By Cesar Cappa and Kris McNaughton, OMAFA
Published December 2025
Introduction
It is not uncommon for strawberry growers to face several difficult questions before establishing new fields. Will the residual herbicides used in the previous rotational crop harm the establishment of new strawberry plants? Could the residues allow for crown establishment but limit runner establishment? Might some herbicides still result in visual damage of the berry?
Growers who have faced these issues understand that strawberries, particularly while establishing perennial systems, are highly sensitive to many herbicides with soil residual activity commonly used in other crops. Failure to consider their lingering impact can have serious consequences.
These growers also recognize an unsettling reality: herbicide carryover is not always predictable. It can be complex to understand and even more challenging to manage. This publication aims to present the most critical information growers should consider in order to make informed decisions and, ideally, avoid herbicide carryover issues altogether.
Herbicide carryover general facts
Herbicide carryover occurs when residues of herbicides, applied to the initial crop, persist in the soil for subsequent crops. These residues may or may not have an economic impact on the following crop, depending on their concentration and the sensitivity of the next crop to that herbicide.
A key concept in assessing herbicide carryover risk is recognizing that carryover is not a static or easily predictable phenomenon. Even within the same field and crop, substantial differences can occur from year to year, or across different areas of the same field within a single season. This variability reflects the many factors that influence both soil half-life and herbicide availability, including the chemical properties of the active ingredient, soil characteristics such as organic matter, cation exchange capacity, clay content, and pH, as well as weather and microbial activity in the soil.
Active ingredient persistence and availability
Soil half-life is the most commonly used indicator of herbicide persistence. In general, a longer half-life indicates slower degradation and a greater potential for phytotoxicity to subsequent crops. However, the mere presence of a herbicide in the soil does not necessarily result in crop injury. If the herbicide remains tightly bound, or adsorbed, to soil colloids, it may be largely unavailable to the following crop, thereby limiting the potential for damage as long as it remains bound.
An important consideration is that herbicide half-life is highly site- and condition-specific. Substantial variation can occur among fields and from year to year, so growers should use caution when interpreting half-life values reported in manuals or online sources.
Soil characteristics
Soil properties strongly influence both persistence and availability of herbicides. These interactions vary across chemical families, making it essential to consider how herbicide families behave rather than all herbicides in general. For instance, soil pH significantly influences the persistence of atrazine in soil but is far less important for pendimethalin (Prowl).
Key soil variables to consider include:
- pH: affects the chemical form, availability, and degradation rate of many herbicides.
- Organic matter, CEC and clay content: influence how much herbicide binds to soil particles. Some herbicide compounds bind more to clay, others to organic matter, and some to neither. Strongly bound compounds are generally less available to cause plant injury but often persist longer in soil because they are less prone to leaching and microbial breakdown.
A less commonly discussed but contributing factor is the composition of soil microbial communities. For example, repeated use of atrazine can select for a population of bacteria that degrade atrazine quickly, reducing its persistence compared to soils where atrazine has never been applied. Microbes can help to breakdown herbicide residues so conditions that affect their numbers (eg. drought, soil pH, cold) can affect how quickly herbicides degrade.
Weather conditions
Temperature and moisture strongly influence how long pesticides persist in the soil. Dry and cold soils slow degradation significantly. Therefore, a dry season during the year the herbicide was applied will greatly increase the likelihood of herbicide carryover into the following spring.
Species and cultivar sensitivity
Plant species, even closely related ones, vary in their sensitivity to herbicides. Additionally, varieties or cultivars, within the same species can also differ significantly with their response. Unfortunately, in strawberries there is limited research evaluating varietal sensitivity to residual herbicides.
Practically, this means growers cannot assume that all strawberry varieties will respond the same way. The possibility of increased sensitivity of untested varieties should always be considered when assessing risk.
One additional consideration is that certain herbicide combinations can act synergistically, increasing the risk of carryover injury to subsequent crops compared with applications of the individual herbicides alone. For example, the inclusion of atrazine with mesotrione (Callisto) can enhance mesotrione phytotoxicity to sensitive crops in the following season.
Label instructions
Unfortunately, many herbicide labels do not provide clear rotational guidelines for horticultural crops, and very few mention strawberries specifically. As a result, growers are often left with a generic statement such as “bioassay required” for an undefined number of years following application.
Bioassays
A bioassay involves growing the next crop in treated soil for at least one growing season to assess the potential for injury. While this is the safest and most strongly recommended approach, growers often want a preliminary estimate of risk before committing time, land, and resources to a full field bioassay.
A less representative, but faster and lower-cost, alternative is to conduct a pot bioassay using sensitive species and, ideally, strawberry crowns grown in containers filled with representative soil samples. When performing this type of bioassay, it is essential to collect truly representative soil samples, as soil characteristics can vary considerably within a field and, as discussed previously, carryover risk can vary accordingly.
For detailed instructions on how to perform this type of bioassay, visit https://onfruit.ca/2022/05/09/before-you-plant-think-about-herbicide-residues/
Laboratory testing
Growers often wonder whether laboratory soil herbicide detection can eliminate the need for bioassays. Although these tests are useful for indicating what remains in the soil, they do not reliably measure the plant-available fraction of herbicides, and crop-specific injury thresholds are not well defined. Therefore, laboratory detection alone cannot reliably predict the risk of crop injury.
Summary table of residual herbicides most relevant information
The following table summarizes essential information that can help assess carryover risk for many commonly used residual herbicides. While this information can support risk assessment and diagnosis of injury symptoms, it does not replace label requirements or the need for proper bioassay testing when required.
An estimation of carryover risk can be made by considering the label’s rotational restrictions, crop sensitivity, and the persistence of the active ingredient. For example, Pursuit is highly persistent and strawberries are very sensitive to it; the label requires a bioassay before planting. This makes it a high-risk option with the potential for multi-year carryover. The same applies to other Group 2 herbicides such as Sandea and Classic, which also show increased carryover risk in alkaline soils.
Important consideration:
Research on strawberry sensitivity to herbicides is extremely limited. The sensitivity information available is often based on trials from different regions, limited cultivars, specific production systems (e.g., plasticulture only), and particular growth stages. Do not assume low risk simply because a product is listed as having “low” sensitivity.
Likewise, persistence values can be extremely variable based on soil properties and weather conditions. Even herbicides classified as low-persistence may persist much longer under drought or cold conditions.
Highlighted cells represent the most important variables to consider, due to their impact, when doing the risk assessment.
Given all the variables previously discussed, it is safe to say that one cannot achieve complete certainty about the presence or absence of economically significant carryover without a proper bioassay. However, better-informed decision-making is certainly possible when growers consider all the factors that can influence herbicide carryover.
Remember to ask yourself:
- Do I know the herbicide application history of the field? Try to find out what was applied, when it was sprayed, and at what rate.
- What are the soil characteristics of the field? Soils with high clay, organic matter, and CECs often adsorb the herbicide more tightly than coarse, sandy soils.
- What were the weather conditions like last year? Cold or dry conditions may mean more of that herbicide is lingering in the soil, making crop injury more likely.
- Do I know if the herbicide applied the previous year has been shown to harm strawberries or if there are known cultivar sensitivity issues?
- Based on the risk assessment, is a bioassay required before planting the field?