Weeds are persistent plants that seem to always find a way back into our gardens and lawns no matter how many times we pull them out or apply herbicides. Understanding why weeds continue to return can help gardeners and homeowners better manage them for the long term.
Weed Adaptations That Enable Persistence
Weeds have adapted in many ways that enable them to survive attempts to remove them and quickly recolonize areas:
- Prolific seed production – Many weeds produce vast quantities of seeds that can remain viable in soil for years. Even if the original weeds are removed, seeds left behind will sprout new weeds.
- Fast growth – Weeds like crabgrass and pigweed grow rapidly, allowing them to quickly grow back after removal. They can also outcompete slower growing landscape plants.
- Underground stems and roots – Perennial weeds spread via underground stems (rhizomes) and roots that are left behind when the above ground portion is removed. New shoots sprout from these structures.
- Spreading seeds – Weeds disperse seeds far and wide via wind, animal fur, flowing water, and other means. New weed infestations easily spread from nearby areas.
- Resilient seeds – Some weed seeds have thick, impermeable coatings that allow them to remain dormant but viable for years in soil until conditions are right for germination.
Large, Persistent Seed Banks
A major factor in the persistence of weeds is the gradual accumulation of weed seeds in soil, known as the seed bank. Weed species produce copious amounts of seeds – often hundreds to thousands per plant. While some seeds germinate right away, others can remain dormant and viable in soil for decades due to specialized seed coatings and dormancy mechanisms. Even shallow cultivation can bring older weed seeds to the surface where they can then germinate. The viable seeds that accumulate in soil essentially act as a reservoir from which new generations of weeds continuously emerge.
| Weed Species | Estimated Seeds Per Plant | Seed Longevity in Soil |
|---|---|---|
| Crabgrass | 150,000 | up to 15 years |
| Common lambsquarters | 72,450 | up to 40 years |
| Curly dock | 30,000 | up to 60 years |
| Common purslane | 240,000 | up to 40 years |
As shown above, each weed plant can produce tens of thousands to hundreds of thousands of seeds, resulting in an enormous and concentrated seed bank after just a few years of uncontrolled growth. With seeds remaining viable for up to decades, this creates a virtually inexhaustible supply of weeds.
Rapid Growth Allows Reinfestation
Weeds are able to quickly reestablish themselves in areas where they have been eliminated because many grow at faster rates than surrounding landscape plants. Crabgrass, for example, can complete its entire life cycle from germination to producing new seed in as little as 6 to 8 weeks. Thus, any bare soil quickly becomes occupied by crabgrass again. The same is true of fast-growing annual weeds like purslane, spotted spurge, and common lambsquarters.
Perennial weeds also rapidly regenerate from bits of root left behind after cultivation and from underground stems that were missed. Yellow nutsedge tubers left in soil after attempts to eliminate the weed can quickly resprout. A single tuber left behind results in a new nutsedge plant. Loosestrife, bindweed, and quackgrass quickly spread through roots and rhizomes to cover bare areas.
Weed Seeds Spread From Many Sources
Weeds seem to appear out of nowhere in gardens and lawns even when there were none the previous year. This continual reintroduction of weeds from outside sources is another factor in their persistence.
Weed seeds are easily spread by wind – some like dandelions have specialized tufts to better disperse their seed through the air. Birds also spread seed as they eat mature weed seeds and then deposit the undigested ones elsewhere in their droppings. For weeds growing near water, flowing water readily transports seeds to new areas. Many weed seeds stick readily to animal fur and human clothes, resulting in longer distance dispersal as people and pets move about.
Weed seeds also spread via contaminated mulch, hay bales, or gravel brought on site for landscaping projects. Equipment used for gardening, mowing, grading, and excavation can carry weed seed from area to area on mud stuck in treads or wheels. The diversity of vectors by which weeds spread their seed makes controlling their spread very challenging.
Weed Competition Impacts Landscape Plants
Once weeds become established, they are difficult to eradicate because of their competitive adaptations that allow them to deprive desired plants of light, moisture, nutrients, and space. Traits like broad leaves, shallow roots, and faster growth confer advantages to weeds in colonizing and thriving in areas meant for landscape plants:
- Shading – Tall weeds like burdock and bindweed grow quickly and cast shade over lower growing landscape plants, blocking sunlight needed for growth.
