Invasive plant species are spreading across landscapes and continents at an alarming rate. These foreign plants escape from gardens and aggressively expand their range, displacing native species. One category of invaders that receives less attention are invasive flowers. Although they may seem harmless or even beautiful, introduced flowers can wreak havoc on ecosystems. Let’s explore some of the most disruptive invasive flowers, how they spread, and what we can do to control them.
What Makes a Flower Invasive?
For a flower to become invasive, two key factors are involved:
Reproduction and Spread
The plants must be able to reproduce prolifically and disperse seeds broadly. Many invasive flowers generate copious amounts of lightweight seeds that travel far on the wind or water. Others spread vegetatively via rhizomes, tubers, or plant fragments. This allows them to quickly colonize new areas.
Escape from Limiting Factors
In their native range, plants are kept in check by herbivores, diseases, and competition. But in new habitats, these limiting factors are often absent. The plants
are suddenly released from control, their populations exploding.
Additionally, invasive flowers typically share aggressive traits like rapid growth, adaptation to disturbance, and allelopathy (chemical inhibition of other plants). Equipped with these abilities, they outcompete native flora.
Notable Invasive Flowers in North America
Many alien flowers have become problematically invasive across parts of North America. Here are some of the worst culprits:
Purple Loosestrife (Lythrum salicaria)
This striking wetland flower with purple spikes escaped from gardens in the early 1800s. It now occurs coast-to-coast, forming dense mono-cultural stands. Loosestrife crowds out native plants like cattails that provide food and shelter for wildlife.
Spotted Knapweed (Centaurea stoebe)
Introduced from Eurasia as a contaminant in alfalfa seed, spotted knapweed infests over 7 million acres in the western US and Canada. The plant displaces native grasses and wildflowers, significantly reducing forage for livestock and habitat for pollinators.
Himalayan Balsam (Impatiens glandulifera)
Originally from Asia, this large annual plant with showy pink flowers has become a major invader of riparian areas, wetlands, and moist forests across North America. It displaces native plants and damages stream ecosystems. Seeds explode from the seedpods, scattering up to 20 feet away.
Yellow Flag Iris (Iris pseudacorus)
The yellow flowers of this Eurasian iris adorn many American wetlands where it has escaped cultivation. But it forms dense colonies, outcompeting native plants. It also alters wetland hydrology and chemistry in ways that exclude other species.
Oxeye Daisy (Leucanthemum vulgare)
Oxeye daisy, famous for dotting fields in Europe, was brought to North America as an ornamental plant. It has now invaded fields, prairies, open forests, and alpine meadows across much of the continent. This aggressive perennial excludes native plants and reduces wildlife habitat.
Himalayan Blackberry (Rubus armeniacus)
Originally from Armenia and India, this prickly shrub has become ubiquitous across the Pacific Northwest since its introduction in the late 1800s. It crowds out native shrubs and wildflowers in open areas and prevents establishment of trees and other plants. Birds disperse the seeds far and wide.
Common Tansy (Tanacetum vulgare)
Brought by European colonists for medicinal uses, common tansy is now a rapidly expanding invader of open disturbed areas like pastures, roadsides, and prairies. Its tactic for dominance is allelopathy – toxins from the roots suppress other plants. This significantly reduces native plant diversity.
Purple Pampas Grass (Cortaderia jubata)
Native to South America, purple pampas grass has been widely planted as an ornamental for its large, purple, feathery plumes. But it readily invades coastal habitats like dunes and wetlands, outcompeting native plants and altering habitat. Vast root systems accumulate, elevating the soil surface.
Canada Thistle (Cirsium arvense)
Despite its name, Canada thistle is native to Eurasia and North Africa. Introduced in the 1600s, it now infests all lower 48 states and much of Canada. Its extensive roots spread aggressively, displacing other plants in prairies, forests, and riparian zones. Patches expand steadily at up to 10 feet per year.
Why Are Invasive Flowers Problematic?
