Home gardens

Why are weeds so competitive with my plants?

Every summer I battle an enemy that can come back from the dead and there seems to be more of them every time I turn around! That’s right, I’m talking about zombies – ok, not zombies: weeds! Weeds, those annoying plants that crop up in my lawn, side walk and garden and are just so darn hard to kill!

Kochia flower
Weeds steal sunlight, water and nutrients from our desired crops. One like Kochia is a prolific weed because each flower produces a seed with more than 50,000 seeds per plant! Credit: Vipan Kumar

Weeds are annoying and hard to get rid of, but why does it matter? Well weeds don’t just look bad, they also make it difficult for you to grow the plants you do want. They steal water and nutrients away from desirable plants. They might shade desirable plants so the seeds don’t germinate or the seedlings cannot access sufficient light and die back. Some weeds are so competitive that they actually murder neighboring plants via production of toxic chemicals. This chemical warfare is termed allelopathy by plant experts.

So, the reality is that weeds are super at competing for sunlight, water and nutrients from our desirable plants. That can cause problems in your home garden or a farmer’s field. What makes weeds such great competitors?

Weeds tend to have a few things in common. First, they are very hardy and can survive in locations that are inhospitable to more desirable plant species. They might have waxy leaves that prevent water loss, which makes them more tolerant of drought and heat. Some have deep rooting root systems that can access water more shallow rooted species can’t. Think of those plants growing in between the cracks in a sidewalk when it’s 100 degrees out and hasn’t rained in a month.

Another common trait of weeds is the ability to produce thousands of seeds. Their seeds also are great at dispersing (think of the dandelion seeds blowing from your neighbor’s yard!) So when you think you’ve done a really good job of cleaning up your lawn and garden, a month later or next season, BAM they’re back from the dead!!  Really successful weeds produce seed that can survive in the soil for years! So if you let those weeds go even one year and they produce seed, expect to be battling them for a long time to come.

Soil is the hope of life
Weeds can grow in inhospitable places, like sidewalk cracks, due to their survival traits. Credit: Shuang Liu

How can you best fight the weeds? Weed prevention is key, so never let weeds go to seed in your yard. Collecting them in a garbage bag may be necessary – some invasive weeds like garlic mustard need to be discarded according to city/town regulations. By not allowing weeds to spread their seeds, you can reduce their numbers.

And pulling weeds early is important. Some weeds are like Heracles slaying the Hydra – you might notice that after you weed whack those pesky plants that they come right back. Like the Hydra which produces a new head where one has been cut off, the tap roots of some weeds such as dandelion allow them to produce a new plant just from the surviving root in the ground. When dealing with these weeds you have to pull them, root intact, from the ground, or use an herbicide that goes down into the roots to kill the entire plant. Many weeds, such as thistle, can also reproduce via rhizomes or stolons. These are under-ground stems which can generate an entire new plant. If you choose to use a hoe in your garden, be aware that every piece of the rhizome has the potential to produce a new plant so all plant material must be removed!

When using herbicides to manage weeds in your garden, correct timing is key. Winter annuals, such as shepherd’s purse, emerge in the fall, survive the winter and then continue to grow and set seed the following summer. Summer annuals emerge in the spring, flower and set seed, then are killed by frost in the fall. Crabgrass and knotweed are examples of common nuisance summer annuals in lawns. To get good control of winter annuals it is often necessary to apply herbicides in the fall when the plants are small, but summer annuals can be controlled in the spring.

While herbicides can be an important part of a weed management strategy, as Benjamin Franklin so aptly put it, “an ounce of prevention is worth a pound of cure.” Using mulches and landscape fabric is a great way to prevent weeds from becoming problematic in the first place. In lawns, make sure to quickly re-seed bare patches and mow high (3”) to allow the grass to out-compete weed species. Remember weeds do best in stressful environments that prevent the growth of your desirable plants. If your garden or lawn isn’t thriving, take a soil sample and submit it for testing to determine if nutrient levels are sufficient to support healthy plant growth. Amendments with compost or chemical fertilizers might be needed. Irrigate during dry weather conditions and select the right plants for the location – shade tolerant species in low light locations, hardy grass species in high traffic lawns, etc.

Whatever your purpose for cultivating plants, whether farm, garden or landscape, weeds make your job harder. The best way to tackle these plants is to know your enemy. Correctly identify the weed, learn about its growth habit (winter vs. summer annual, seed reproducing, rhizome reproducing etc.) and then develop your management plan from there. Just remember to get a jump on it! These fast growing, tough, highly contagious plants wait for nobody.

Answered by Audrey Kalil, North Dakota State University

Learn more about weed management

What other ways are agronomists finding to control weeds? Read about flameweeding research, and robotic weeder research. Researchers are also studying the correct temperature of sugar-cane burns to reduce seed regrowth.

To read more about a weed scientist in action, read about weed scientist Anita Dille, or watch Anita’s video below:

About us: This blog is sponsored and written by members of the American Society of Agronomy and Crop Science Society of America. Our members are researchers and trained, certified, professionals in the areas of growing our world’s food supply while protecting our environment. We work at universities, government research facilities, and private businesses across the United States and the world.

5 replies »

Leave a Reply