Why Do Weeds Grow?

Why do weeds grow so fast and easy? For weeds to grow in your yard, there must be seeds or live roots in the soil and proper germination and growth conditions. Unfortunately, weeds don’t need much to sprout and grow. 

Any problem in your lawn is an opportunity for resilient weeds to thrive. Clovers grow in low-nitrogen soil. Spurges survive in nematode infestations. Dollar weed loves soggy soil. To keep weeds at bay, all you have to do is keep the soil and grass healthy. 

Read this article to learn why weeds grow in your lawn and how to make the yard less weed-friendly.

What are weeds? 

The Weed Science Society of America (WSSA) defines a weed as a “plant that causes economic losses or ecological damage, creates health problems for humans or animals, or is undesirable where it is growing.” 

Most weeds crowd turfgrass and ornamentals and are unwanted in lawns. But there are some exceptions. For example, white clover is considered a lawn weed when it grows unbidden among the grass. When planted as a groundcover or in a mixture with grass for more resilient lawns, clover is a desired plant and stops being a weed.

Why are weeds bad for your lawn? Technically, weeds are not bad for your soil or your lawn’s ecosystem. Some enrich the ground with nutrients, others help stabilize erosion-prone areas, and flowering weeds are valuable food for pollinators. In some cases, weeds are more beneficial for the environment than turfgrass.

However, weeds do ruin homeowners’ plans of having that perfect, neat, grass-only turf in more than one way:

• Weeds come in various shapes, sizes, and colors, turning a neat, green turf carpet into an eye sore. Broadleaf weeds are especially disturbing in a uniform turf.
• Most weeds are fast growers and crowd the grass. If allowed to grow tall, they shade newly sprouted turf seedlings and even mature grass plants.
• Weeds compete with turf for water and nutrients, depriving the grass of valuable resources.
• Invasive weeds such as tropical chickweed, chamber bitter, fig buttercup, and purple nutsedge spread aggressively and can quickly take over entire lawns. 
• Some weeds are dangerous to humans and animals and should not be allowed to grow around the house. Poison ivy and stinging nettle can cause nasty skin rashes; Jimson weed and water hemlock are poisonous if ingested. 

Why do weeds grow on your lawn?

Geoff Lawton, a world-renowned permaculture consultant, says weeds are a symptom of problems in the local ecosystem.

“In any square meter of ground, there can be thousands of different types of seeds that can germinate, but the germination conditions determine what does germinate,” he explains. For example, “When you have compaction, you get decompaction weeds that germinate – dandelions in lawns.” 

Weeds are highly adaptable, resilient, opportunistic plants and will take advantage of any problem in your lawn that makes the grass less competitive. If weeds grow in your lawn in large numbers, one or more of the following situations is happening:

Your lawn has compacted soil

Soil compaction happens due to various factors, such as:

• Clay, dense soil 
• Intense foot traffic and using heavy lawn care machinery
• Heavy rainfall
• A few consecutive seasons without core aeration – soil aeration loosens compacted soil, promoting deep roots and better nutrient absorption

When your lawn has at least a few areas with highly compacted soil where the turf is thinned or dead, weeds come to the party. Goosegrass, dandelions, knotweed, and plantain are just a few species that can tolerate compacted soil much better than turfgrass and will soon start to grow if their seeds are present in your lawn.

You skipped on overseeding and repairing bare patches

Even with perfect lawn care, some turfgrass gets damaged in summer or winter when the weather provides less suitable growing conditions. This means thinning turf here and there, maybe some small bare spots or larger dead patches. 

If you don’t solve these lawn problems in time, weeds will rush to sprout and take over all the thin and bare spaces. 

Scalping the lawn or its edges

When you’re tempted to scalp the lawn in the spring only to force the grass to grow faster, think back to the last season and whether or not you had a summer weed problem. Dandelions, pennycress, henbit, and yellow rockets are just a few weeds that sprout early in the season and can take over a lawn mowed too short.

Short turfgrass is less dense and leaves more soil exposed to sunlight, where weed seeds can germinate. It is also more vulnerable to drought, fungal diseases, and lawn pests and can lead to a thin, patchy lawn in the summer. 

Pro tip: Trim the lawn edges carefully to avoid scalping the grass along the pavement. It’s a top spot for weed infiltration in your lawn.

Cracks in the pavement

Pavement walkways on your property get too hot in the summer and too cold in the winter for grass or ornamentals to thrive around them. 

But not for weeds. Crabgrass and quackgrass are two warm-season weeds that can easily thrive in cracks in your driveway and walkways, ignoring the scorching summer temperatures. In northern yards, chickweed, a cool-season weed, has adapted to survive between concrete slabs and bricks, even when ice-covered.

