Exploration of Shredded Cardboard as a Mulch and Compost Resource to Improve Soil Health and Water Management by Urban Growers in the Twin Cities

View the project final report

Commodities

• Fruits: berries (strawberries)
• Vegetables: beans, collard greens

Practices

• Crop Production: municipal wastes, mulching
• Education and Training: demonstration, display, extension, farmer to farmer, on-farm/ranch research, participatory research, technical assistance
• Natural Resources/Environment: soil stabilization
• Pest Management: mulches - general, physical control, weed ecology
• Production Systems: urban agriculture
• Soil Management: nutrient mineralization, organic matter, soil analysis
• Sustainable Communities: leadership development, urban agriculture

Title: Exploration of Shredded Cardboard as a Mulch to Improve Plant and Soil Health in Urban Farms and Gardens in the Twin Cities Metropolitan Area, MN 

Problem Description: Mulching is utilized by urban growers to build soil health, mitigate weed pressure, and reduce water use. Despite these benefits, many mulching materials have become too expensive for growers to purchase and transport in the quantities required for an urban garden or farm. Our community-university research team identified shredded cardboard as a potential new mulch material that is accessible and affordable because of the increase in cardboard packaging for home delivery during the COVID-19 pandemic. The overall objective of this collaborative project is to assess the performance of shredded cardboard mulch in urban farms and gardens.

Key Outcomes:  Researchers, extension educators, and urban growers will increase understanding about the efficacy of shredded cardboard mulch and its impact on plant health, water use, and soil fertility. Researchers/educators will leverage grower experiences to develop mulching guidance and education resources. Growers will implement mulching practices to increase yields while stewarding environmental resources, such as water and soil nutrients.  

Approach: Activities were designed collaboratively by the community-university research team and informed by feedback from twenty growers who participated in a pilot study of shredded cardboard mulch in summer 2021. Growers observed that while mulch reduced weed pressure and maintained soil moisture, crops were more susceptible to chlorosis (leaf yellowing). Thus, we will conduct replicated research trials at the University of Minnesota to compare the effects of shredded cardboard mulch, straw, and no mulch on yield; nitrogen cycling; and soil characteristics (Activity 1). Based on grower interest in conducting research and requests for support to implement mulching, irrigation, and fertility management practices, we will facilitate complementary grower trials at six community gardens and facilitate co-learning gatherings to build shared knowledge (Activity 2). Finally, we will connect shredded cardboard mulching practices to efforts by establishing an industry advisory panel, conducting a literature review, and facilitating three tours per year of sites such as cardboard production, recycling, and composting facilities, garden networks, or community organizations addressing climate change adaptation (Activity 3).

Relevance: In combination with parallel projects to explore local production and distribution of shredded cardboard, this project will contribute to increasing the availability, affordability, and accessibility of a new mulching option, which would benefit small-scale growers throughout the North Central Region.

Comprehensive Project Summary: 

