The competition between grass and weeds in lawns and fields is a common occurrence. Many gardeners and landscapers aim to maintain a lush green lawn or pasture of grass, relatively free of weeds. But weeds seem to always find their way into grassy areas and can sometimes take over. This raises the question – if left alone, will grass eventually choke out weeds, or will the weeds prevail?
– In general, given enough time, grass will eventually choke out weeds in a lawn or field. But it depends on the type of grass, the growing conditions, and maintenance practices.
– Grass primarily chokes out weeds by shading them from sunlight and outcompeting them for water, nutrients and space. A thick, healthy stand of grass will limit resources available to weeds.
– Weeds tend to thrive in thin, patchy lawns or fields, exposed soil, poor drainage and low fertility. Improving grass density, growth and vigor through proper care will tip the balance in favor of the grass.
– Occasional weeding, overseeding, mowing, watering, fertilizing and soil amendment helps grass choke out and prevent weeds over time. But weeds may still pop up sporadically in even the healthiest grass stand.
– Certain aggressive grass types like Bermuda and Zoysia grass are particularly good competitors against weeds when properly established and cared for. But weeds can still sometimes invade.
– Preventing weeds from going to seed will help deplete the weed seedbank over time and limit future weeds. But dormant weed seeds can persist in soil for years.
– Completely weed-free grass is difficult to achieve long term. But proper lawn and field management can shift the balance heavily in favor of the desired grass and significantly limit weed pressure.
How Grass Chokes Out Weeds
Grass primarily chokes out weeds through the following mechanisms and competitive advantages:
An established, dense stand of grass will form a thick canopy that shades the soil surface and prevents weed seeds from germinating and establishing. Weed species tend to be more intolerant of shade than grasses, so shading gives grass an important competitive edge.
Once established, grasses are very effective at absorbing water, nutrients and root space in the soil, depriving potential weeds of these essential resources. Grass root systems are usually much larger and more extensive than roots of common lawn and field weeds.
Many turf and forage grass varieties grow rapidly and produce abundant aboveground biomass. This allows the grass to quickly fill in bare spots where weeds might otherwise take hold. Fast regrowth also contributes to the shading capability of grasses.
Grasses evolved to withstand grazing pressure from herds of herbivores. Therefore, they can tolerate regular mowing, whereas weeds are less adapted to survive repeated cutting or grazing. Mowing gives grass a competitive edge.
Healthy grass stands produce abundant viable seeds. As these seeds continually self-sow into the existing grass, they enhance its density. Weeds produce fewer seeds in grassy areas due to resource competition.
Some grass species, particularly ryegrass, produce allelopathic compounds that chemically inhibit the germination and growth of weeds. This allelopathy contributes to weed suppression.
Factors That Affect Competitive Ability of Grass
Several key factors influence the degree to which grass can effectively choke out weeds:
Grass Species and Variety
Some grasses are inherently more competitive against weeds. For example, fine fescues tolerate shade well but compete poorly with weeds. Improved turfgrass and forage cultivars have been bred for vigor and density.
Thin, nutrient-poor or compacted soils favor weeds. Optimizing light, fertility, drainage and soil health tips the balance toward grass. Grass also competes better in its preferred pH range.
A thick, lush grass stand leaves little room for weeds. But thin, patchy areas allow weeds to gain a foothold. Overseeding can help fill in bare spots.
Height of Cut
Mowing too low stresses grass and gives weeds an opening. Following proper mowing height guidelines removes weeds while optimizing grass growth.
Irrigation and Rainfall
Adequate moisture prevents drought stress in grass and enables rapid growth to close in bare areas. Excess water can sometimes favor weeds in poorly drained areas.
Light weed infestations are more easily controlled by grass competition than heavy infestations and pervasive weeds like bindweed and nutsedge. Starting with a weed-free area gives grass the advantage.