Wet or soggy areas in your lawn

Overwatering, drainage issues, or low spots collecting water in your lawn can all cause overly moist soil. When this happens, and wet and soggy areas appear across your lawn, turfgrass suffers, but a few weeds will break out the champagne in celebration. 

Doveweed, dollar weed, yellow nutsedge, and Virginia buttonweed are four common examples of moisture-loving weeds. They will happily make a home anywhere on your lawn where the soil gets too much water.

Drought or dry areas in your lawn

Is dry soil less exposed to weeds? Not at all. There are plenty of weed varieties out there that can’t wait for drought to wilt the grass in your lawn to take its place. Goosegrass, spotted spurge, plantain, and Florida pusley are just a few weeds you might discover on dry spots in your turf.

Unbalanced soil pH

Turfgrass prefers a slightly acidic soil pH between 6.5 and 7.0. Where the soil pH is too alkaline or acidic for turf to thrive, grass fails to grow properly, and weeds come running to cover the soil. 

Yellow sorrel, clover, broadleaf plantain, and common Bermudagrass are adapted to grow in alkaline soils with a pH of over 7. If your lawn has a low pH, crabgrass, prostrate knotweed, and sheep sorrel might spring up among the weakened grass.

High salinity 

Because of de-icing winter treatments, spring lawns often find themselves spotted with salty areas along sidewalks and driveways. Coastal lawns are also exposed to salt spray, in their case brought by Mother Nature from the sea.

If the salt is not flushed out from the soil soon enough, salt-tolerant weeds might grow instead of the desired turfgrass. Purslane, lambs quarters, common ragweed, tassel flower, and barnyard grass often grow in saline soils. 

Shady areas

Most turfgrass species need at least 6 hours of direct sunlight to thrive. They grow leggy and thin in the shade – under trees, near tall shrubs, or buildings. If you have shaded areas in your lawn where the turf is struggling, you can expect weeds like ground ivy, violets, nimblewill, or basketgrass to appear.

Problems with soil fertility

Homeowners often overfertilize their lawns to make the turf a deeper green, not realizing that the excess nitrogen is feeding the weeds (also risking fertilizer burns on the turfgrass). Most weeds love high soil fertility and start to grow immediately after fertilization if the turf is not dense enough to crowd them.

Poor soil is also prone to weeds. If not enough nutrients are available for establishing and growing healthy turfgrass, weeds can easily become more competitive. Clovers, sandburs, beggarweeds, ragweed, and creeping indigo are a few examples of weeds that grow well in low-nitrogen soils.

Nematode infestations

If spurges, Florida pusley, and prostrate knotweed are growing in your lawn, it might be because the turf is infested with parasitic nematodes. While beneficial nematodes are valuable for controlling lawn pests, parasitic species can harm the turfgrass, resulting in a stressed, thin lawn open to weed growth. 

Prostrate knotweed, spurges, and Florida pusley have a high tolerance to nematode infestation and often grow where parasitic nematodes damage the grass. 

Where do weeds come from

Sometimes, it seems like weeds appear out of nowhere. One season, your lawn is clean and neat; the next, weeds pop up in all shapes and sizes. So, how do weeds spread, and what can you do about it?

How weed seeds get on your lawn

Weeds spread mostly through seeds, but some also use their vegetative parts: roots, rhizomes, and stolons. While pieces of roots and stolons ensure spreading the weed on a small surface, weed seeds can travel long distances using:

• Wind: The wind easily carries small seeds like dandelions, horseweed, and common milkweed.
• Animals and birds: When eaten, most seeds pass through the digestive tract of animals and can land on your lawn in a small “fertilizer” clump. 
• Water: Heavy rain and runoff remove weed seeds from the soil and carry them on the street or nearby areas. Seeds that get into the drainage system can travel far.
• People: Weed seeds can easily cling to clothing and the soles of shoes, especially when muddy outside.
• Equipment: Lawn care equipment and the wheels of bikes or cars often bring home various plant seeds collected around the neighborhood. 
• Grass and flower seeds: Unfortunately, some turf and flower seed packages also come with weed seeds. 

Weeds grow when you till the soil and bring old seeds to the surface, where warmth and sunlight trigger germination. Weed seeds are tough and can last for years in the soil. 

“For example, seeds of common ragweed can survive nearly 40 years in undisturbed soil. Only a portion of the seeds in the soil germinate each year, leaving a reserve for future years,” says Linda Naeve, program specialist at the Iowa State University Extension.

Types of lawn weeds 

Not all weeds are the same. Some rely solely on seeds to spread, while others live on through their roots year after year. Here are the main types of weeds you’re dealing with based on their life cycle.

Annual weeds: Annual weeds have the shortest life cycle, lasting only a year. They sprout from seed, grow leaves, flower, spread seeds, and die in 12 months or less. Common annual weeds include:

• Oxalis
• Chickweed
• Hairy bitter grass
• Groundsel

Biennial weeds: Biennial weeds grow only a leafy plant during the first year and develop a flowering stalk and seedhead during the second year. Once they flower and spread the seed, they die. 

