“Weed and Feed” products combine a fertilizer to promote healthy turf growth with a broadleaf herbicide designed to eliminate common lawn weeds. Aggressive, perennial species like Canada thistle and the biennial bull thistle are frequently targeted by homeowners seeking a simple solution. Whether this single-step product can kill established thistles depends heavily on the herbicide’s active ingredients, the timing of the application, and the thistle species involved. While Weed and Feed can suppress or kill young thistles, achieving complete, long-term control often requires effective chemical combinations applied at the perfect time.
Understanding Weed and Feed Herbicide Components
The effectiveness of a Weed and Feed product against thistles is directly tied to the types of herbicides it contains. These products typically use selective, post-emergent herbicides absorbed through the leaves of broadleaf plants, sparing the surrounding grass. These herbicides are synthetic auxins, chemicals that mimic natural plant growth hormones, causing the weed to grow uncontrollably and die.
Many common formulations rely heavily on 2,4-D, which is effective for a wide range of broadleaf weeds, including thistles in their young, rosette stage. However, 2,4-D alone often provides only partial control of deeply rooted, established perennial thistles like Canada thistle. The plant’s extensive root system can survive the initial top-growth damage and quickly send up new shoots.
For improved control, the most effective Weed and Feed mixtures include a combination of herbicides, such as 2,4-D combined with dicamba or triclopyr. Dicamba and triclopyr are more potent against hard-to-kill weeds and enhance the systemic movement of the chemical down into the root system. This multi-herbicide approach significantly increases the likelihood that the chemical will successfully travel throughout the entire plant structure. This systemic action is necessary to disrupt the plant’s entire physiology.
Why Thistles Are Difficult to Eradicate
Thistles, especially perennial species such as Canada thistle, possess biological characteristics that make them difficult to eliminate with a single treatment. Their primary defense mechanism is an extensive, aggressive root system made up of deep taproots or creeping rhizomes. These underground structures store significant energy and nutrients, enabling the plant to recover even after the above-ground foliage is destroyed.
When a systemic herbicide only kills the leaves and stem, the plant draws on its root reserves to sprout new growth, often within a few weeks. The creeping rhizomes of Canada thistle can spread horizontally for many feet, allowing a single plant to colonize a large area rapidly. This network of roots must be fully poisoned to achieve permanent eradication.
Additionally, the leaves of mature thistles often have a thick, waxy coating, which can repel granular or liquid herbicides. This waxy layer prevents the herbicide from being absorbed efficiently into the plant’s vascular system. If the chemical is not absorbed effectively, it cannot travel down to the root system in a lethal concentration. This necessitates specific application methods and timing to maximize leaf absorption.
Application Timing and Effectiveness Against Thistles
The timing of the Weed and Feed application is the most important factor in determining its long-term effectiveness against thistles. Since the goal is to kill the entire plant, the systemic herbicide must be applied when the plant is naturally moving resources downward, a process known as translocation.
For perennial thistles, the best time for application is in the late summer or early fall, when the plant is preparing for winter. During this period, the thistle actively moves sugars and nutrients from its leaves down to the root system for storage. Applying the systemic herbicide at this time carries the chemical directly to the root network.
A secondary window exists in the spring, when the thistle is in its rosette stage and actively growing, but before it bolts or develops flower buds. If the herbicide is applied too late in the spring or during the heat of summer, the plant is primarily moving energy upward for flowering and seed production. This results in only a temporary burn-down of the top growth, meaning the herbicide fails to reach a lethal concentration in the roots.
Because the extensive root system of a perennial thistle requires a significant amount of herbicide to exhaust its reserves, a single application of Weed and Feed is usually insufficient. Multiple treatments over several seasons are necessary to fully control a well-established thistle patch. This process involves systematically weakening the root system until it can no longer support new growth.
Specialized Methods for Thistle Control
When traditional Weed and Feed proves insufficient, more specialized methods are available for targeted control. Instead of a broadcast application, spot treatment with a concentrated liquid systemic herbicide offers a much higher dose directly to the weed. Herbicides containing ingredients like glyphosate or triclopyr can be mixed to a strong concentration and applied directly to the thistle foliage.
For established, large thistles, applying the concentrated herbicide directly to a freshly cut stem or a large rosette significantly improves root translocation. This method bypasses the waxy leaf cuticle and ensures the chemical is absorbed quickly into the plant’s vascular system. When using a non-selective product like glyphosate, caution must be taken, as it will kill any plant it contacts, including surrounding grass.
Non-Chemical Control
Non-chemical control methods can also be effective, especially when combined with chemical treatments. Repeated mowing or cutting of the thistle at ground level, particularly when the plant is in the bud stage, forces the plant to use up its stored root energy to regrow. Consistent cutting starves the root system over time. Hand-digging can be effective for annual or biennial species like bull thistle with a taproot, provided the entire root is removed to prevent regrowth.