- Water depletion – Weeds with dense, shallow roots absorb surface water before it can reach the roots of landscape plants.
- Nutrient theft – Fast-growing weeds uptake soil nutrients like nitrogen faster than desired plants.
- Allelopathy – Some weeds like spotted knapweed release natural chemicals (allelochemicals) that inhibit other plant species.
- Crowding – Low growing weeds strip bare areas of open space needed by landscape plants to become established and thrive.
This ability to quickly establish, grow fast, and compete aggressively makes weeds difficult to permanently eliminate from landscapes once they become prevalent.
Persistence Through Diversity
Weed populations have a high degree of genetic and trait diversity that provides resilience. Any weed population contains some individuals that can withstand or avoid whatever control methods are used. When the rest of the population is eliminated, these hardy individuals survive to regenerate a new population. Some examples:
- Applying the herbicide glyphosate repeatedly causes resistance to evolve in some weeds, making the chemical ineffective.
- Tilling weeds exposes new seeds to light and kills surface weeds, but deep rooted perennials survive to regrow.
- Mowing weeds encourages low-growing, mowing tolerant varieties to dominate.
- Hand weeding removes surface weeds but not deep rooted ones that then flourish.
This filtering effect leaves the most hardy weeds to reproduce. Over time, the weed population becomes increasingly difficult to manage.
Prevention: Stopping an Ever-Growing Weed Problem
While weeds may never be completely eradicated from a landscape, steps can be taken to prevent existing weed problems from becoming unmanageable:
- Use weed barrier fabrics when building new planting beds to block light and prevent weeds from germinating.
- Mulch beds thickly to block light from reaching weed seeds.
- Plant closely spaced, dense plantings to leave little open space for weeds.
- Pull weeds early before they go to seed to prevent seed bank build up.
- Cut off seed heads of perennial weeds to prevent spread.
- Clean equipment after working in weedy areas to avoid spread.
- Use weed-free soil, gravel, and mulch to avoid introduction.
- Scout for new weed infestations early and eliminate while still small.
While more work up front, taking preventative steps will pay off long term by avoiding uncontrollable infestations. Be vigilant and proactive against weeds!
Control: Managing Established Weed Populations
For landscapes already overrun with weeds, all hope is not lost. A combination of tools and techniques can provide control even when weeds seem persistent:
- Cultural controls – Improve soil health and fertilization to help desired plants outcompete weeds.
- Mechanical removal – Regularly hand pull, hoe, or till weeds to disrupt growth. Repeat weekly in first year.
- Mulching – Use 3-4 inches of mulch to block light and suppress weeds.
- Solarization – Clear an area, water thoroughly, and cover with clear plastic for 6-8 weeks to heat kill weeds.
- Herbicides – Selectively spot spray weeds while avoiding desired plants.
- Smothering – Cover weedy areas with landscape fabric, boards, or other light-blocking material.
It will likely take multiple seasons of persistent management to significantly reduce established weed populations. But over time, vigilant control efforts will pay off and make weeds far more manageable.
Key Takeaways on Weed Persistence
- Weed seeds accumulate in soil seed banks, remaining viable for many years.
- Specialized adaptations allow weeds to thrive, spread, and quickly regenerate after removal.
- Prevention is critical to avoid uncontrollable infestations in landscapes.
- Integrated, multi-option control strategies are most effective on established weeds.
- Persistence is key – weeds can be managed but will likely never be permanently eliminated.
Conclusion
Weeds demonstrate impressive resilience and seem to always find a way to return and infest gardens and lawns. Understanding their competitive advantages, reproductive strategies, and dispersal mechanisms provides insight into why weeds continue to plague homeowners and professionals alike. While they may never be completely vanquished, vigilance and persistently utilizing a diversity of control techniques can significantly reduce weed problems and prevent them from spiraling out of control. Never give up – with concerted effort over time, weeds can be managed to acceptable levels through prevention and control.
