Thistle is the common name for a diverse group of flowering plants characterized by their prickly nature. These plants thrive particularly well in disturbed areas like pastures, roadsides, and fields. Thistles are easily recognized by their distinctively spiny leaves and stems, which serve as a defense mechanism. They typically produce globe-shaped flower heads, most commonly appearing in shades of purple or pink.
Identifying Features and Common Varieties
Thistle species are identified by their life cycles and root systems, which determine the most effective removal strategy. Two widespread and problematic types are Canada Thistle (Cirsium arvense) and Bull Thistle (Cirsium vulgare).
Canada Thistle
Canada Thistle is a perennial plant that reproduces aggressively through both seeds and an extensive, creeping root system known as rhizomes. Its leaves are generally hairless and wavy with spiny edges, but its stems are smooth, lacking spines. The flowers are relatively small, growing in clusters at the tips of branches.
Bull Thistle
Bull Thistle is a biennial, completing its life cycle over two years. In its first year, it forms a low-growing rosette of leaves, and in the second year, it sends up a tall flowering stalk. Bull Thistle is much spikier than Canada Thistle, featuring stiff hairs on its foliage and sharp, spiny wings running down the stem. It reproduces only by seed and is anchored by a deep taproot, unlike the Canada Thistle’s creeping rhizomes.
Why Thistles Are Considered Weeds
Thistles are considered weeds due to their aggressive growth and negative impact on agriculture and natural ecosystems. Their competitive nature allows them to quickly outcompete desirable plants for sunlight, water, and nutrients. This competition reduces crop yields and diminishes the quality of forage in pastures.
The sharp spines of thistles can injure livestock, deterring grazing and potentially leading to infections. Perennial varieties, such as Canada Thistle, are challenging because their deep, lateral root systems are difficult to eradicate. These extensive root networks allow them to form dense patches that displace native vegetation and reduce biodiversity.
Methods for Effective Thistle Management
Effective thistle management requires a multi-year commitment and a strategy tailored to the specific species and size of the infestation. The primary goal is to prevent seed production and continually deplete the plant’s root energy reserves. Bull Thistle is generally easier to control manually than the rhizomatous Canada Thistle because it reproduces only by seed and has a taproot.
Physical Control
Physical removal methods are effective for biennial species like Bull Thistle, especially when the plant is in its first-year rosette stage. Hand-pulling or digging must remove the entire taproot to prevent regrowth. For perennial Canada Thistle, repeated mowing or cutting the plant when it is beginning to bud can suppress it by preventing flowering and seed set. This must be done consistently to starve the extensive root system. Tilling is generally not recommended for Canada Thistle because breaking up the rhizomes promotes the growth of many new plants from the root fragments.
Chemical Control
Chemical control involves using systemic herbicides, which are absorbed by the leaves and transported down to the roots to kill the entire plant. The most opportune time to apply these chemicals, such as glyphosate or selective broadleaf herbicides, is in the late summer or early fall. During this period, the plant naturally moves energy reserves from the leaves down to the roots in preparation for dormancy. This allows the herbicide to be translocated more effectively into the deep root system. Herbicides should be applied when the plants are actively growing and have reached the bud stage, but before they have fully flowered.
Cultural Control
Cultural control strategies focus on increasing competition against the thistle to suppress its growth. Planting competitive cover crops, such as dense stands of sorghum-sudangrass hybrids, can shade out and physically choke the thistle plants. Maintaining healthy, dense turf or pasture grasses also minimizes the open ground available for thistle seeds to germinate and establish themselves. In some agricultural settings, biological control agents, such as specific insects, may be introduced to damage the plants, but this approach requires careful study to ensure it does not negatively impact native thistle species.