Common examples of biennial weeds are:

• Wild carrots
• Prickly lettuce
• Clover
• Evening primrose
• Burdock
• Common mullein
• Moth mullein

Perennial weeds: Coming back season after season from their roots, perennial weeds in your yard can last many years and are the hardest to kill. They have long taproots deep into the soil, ensuring survival across harsh winters and scorching summers. 

Perennial weeds are spread by seed but also by stolons and rhizomes, making them harder to remove once they set foot in a garden.

Common perennial weeds are: 

• Ground ivy
• Thistle
• Quackgrass
• Dandelions
• Nutsedge
• Chicory

How to prevent weed growth and spread

All you have to do to control weeds is keep the grass healthy and thick. Here are the main lawn maintenance strategies to deploy for a weed-free lawn:

Keep the soil healthy and balanced

Aerate at least once every other year: Weeds can thrive in compacted soil, but most turfgrass types suffer from stone-hard dirt around their roots. If your lawn has soil compaction, core aerate during fall or spring to ensure your grass grows healthy and dense and keeps weeds in check.

Test and amend the soil: Test your soil’s pH and nutrient levels once every two years. Balance soil pH with lime or sulfur and improve topsoil quality with the right soil amendments and fertilizers. 

Fertilize the turfgrass only during the growing season: Dormant grass feeds less than growing grass, and all the remaining nutrients provided during dormancy become food for weeds. Use slow-release fertilizers to prevent excess nitrogen.

Maintain a thick, dense lawn

Overseed the lawn yearly: Even healthy, well-cared-for grass tends to thin during the year due to drought, heat, or aging, becoming vulnerable to weeds. Keep your lawn dense and thick by overseeding yearly in the fall. Dense turf keeps the soil shaded, limiting weed germination and crowding young weeds before they manage to grow strong.

Patch the bare spots in a timely manner: Hot, dry summers and soggy, cold winters often leave bare patches on your lawn. Weeds can’t wait to cover them, so make a habit of repairing any patches as soon as the weather allows it.

Follow a proper lawn care routine

Water the grass deeply and less often: Less frequent watering promotes deeper roots, better resistance to drought, and a more balanced moisture level in the soil. Use Lawn Love’s “The Definitive Guide to Watering Your Lawn” to adjust your irrigation routine.

Mow the lawn correctly: Cut the grass taller to promote deeper roots and more resilient turf. Avoid trimming more than 1/3 of the blade’s length. Don’t mow wet grass. Learn more about proper lawn mowing from our guide, “8 Lawn Mowing Tips and Tricks”.

Use effective weed control methods

Mow weeds before making flower heads: Trim weedy areas in your lawn as soon as you see flowering buds. This prevents seed heads from forming and seeds from spreading even more across the lawn.

Hand pull weeds when in small numbers: Do your best to extract the entire root for perennials and biennial weeds. Otherwise they might regrow.

Apply a suitable herbicide: Pre-emergent weed killers are more effective on annual and biennial weeds than perennials. Annuals can be controlled with contact and systemic post-emergent herbicides, while perennials require a systemic post-emergent weed killer to infiltrate the entire plant. 

Plant the right turfgrass

Only buy grass seeds from reliable sources: It’s the best way to ensure the grass seeds you’re spreading are not mixed with weed seeds.

Choose a turf type suitable for your lawn: Plant warm-season grasses like Bermudagrass, St. Augustine, or Zoysia if you live in the South and cool-season grasses like fescues and ryegrass if you live up North. Consider a shade-tolerant variety for the shady areas on your lawn.

FAQ about weed growth

Why do weeds grow randomly on your lawn?

Weeds grow wherever seeds are brought by wind, lawn mowers, or animals and find favorable conditions. This often means bare spots in your lawn spread randomly on compacted areas or zones with too much or too little sun exposure.

Why do weeds keep coming back?

If weeds keep coming back into your lawn after hand pulling and herbicide applications, it can be because:

• Your lawn provides them with favorable growth conditions (thin grass, poor soil, improper watering, etc.).
• You don’t use the proper weed control methods and products. 
• You use the right herbicides but don’t apply them correctly and at the right time.

This is a situation when hiring a lawn care professional is recommended.

Why do weeds grow so fast?

Weeds are better at grabbing nutrients and water from the soil. This helps them grow a little faster than turfgrass and crops.

When to call the lawn professionals?

If your lawn has a weedy history, don’t wait for these leafy pests to take over your turfgrass. Use the Lawn Love website to find a lawn care company to perform all the pre- and post-emergent treatments and keep your lawn weed-free year-round!

Main Photo Credit: Tony Alter | Flickr | CC BY 2.0