Thistles are flowering plants that have sharp, prickly leaves and stems. They are found throughout the world and considered weeds in many areas. However, despite their “weedy” reputation, thistles play an important ecological role by providing food and habitat for pollinators such as bees, butterflies, and hummingbirds.
What are some common types of thistles?
There are over 120 species of thistles found worldwide. Some of the most common types in North America include:
- Canada thistle – Widespread across North America with pink-purple flowers. Introduced from Europe.
- Bull thistle – Large biennial thistle with purple flowers. Native to Europe and Asia.
- Tall thistle – Blue flowering thistle native to eastern North America.
- Yellow starthistle – Bright yellow flowers. Invasive species in western North America.
- Musk thistle – Pink-purple flowers. Native thistle in North America.
- Scotch thistle – Spiny thistle with large purple flowers. Native to Europe and Asia.
Why are thistles considered beneficial for pollinators?
Although often viewed as pesky weeds, thistles provide a number of important benefits for pollinating insects:
- Nectar source – Thistles produce abundant nectar that attracts pollinators.
- Pollen source – Bees use thistle pollen to feed developing larvae in the hive.
- Long flowering period – Many thistle species flower for weeks, providing a prolonged food source.
- Habitat – Thistles help provide shelter and nesting sites for pollinators.
- Nutrition – Thistle pollen contains high levels of protein, fat, and nutrients needed by pollinators.
In areas where thistles grow in large numbers, they can form an important part of the habitat for bees, butterflies, and other flower-visiting insects.
Thistles as a food source
The nectar and pollen from thistles provide an important food source for many pollinating insects. Different species feed on thistles, including:
- Honey bees – Gather nectar and pollen from thistles to take back to the hive.
- Bumble bees – Feed on thistle nectar and use thistle pollen to feed developing larvae.
- Butterflies – Sip nectar from thistle flowers.
- Hummingbirds – Drink nectar from thistles with tubular flowers.
- Solitary bees – Specialist bees such as the thistle long-horned bee specifically forage on thistle pollen.
Thistles bloom over an extended period which offers a reliable, long-lasting source of food. The high protein content of thistle pollen also makes it particularly nutritious for bees.
Shelter and nesting habitat
In addition to food, pollinators need suitable habitat for nesting and shelter. Features of thistles that help provide habitat include:
- Dense patches – Thickets of thistles provide shelter from predators and the elements.
- Undisturbed land – Many thistles grow in untilled or unmowed areas which solitary bees need for nest sites.
- Stems and foliage – Bees will nest in hollow thistle stems or use leaves to line nests.
- Few pesticides – Weedy thistles often grow where pesticides aren’t sprayed, providing safe habitat.
Allowing some thistles to grow undisturbed provides necessary habitat diversity for bees, butterflies, and other pollinators.
What are some examples of pollinators that feed on thistles?
Many different insect species visit thistle flowers in search of nectar or pollen. Some main pollinator groups that utilize thistles include:
Bees
- Honey bees
- Bumble bees
- Mining bees
- Mason bees
- Leafcutter bees
- Long-horned bees
- Carder bees
Bees are the most important pollinators of thistles. Bumble bees in particular rely heavily on thistles. Specialist bees like the long-horned bee have evolved to specifically gather pollen from thistles.
Butterflies and Moths
- Painted lady
- Red admiral
- Common buckeye
- Checkerspots
- Skippers
- Hummingbird moth
Butterflies and moths drink nectar from thistle flowers with their long, straw-like proboscises. Some moth caterpillars even feed on thistle leaves.
Flies
- Hoverflies
- Tachinid flies
- Bee flies
Though not as efficient pollinators as bees, flies do transfer some thistle pollen from plant to plant.
Hummingbirds
Hummingbirds use their long beaks to drink from tubular thistle flowers. Examples include the rufous hummingbird and broad-tailed hummingbird which feed on thistles in the western United States.
Beetles
Various flower beetles, soldier beetles, and checkered beetles may feed on thistle pollen and aid in minor pollination.
How does thistle growth habit and morphology help attract pollinators?
Thistles have several key morphological adaptations that make them attractive food sources for pollinators:
Flower heads
Thistles produce dense clusters of many small tubular flowers called florets. Having numerous florets in one location makes it more efficient for pollinators to feed and move between flowers.
Flower color
Thistle florets often have brightly colored pink, purple, yellow, or white petals which help attract pollinators. Some thistles even reflect ultraviolet light only visible to bees.
