Prickly lettuce, scientifically known as Lactuca serriola, represents a challenge for home gardeners and landscapers. This aggressive plant, sometimes called compass plant due to its unique leaf orientation, is a fast-growing weed that competes vigorously with cultivated plants for water and soil nutrients. Its resilience and capacity for prolific seed production make it difficult to manage once established. Understanding the plant’s life cycle and physical characteristics is the first step toward successful long-term management.
Identifying Prickly Lettuce and Understanding Its Growth Cycle
Identification begins by recognizing the plant’s distinctive physical characteristics, particularly the spines. Sharp prickles line the edges of the leaves and run along the underside of the main leaf vein. Breaking any part of the stem or leaf will immediately release a milky, white sap.
A unique trait of mature prickly lettuce is its tendency to twist its leaves vertically, aligning the leaf edges toward the sun’s direction. This behavior gives the plant its alternative name, compass plant, as the leaves often orient themselves along a north-south plane. The plant starts as a low-lying rosette of leaves before transitioning to its upright form.
Prickly lettuce can function as either an annual or a winter annual/biennial, completing its life cycle in one or two growing seasons, depending on when seeds germinate. For effective management, timing is dictated by the transition from the rosette stage to the bolting stage. The plant is easiest to control when it is small and growing close to the ground.
Bolting occurs when the plant sends up a tall, rigid flower stalk, usually during summer. Once this stalk develops, the plant rapidly produces numerous small, yellow flowers that mature into seed heads. A single, mature plant can produce thousands of seeds, ensuring its proliferation in subsequent seasons. Controlling the weed before it reaches this reproductive stage is the most important factor in preventing large-scale infestations.
Manual Removal Techniques and Cultural Prevention
For those preferring non-chemical methods, manual removal is effective, provided it is executed correctly and consistently. Hand-pulling is most successful when the prickly lettuce is still in the small rosette stage, before its taproot has fully developed. Pulling should be done when the soil is thoroughly moist, which helps the soil release the root structure more easily.
The taproot of Lactuca serriola can grow deep, making complete extraction challenging, and it is brittle. If the root breaks and the crown remains in the soil, the plant can regrow, necessitating follow-up. The goal is to remove the entire root system to eliminate the possibility of regeneration from the root crown.
If pulling the entire root proves difficult, an alternative manual method is to sever the taproot below the soil surface. Using a garden hoe or a sharp knife, cut the stem approximately one or two inches beneath the crown where the leaves meet the root. This action effectively separates the photosynthetic part of the plant from the root structure, preventing the plant from drawing nutrients and regrowing.
Cultural Prevention Strategies
Long-term cultural prevention strategies focus on creating an environment unfavorable for prickly lettuce germination. Heavy mulching is an effective non-chemical barrier. Applying a layer of organic material, such as wood chips or shredded bark, at a depth of three to four inches, physically blocks light necessary for the weed seeds to sprout.
Maintaining a dense, healthy lawn is another preventative measure against this weed. A thick canopy of turfgrass shades the soil, inhibiting the germination of prickly lettuce seeds, which require light to sprout. Proper watering and fertilization help the desirable grass outcompete any newly emerging seedlings.
Minimizing soil disturbance in garden beds helps keep dormant seeds buried deep where they cannot germinate. Prickly lettuce seeds can remain viable in the soil for one to three years. Tilling or aggressive raking can bring these seeds to the surface, so limiting these activities helps reduce the overall seed bank near the soil surface.
Effective Herbicide Applications
Chemical control offers an effective option for managing larger infestations of prickly lettuce, particularly in turfgrass areas. The success of any chemical application depends on precise timing. Herbicides are most effective when the weed is small and actively growing in its low-lying rosette stage, either in the fall or early spring.
Treating the plant before it bolts ensures that the chemical can be easily translocated throughout the entire system before the stem hardens. Once the plant has bolted and developed a rigid stalk, it becomes more tolerant to common chemical treatments. Applications timed to the rosette stage offer the greatest opportunity for complete control.
Prickly lettuce is classified as a broadleaf weed, making it susceptible to common post-emergent herbicides. Look for products that contain active ingredients such as 2,4-D, dicamba, or triclopyr, often sold as combination broadleaf weed killers. These chemicals interfere with the plant’s natural growth hormones, causing lethal growth.
When applying these chemicals, exercise caution, especially when treating weeds near desirable plants. Herbicides containing dicamba or 2,4-D can easily drift or vaporize in warm conditions, potentially causing damage to nearby flowers or vegetables. Always follow the label instructions regarding wind speed and temperature limits to minimize off-target movement.
For application within turfgrass, many combination products are formulated to selectively kill the broadleaf weeds while leaving the grass unharmed. In vegetable gardens, a non-selective product containing glyphosate may be necessary, but this requires careful spot-treatment to avoid killing surrounding plants. Always ensure the chosen product is explicitly labeled for the specific application area.