your tool, lightly scrape around your plants and in the areas between the rows. This eliminates weed seedlings when they are still too small to pull by hand.

• Do not chop or scrape too deeply with your tool, or you may harm the roots of your vegetable plants.

• It is difficult to keep underground roots and stems from invading the edges of your garden. Most edging materials only extend a few inches into the soil, while many plants spread by underground parts that can be more than a foot deep. Always look for these weeds.

• Almost any kind of mulch can help you battle weeds. Plastic mulches that help warm the soil can also keep weeds from emerging.

• If you use plastic mulch, check the holes cut into the mulch for plants, and pull the weed seedlings that may be growing alongside your vegetable plants.

Preparing a new vegetable garden site

Converting a piece of ground from lawn, weed patch or grassland to a vegetable garden can be challenging.

• A weedy site, such as a vacant lot, may have a mix of perennial plants and annuals.
• Its soil will probably have an abundance of weed seeds ready to germinate.
• A lawn area will have a high population of perennial grass species well-adapted to the site.
• Unless all the perennials are absent, they will continue to emerge in your new vegetable garden.

First, kill the plants

• Start by using a broad-spectrum herbicide such as glyphosate to kill all the vegetation.
• If you are trying to kill a lawn, one or two applications of herbicide may be enough to kill the grass.
• Herbicide applications should be 2 to 3 weeks apart.
• If the site is weedy or wild, you may have to spray many more times.
• You can also cover the future garden plot with heavy plastic sheeting, thick layers of newspapers or old carpeting. After an entire growing season, the perennial plants under the covering will have died and you will be able to start your garden.
• Another option is physically removing the sod with a shovel, be sure to get all the bits and pieces of your former lawn out of your new garden.

When to plant

• If you start trying to establish the new garden site in spring, you may not be able to plant anything until at least midsummer, or even until the following year.
• Planting vegetables too early, before you are sure all the perennials are absent, can lead to a very frustrating gardening season.
• If you start killing existing vegetation later in the season, hold off on planting the following year until you are sure that you have eliminated the population of perennial plants.

Using herbicides in the vegetable garden

Most herbicides are not recommended for use in the vegetable garden.

Remember that federal law governs the use of all pesticides.

Anyone applying pesticides must use them in accordance with the instructions and restrictions on the label.

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• Some gardeners use a non-selective, post-emergence herbicide, such as glyphosate.
• This type of herbicide kills existing weeds before planting seeds or transplants.
• You can use some types of glyphosate in the vegetable garden to kill weeds that have emerged and are actively growing.
• Keep the spray off the vegetable plants as it will harm or kill them.
• Check the product label to be sure you can use it in a vegetable garden. Follow the instructions carefully.

• To prevent new weeds from coming up, some gardeners use an herbicide containing trifluralin, also known as Treflan. This granular product has many trade names.
• Apply and water it into the soil before weeds emerge, to prevent weed seeds from germinating.
• It has no effect on existing weeds, and does not control germination of all weeds that could come up in a Minnesota garden.
• Trifluralin can also prevent vegetable seeds from emerging. Results when using trifluralin will vary, from good control to poor—or no control at all.
• Trifluralin breaks down through a series of biological processes that depend on temperature, sunlight, moisture and soil type. Normally, it will completely degrade within 3 months after application.
• It is also possible that the chemical could remain in the soil and prevent emergence of future vegetable crops.
• Note that beets, chard, corn and lettuce are not on the trifluralin label. Do not use that herbicide near those crops.
• Check the product label to be sure you can use the product near the plants in your garden. Follow the instructions carefully.

• Corn gluten meal is a naturally derived, pre-emergent herbicide that contains 10% nitrogen.
• It may or may not control weeds.
• A byproduct of producing ethanol from corn, this substance does not contain the wheat protein to which some people have sensitivities. Instead, it is a complex of unrelated corn proteins named "gluten."
• In research at Iowa State University, corn gluten meal inhibited germination of many weed seeds. Later studies in Oregon found no benefit from using corn gluten meal.
• In a vegetable garden, repeated treatments of corn gluten meal could be more effective against weeds. It is also possible that the corn gluten meal could negatively affect vegetable seed emergence.

Know your enemy

It may be helpful to identify weeds that cause trouble in your garden. If you can recognize problem weeds before they establish, you may have an easier time keeping weeds from overtaking your garden.

CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.

Reviewed in 2018

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Source: https://extension.umn.edu/planting-and-growing-guides/controlling-weeds-home-gardens

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