Nectar
Abundant, slightly sweet nectar induces pollinators to feed. Nectar is produced at the base of the tubular florets.
Pollen
Thistles produce copious amounts of protein-rich pollen grains from the anthers of each floret. Bees harvest this pollen to feed developing larvae.
Flower height
Thistles have tall stems that lift the flower heads up and make them accessible. Flower heights range from several inches to five feet or more.
Prolonged blooming
Individual plants and whole thistle stands flower for weeks to months, ensuring a consistent food supply.
Through these adaptations, thistles are able to effectively provide for the nutritional needs of pollinating insects.
What role do thistles play in their ecosystem?
Although often considered annoying weeds, thistles fill an important ecological niche:
- Food web support – Thistles provide food for pollinators as well as other wildlife including seed-eating birds like goldfinches.
- Soil health – Extensive thistle root systems aerate soil and help prevent erosion.
- Biodiversity – Thistles increase plant community diversity in areas dominated by grasses.
- Shelter – Thistle patches provide cover for small mammals and other wildlife.
- Seed dispersal – Seed-eating birds spread thistle seeds over the landscape.
- Nitrogen cycling – As thistles die back they return nitrogen to the soil to fertilize other plants.
Allowing some thistles to grow contributes to the health of the broader ecosystem.
Can thistles ever be considered invasive or problematic weeds?
In some situations, thistles can proliferate and become detrimental invasive weeds. Reasons thistles sometimes become invasive include:
- Lack of natural predators and parasites to keep them in check.
- Extensive wind-borne seed dispersal.
- Rapid growth that outcompetes other plants.
- Abundant nectar rewards encourage pollinators to spread seeds.
- Deep taproots with extensive nutrient stores enable fast growth.
- Spiny defenses make thistles unpalatable to herbivores.
However, the threat posed by thistles is often exaggerated. Most thistles are not aggressive invaders when part of a healthy, diverse ecosystem.
Problematic invasive thistles
Invasive thistle species of greatest concern include:
- Canada thistle – Widespread across North America.
- Bull thistle – Problematic in western North America.
- Musk thistle – Invades pastures and rangeland.
- Yellow starthistle – Serious invader in western states.
Control methods like mowing, herbicides, or biocontrol insects may be warranted in cases where thistles dominate and reduce biodiversity.
Should thistles sometimes be encouraged in habitats?
Management plans that encourage some thistles can benefit pollinators. Strategies include:
- Allowing non-invasive thistles to grow undisturbed in sections of parks, meadows, or rangeland.
- Mowing or spraying thistles on a rotation leaving some intact each year.
- Introducing thistle species to planted pollinator gardens.
- Avoiding broadcast spraying of thistle stands which would eliminate pollinator habitat.
With selective management, thistles can enhance spaces as pollinator food and shelter sources without becoming dominant weeds.
Tips for encouraging thistles
Ways to intentionally incorporate thistles include:
- Allow native thistles to thrive in designated unused areas.
- Hand seed native thistles along habitat margins and roadsides.
- Cultivate non-aggressive thistles like bull or swamp thistle in pollinator gardens.
- Use thistles as ornamental plants in landscape beds and borders.
- Manually control thistles that spread into unwanted areas.
Encouraging thistles in targeted areas helps sustain pollinators while minimizing weed risk.
Summary of key points
To summarize the key points:
- Thistles provide abundant nectar and nutritious pollen that supports diverse pollinators.
- Bees, butterflies, and other insects all gather food from thistle flowers.
- Thistle stands offer shelter and nesting habitat for pollinators.
- Flower and growth adaptations make thistles attractive to pollinators.
- Thistles fill roles in the ecosystem food web and nutrient cycling.
- A few thistles are problematic invaders, most are not aggressive weeds.
- Allowing some thistles to grow benefits pollinators greatly.
Rather than being mindlessly eliminated as weeds, thistles should be recognized and managed as the important pollinator plants that they are in most situations.
Conclusion
Thistles have developed a poor reputation as troublesome weeds. However, the pollination services thistles provide far outweigh their minor nuisance. Their nectar and pollen sustain a wide array of bees, butterflies, and other essential pollinators that might otherwise struggle to find adequate food sources. Thistles help maintain pollinator diversity and abundance in both natural and agricultural ecosystems. Allowing some thistles to thrive in our landscapes demonstrates an appreciation of their ecological value. With their unique natural history and myriad benefits to other species, thistles represent a fascinating and ecologically important group of plants.
