Smartweed (genus Persicaria) is a persistent, common broadleaf weed that rapidly colonizes gardens, flower beds, and turf areas. These species are prolific seed producers and thrive in disturbed or moist environments, quickly establishing a dense presence. Effectively managing an infestation requires a multi-faceted approach, starting with accurate identification, followed by manual removal or chemical treatment, and long-term preventative measures.
Identifying Smartweed Species and Growth Habits
Smartweed species are identified by their lance-shaped to elliptical leaves arranged alternately along their stems. A telltale sign, common on species like Ladysthumb (Persicaria maculosa), is a dark, often purplish or black, thumbprint-like mark near the center of the leaf blade. Another defining characteristic is the presence of an ocrea, a swollen, papery membrane that wraps around the stem at the junction of the leaf.
Most common yard invaders, such as Pennsylvania smartweed (Persicaria pensylvanica), are summer annuals that germinate in the spring and die after producing seeds in the fall. These annuals reproduce exclusively by seed, with a single plant capable of generating over 20,000 seeds that remain viable in the soil for years. Other species, like Swamp Smartweed (Persicaria setacea), are perennial and regrow from established root systems. Smartweed prefers damp areas, but it can adapt to most sunny or partly shaded, disturbed soil sites.
Non-Chemical Removal and Site Management
For smaller infestations, physical removal is an effective strategy, especially before the plants produce their pink or white flower spikes and set seed. Hand-pulling is easiest when the soil is moist, as smartweed typically has a fibrous root system or a small taproot. For annual species, removing the entire plant before it flowers stops the current generation from spreading more seeds.
Perennial smartweed species require the complete removal of all root fragments to prevent regrowth. For a larger patch, solarization can be used by covering the area with clear plastic sheeting for several weeks during the hottest part of the summer. This technique uses the sun’s heat to kill both existing weeds and the seeds in the top layer of the soil. Adjusting irrigation is also important, since smartweed thrives in consistently wet soil. If the weed is in a lawn, increasing the mowing height encourages dense turf growth, which shades the soil and prevents new smartweed seedlings from germinating.
Effective Herbicide Applications
When smartweed infestations are widespread, chemical control may be necessary. Post-emergent broadleaf herbicides are the most common choice for killing actively growing smartweed plants. Look for products containing a combination of active ingredients like 2,4-D, dicamba, and mecoprop (MCPP). This three-way mixture offers broad control over many common broadleaf weeds, including smartweed. These chemicals are selective, meaning they target broadleaf plants while leaving turfgrass largely unharmed.
The best time to apply post-emergent herbicides is when the smartweed plants are young and actively growing, but before they set seed. These ingredients act as synthetic auxins, or growth hormones, causing the weed to grow uncontrollably and eventually die. Always follow label instructions precisely. Be cautious with dicamba products near desirable trees and shrubs, as it can be absorbed through their roots and cause damage. For long-term suppression, apply a pre-emergent herbicide in the early spring to prevent the seeds from germinating.
Preventing Future Smartweed Infestations
Stopping smartweed from returning requires addressing the conditions that allowed it to flourish in the first place, primarily through improving drainage and creating physical barriers. Since smartweed seeds germinate best in disturbed, exposed soil, covering bare ground is an effective preventative measure. Applying a thick layer of organic mulch, such as wood chips or composted bark, to a depth of two to three inches denies sunlight to the soil, which significantly reduces seed germination.
Alternatively, landscape fabric can be used beneath mulch or stones to create a permanent physical barrier. Another long-term strategy involves establishing a dense stand of desired plants, such as groundcover or thick turfgrass, which outcompetes smartweed for resources and sunlight. Improving soil aeration and drainage will reduce the moist conditions that smartweed prefers, making the environment less hospitable for future infestations.