• Problem Addressed and Solution Pursued: Urban growers in the Twin Cities face barriers to accessing affordable, effective mulching materials. Traditional mulches like straw and wood chips can be cost-prohibitive for purchase and transport in quantities needed for urban gardens and farms. Simultaneously, the COVID-19 pandemic dramatically increased cardboard packaging waste from home delivery services, with only 50% recovered through residential recycling programs—creating both a waste management challenge and a potential resource opportunity. Our community-university research team identified shredded cardboard as a potential solution: an accessible mulch alternative that could simultaneously address urban growers' resource needs and reduce waste streams through recycling and waste industry placement.
• Research Approach: We conducted a comprehensive two-year study (2023-2024) combining controlled research trials with community-based participatory research across multiple urban agriculture sites. The research design included:
  • Controlled Trials: Replicated experiments at the University of Minnesota St. Paul campus comparing shredded cardboard mulch, straw mulch, and bare soil treatments across three crops (dry beans, collard greens, strawberries). We measured crop yield and quality, soil fertility parameters (carbon, nitrogen, phosphorus), soil moisture and temperature, weed biomass, and weeding labor time.
  • Community Exploration: Parallel trials at six community garden sites with participating growers who received supplies, training, and ongoing support. Community observations were systematically collected through submission forms documenting real-world performance across diverse growing conditions.
• Educational Approach and Farmer Learning Outcomes: Our educational approach centered on co-learning frameworks that honored multiple ways of knowing. We implemented:
  • Community-Responsive Programming: 15+ co-learning events annually, pairing mulch education with seasonal topics selected by community members (pest management, food preservation, climate adaptation). Events included hands-on activities and take-home materials.
  • Relationship-Building: 30-minute food-sharing components at gatherings, support for community co-facilitators, and recurring visits to build trust and sustained engagement within garden communities.
  • Practical Skill Development: Demonstration of application techniques, recordkeeping training, soil testing interpretation, and irrigation management to complement mulching practices.
  • Systems Understanding: Tours and educational content connecting on-farm practices to broader cardboard production and recycling industries.
  • Learning Outcomes: Participants across both years developed increased understanding of mulching and practical application skills. Community gardeners implemented cardboard mulching techniques and provided valuable feedback that refined best practices.
• Research Conclusions: 
  • Crop Performance: Over two years, shredded cardboard mulch showed no significant negative impact on marketable yield compared to conventional alternatives. However, application timing appears to be critical, as early-season application near newly planted crops may have reduced establishment success. Some community observations noted plant yellowing and reduced growth when mulch was applied too close to young plants or incorporated into soil.
  • Weed Management: Cardboard mulch significantly reduced both weed biomass and weeding time when properly applied in uniform 1-2 inch layers. However, effectiveness was highly dependent on application technique and thin or non-uniform coverage showed poor weed suppression, and benefits diminished over time requiring mid-season reapplication.
  • Water conservation: Moisture retention benefits were context-dependent. Under frequent irrigation, cardboard showed minimal advantages over other treatments. Benefits were most apparent during drought conditions with limited irrigation—a scenario that may reflect water source uncertainties in some urban and community gardens.
  • Soil nutrient availability: No significant effects on soil N and P availability were detected over the two-year study period. However, we were not able to study how potential nitrogen immobilization effects may accumulate over time given the limited time frame, or the long-term decomposition patterns of shredded cardboard mulch. 
  • Contaminant concerns: Based on analysis of shredded cardboard mulch and straw mulch materials in our study, we found no major evidence for contamination concerns at levels of application used in this work. Elemental analysis using complete digestion of materials showed that the cardboard mulch in our study (which did contain some colored inks) generally had higher levels of metals than straw mulch, but exceptions occur, for example cardboard mulch had lower levels of cadmium (Cd) than straw mulch, and concentrations of arsenic (As) were below the lower limit of detection in both mulching materials. Several metals (aluminum (Al), nickel (Ni), copper (Cu), lead (Pb), and iron (Fe)) were significantly higher in concentration in cardboard relative to straw mulch, however the risk of elevating soil levels of these elements above background is negligible to very low, given that concentrations of these elements in the cardboard mulch are still 2-3 orders of magnitude lower than that found in the soil at background conditions.
  • Community observations: Mixed experiences were reported, with some growers noting plant stress, yellowing, and reduced growth particularly with newly established plants. There were anecdotal observations of earthworm incorporation observed at community sites, while potentially beneficial, also necessitated additional labor for mulch replenishment and raised questions about whether this represents a sustainable system or requires ongoing inputs.
• Farmer Adoption Actions and Behavior Changes:
  • More than 50 growers participated across six community sites, however adoption was variable and often required ongoing support. Some growers discontinued use after experiencing plant establishment problems or finding the technique and material unfamiliar. Through trial and error, successful adopters developed specific protocols including optimizing application depth (1-2 inches), timing relative to crop establishment, and experimenting with nitrogen supplementation. Some community gardeners became advocates and shared techniques with neighbors, though adoption beyond direct project participants remained limited. This project created educational materials though widespread dissemination and implementation outside of the project remain to be demonstrated. Partnerships have been established with local recycling and cardboard industries, but economic viability and supply chain sustainability at scale were not investigated in this project (which was focused solely on soil and agronomic outcomes). 

Researchers, extension educators, and urban growers will increase understanding about the efficacy of shredded cardboard mulch and its impact on plant health, water use, and soil fertility. Co-learning gatherings will deepen relationships between researchers and growers; build shared knowledge; and increase grower capacity, all of which will support action outcomes (people). Researchers/educators will leverage grower experiences to develop mulching guidance and education resources. Growers will implement mulching practices to increase yields (profitability) while stewarding environmental resources, such as water and soil nutrients (planet). Participating in tours will highlight connections between research, grower practices, environmental sustainability, and climate change.