Beautiful as they may be, introduced flowers can cause extensive ecological harm when they invade natural areas. Here are some of the major concerns:
- They displace native plants – Invasive flowers use their robust traits like prolific seed output and allelopathy to take over sites, reducing native plant cover and diversity.
- They alter ecosystem functions – Different invasive flowers change factors like soil chemistry, light availability, nutrient cycling, and hydrology to favor themselves over natives.
- They reduce pollination – Some invasive flowers attract generalist pollinators like bees, but provide little reward. This can reduce pollinator services to native plants adapted to specialized pollinators.
- They degrade wildlife habitat – Monocultures of invasive flowers support far less diversity of insects, birds, and other wildlife than the native plant communities they replace.
- They increase costs – Management efforts to control invasive flowers and mitigate their impacts are extremely expensive for agencies and land managers.
- They decrease property values – Infestations of certain invasive flowers like kudzu and Himalayan blackberry can reduce property values.
- They harm ecosystem services – Displacement of deep-rooted native wildflowers by shallow-rooted invaders can increase erosion and flood damage and lower water table levels.
In general, invasive flowers reduce biodiversity and ecosystem health of natural areas. Their beauty masks detrimental impacts.
Pathways of Introduction and Spread
Invasive flowers get introduced to new continents and habitats through diverse pathways:
- Intentional horticultural introductions – Plants imported for their ornamental merits that later escape cultivation. Examples: purple loosestrife, yellow flag iris, purple pampas grass.
- Contaminants of crop/pasture seeds – Weed seeds unintentionally introduced with imported seeds and grains. Example: spotted knapweed in alfalfa seed.
- Medicinal plant introductions – Plants brought for herbalism and folk medicine. Example: common tansy.
- Accidental transport – Seeds stuck in shipping crates, vehicles, cargo, etc. across borders. Example: oxeye daisy in ballast soil of ships.
- Landscaping/erosion control plantings – Introduced for landscaping, but later escaped. Example: Himalayan blackberry originally planted as living fence rows.
- Garden escapes – Cultivated plants that escaped yards and gardens. Most only become invasive in some areas but not others. Example: creeping buttercup.
After introduction, further spread occurs by both natural and human vectors:
Natural Spread
- Wind dispersal of seeds
- Water transport of seeds
- Seed dispersal by birds and other wildlife
- Vegetative propagation from plant fragments and roots
Human-Assisted Spread
- Transport along highways and trails
- Spread via mowing, tilling, and soil disturbance
- Intentional plantings for landscaping, livestock forage, or erosion control
- Accidental spread with nursery plants and seed mixes
Invasive Flower Management Strategies
Controlling invasive flowers requires integrated management plans using multiple complementary approaches:
Manual Removal
Pulling, digging, or hand-cutting invasive flowers can be effective for small infestations. Care must be taken to remove all roots and fragments that may resprout. Repeated removal is needed.
Mowing/Cutting
Frequent mowing or cutting can slow growth and prevent seed production in some species. But plants may regrow from roots unless also treated with herbicides.
Grazing
High-intensity targeted grazing provides partial control of certain invasive flowers. Goats, sheep, and cattle preferentially browse on invasive plants over native vegetation.
Herbicides
Applying systemic herbicides like glyphosate to foliage or stems provides effective control of many invasive flowers. However, impacts on pollinators and native plants must be minimized through careful application.
Prescribed Burns
Fire suppresses cool-season invasive flowers while favoring native warm-season grasses and wildflowers adapted to periodic burns. But multiple post-burn herbicide or mowing treatments are usually needed.
Biocontrol Insects
Host-specific insects that damage or parasitize invasive flowers can provide long-term suppression when established. Examples include thistle head weevils and leaf beetles that target purple loosestrife. Rigorous screening ensures they pose negligible risk to natives.
Landscape Fabric
Placing permeable landscape fabric over infestations smothers plants by blocking light. It provides lasting suppression of resprouting when used in conjunction with removal methods.