Lawn Care and Maintenance Practices for Weed Control
The following cultural practices and maintenance techniques enable grass to better choke out weeds over time:
– Mow at proper height for grass species
– Keep mower blades sharp
– Vary mowing direction
– Leave clippings to return nutrients
Overseeding and Topdressing
– Overseed thin or bare areas in grass
– Rake in starter fertilizer with overseeding
– Topdress with compost to level lawn
Aerating and Dethatching
– Core aerate compacted soil
– Dethatch to remove excessive thatch layer
– Allows proper infiltration of water, air and nutrients to roots
– Use soil test to determine fertilizer needs
– Apply appropriate fertilizer formulation and rate
– Avoid excessive nitrogen which can sometimes favor weeds
– Apply in splits according to grass growth rate
– Water deeply and infrequently
– Avoid frequent, light watering
– Adjust sprinklers to prevent overspray on hardscapes
– Allow soil to partially dry between watering
– Hand pull large weeds before they set seed
– Spot treat with selective herbicide as needed
– Reseed treated areas
– Use preemergent herbicide to prevent new weeds
– Incorporate compost to increase organic matter
– Apply lime to correct low soil pH if needed
– Gypsum can improve sodic or compacted soils
Preventing Weeds from Going to Seed
An important goal in weed control is preventing existing weed plants from going to seed. This helps gradually deplete the weed seedbank in the lawn or field over time. Recommended practices include:
– Mow frequently enough to prevent weed seedheads from forming
– Hand pull large weeds before flowers or seedheads develop
– Spot treat with herbicide in order to kill weeds prior to seed set
– Avoid excessive thatch buildup, which can harbor weed seeds
– Apply preemergent herbicide in spring to inhibit germination of summer annuals
– Prevent soil disturbance which can uncover dormant weed seeds
– Clean mowing equipment after use in weedy areas to prevent spread
Preventing seed production and dispersal is key to shifting the weed seedbank equilibrium toward desired grasses over time rather than weeds.
Persistence of Weed Seeds in Soil Seedbank
Although preventing weed seed production is helpful, it is difficult to completely deplete the weed seedbank in established lawns and fields. Many weed species have seeds that can persist in the soil for extremely long periods in a dormant state. Reasons for long weed seed persistence include:
– Tough, impermeable seed coats that protect seeds from deterioration
– Ability of some seeds to remain dormant but viable for many years or decades
– Successive generations of weed seeds accumulating in soil over time
– Dormant seeds getting buried deeper in the soil profile away from conditions that trigger germination
– The vast number of existing weed seeds already present in most soils
Due to these factors, weed seeds can potentially persist for 3-4 years or sometimes much longer. While competing grass may prevent new weed seeds from establishing, pre-existing dormant seeds can still occasionally sprout and emerge even in mature, healthy grass stands.
Examples of Weed Seed Longevity in Soil
|Seed Longevity in Soil
|over 50 years
|up to 20 years
|up to 40 years
As this table shows, even short-lived annual weeds can persist for years in the seedbank, while some perennial weed seeds remain viable for decades. Even in a lush, weed-free lawn, dormant weed seeds remain waiting for an opportunity to emerge.
Vigilance and Persistence Against Weeds is Key
While grass can effectively choke out and outcompete weeds over time under the right conditions, achieving a completely weed-free lawn or grassy field is unlikely, especially considering the persistence of buried weed seeds. The occasional emergence of some weeds should be expected even in the most robust, competitive grass stands.
However, with proper establishment, on-going maintenance, and quick action against new weed infestations, healthy grass will keep weed pressure to a minimum. The competition between grass and weeds demands perpetual vigilance and a commitment to care for the desired grass species over the long term.
Letting grass go unmanaged will allow weeds to quickly gain a foothold. But focusing on growing a dense, vigorous sward of grass suited to the site through correct planting, mowing, fertilization, soil care and weed control will tip the scales heavily in favor of the grass over time.
While the weed seedbank persists, reducing new weed seed production combined with promoting healthy grass growth will gradually shift the competitive balance toward the desired grass species becoming dominant in the landscape.
– Grass can effectively choke out weeds through competitive advantages, but typically requires proper maintenance and care.
– Weeds thrive in thin, stressed grass stands. A thick, healthy lawn or field leaves little room for weeds.
– Shading, resource competition and allelopathy allow grass to suppress weeds. But weeds are adapted to exploit bare ground and low fertility.
– Overseeding, fertilizing, aerating, watering and mowing properly all help grass outcompete weeds.
– Preventing weed seed production and dispersal helps deplete the seedbank over time. But dormant seeds can persist for years in the soil waiting to sprout.
– Completely eliminating weeds long term is difficult. Expect occasional breakthroughs. But healthy grass will limit weed pressure and regrow to dominate.
– Maintaining vigorous grass growth and stopping new weed infestations quickly leads to weed decline over years.
The competition between grasses and weeds in lawns, pastures and fields is eternal. But grass can gain the upper hand over weeds when properly managed and cared for. A dense, vigorous stand of grass suited to the site and intended use, along with quick control of any emerging weeds before they set seed, will gradually diminish weed pressure over time. While requiring continued effort, focusing on growing healthy grass will lead to substantial weed decline in the long run. Lawns and fields may never be completely weed free, but proper practices will enable the grass to choke out and dominate weeds to the extent possible.