Revegetation
Replanting competitive native grasses and wildflowers ontreated sites helps resist reinvasion through “biotic resistance.” Robust native communities utilize resources needed by invaders.
Ongoing monitoring and maintenance are critical following any control efforts. Prevention through early detection and public education is the most cost-effective management. Prioritizing eradication of new invaders before they establish saves future effort and expense.
6 Key Tips to Prevent and Control Invasive Flowers
- Learn to identify key invasive flowers in your region and regularly inspect your property for invaders. Early detection and rapid response are critical.
- Clean equipment, boots, and tires when moving between areas to prevent spread of seeds and plant fragments to uninfested sites.
- Never dump or relocate yard debris like grass clippings and garden waste which may contain invasive propagules. Bag and seal them for disposal instead.
- Avoid planting known invasive flowers. Select non-invasive alternatives and native plants instead to support biodiversity.
- Monitor restoration sites for invasive flowers and quickly treat any new populations. Maintain robust native plant communities that resist invasion.
- Participate in volunteer stewardship efforts like invasive plant pulls to help protect natural areas from floral invaders while raising public awareness.
Frequently Asked Questions
What makes invasive flowers able to displace native plants so effectively?
Invasive flowers possess certain biological traits that give them an advantage, like prolific seed output, rapid growth, adaptation to disturbance, and allelopathy. They are also free from natural enemies like specialist herbivores that keep them in check in their native range.
If an invasive flower is pollinated in its new range, can it evolve to become even more invasive?
Yes, when an introduced flower is pollinated and sets seed in its exotic range, the next generation can be better adapted to those conditions. With continued reproduction, invasive traits like seed output and competitiveness may escalate over time through natural selection.
How do invasive flowers reduce pollination of native plants?
Some invasive flowers attract bees and other generalist pollinators, but provide little or no nectar and pollen as a reward. Pollinators waste visits on these plants rather than seeking out native flowers adapted to specialized pollinators. This disrupts pollination services native plants rely on.
Can invasive flowers alter soil chemistry to favor themselves?
Certain invasive flowers release allelopathic chemicals from roots or decaying leaves that suppress competing plants. Some invaders like knapweeds also increase available soil nitrogen, favoring introduced plants adapted to high nutrient levels over native plants specialized for low fertility soils.
How do invasive aquatic flowers like water hyacinth cause problems in waterways?
Fast-growing invasive aquatic flowers form dense mats that reduce oxygen levels and light penetration, degrading water quality. They also block water flow, worsen flooding, impede navigation, entangle boat propellers, and displace native aquatic plants.
Are invasive flowers a problem in ecosystems other than fields and forests?
Yes, they degrade natural habitats of all types. Invasive flowers colonize wetlands, displace alpine wildflowers, infest prairies, and overrun sand dunes. Coastal ecosystems are especially vulnerable to floral invaders that alter dune ecology. Even cities are afflicted by invaders of urban green spaces.
What can homeowners do to remove invasive flowers from their yard without using herbicides?
Manual removal by digging, hand-pulling, or cutting can eliminate small infestations, but the entire root system must be removed to prevent regrowth. Spreading mulch or landscape fabric helps suppress regrowth. Frequent mowing and monitoring for new sprouts is required. Planting competitive native groundcovers also helps crowd out invaders.
Conclusion
In their native ranges, wildflowers add beauty and biodiversity. But moved across the globe, many become disruptive invaders. Invasive flowers like purple loosestrife and Himalayan blackberry spread aggressively, displace native flora, and degrade wildlife habitat. Robust reproduction, escape from natural enemies, and trait advantages drive their dominance. Invasive flowers decrease biodiversity, alter ecosystem function, and are costly to control. Preventing accidental introductions and dispersal, early detection of new infestations, and integrated management are key to protecting ecosystems from floral invaders. With vigilance and care in transporting, planting, and promoting ornamental plants, we can enjoy their beauty without unleashing ecological havoc.