The popularity of container gardening has transformed balconies, patios, and small urban spaces into productive vegetable and herb patches. Plastic containers are often the material of choice for this type of gardening due to their low cost, durability, and lightweight nature. However, the convenience of plastic is often accompanied by concerns about chemical transfer, or leaching, from the container walls into the soil and subsequently into edible plants. This dilemma centers on ensuring the health and safety of the food being grown, requiring a clear understanding of which plastics are appropriate for producing vegetables and herbs.
Understanding Plastic Resin Codes
Selecting a safe plastic container begins with understanding the Resin Identification Code (RIC), the number enclosed by the recycling triangle symbol. This code identifies the specific polymer used and guides the material’s chemical stability and potential for leaching. Heat and sunlight exposure are the primary concerns, as they can cause chemical additives like Bisphenol A (BPA) and phthalates to migrate out of the plastic structure.
The plastics considered safe for growing food are High-Density Polyethylene (#2 HDPE), Low-Density Polyethylene (#4 LDPE), and Polypropylene (#5 PP). HDPE and PP are chemically stable and resist heat and UV radiation, making them reliable choices for outdoor use in direct sunlight. Containers explicitly labeled “food-grade” are manufactured for food contact and are preferable for gardening edibles.
Conversely, several plastic types should be avoided. Polyethylene Terephthalate (#1 PET), used for single-use water bottles, can leach antimony, especially when exposed to heat. Polyvinyl Chloride (#3 PVC) often contains phthalates, plasticizers known to migrate from the material.
Polystyrene (#6 PS) and the “Other” category (#7), which includes polycarbonate, should also be avoided. Polycarbonate plastics are a common source of BPA, which can leach into the soil. Selecting containers with a safe RIC code is the foundational measure for minimizing chemical exposure.
Practical Steps for Minimizing Chemical Leaching
Even when using plastics with safe resin codes, environmental factors can accelerate chemical migration. High temperatures and intense sunlight are the primary catalysts for plastic degradation, breaking down polymer bonds and releasing additives. Positioning plastic containers in areas that receive afternoon shade significantly reduces the internal soil temperature and slows this chemical process.
Watering and drainage also influence leaching risk. Since migrating chemical compounds are often water-soluble, prolonged contact between the container walls and stagnant water increases the potential for transfer. Ensuring adequate drainage by drilling several holes in the container’s base prevents water from pooling and maintains optimal soil moisture.
If using a non-food-grade plastic container, a physical barrier can be employed. Lining the interior walls with a food-grade material, such as a thick pond liner or a specialized fabric grow bag, creates an impermeable layer between the soil and the container plastic. Before using any repurposed plastic container, thoroughly clean and sanitize it, typically with a diluted solution of one part bleach to nine parts water, to remove previous chemical residue or harmful pathogens.
Comparing Plastic to Other Container Options
While plastic offers convenience, other materials provide different benefits and safety considerations for edible gardening.
Terracotta and Clay
Terracotta or unglazed clay containers are highly porous, allowing air and moisture to pass through the walls. This promotes root health and provides a natural cooling effect through evaporative heat loss. However, this porosity means terracotta dries out quickly and requires more frequent watering than plastic.
Wood
Wood containers, such as raised beds, are attractive and customizable, but chemical treatments pose a safety concern. Older pressure-treated wood (pre-2004) often contains Chromated Copper Arsenate (CCA), which can leach arsenic into the soil. Modern pressure-treated lumber uses safer compounds like Alkaline Copper Quaternary (ACQ). Untreated, naturally rot-resistant woods like cedar or redwood are generally preferred for food production.
Metal
Metal containers, particularly galvanized steel, are durable and increasingly popular. Galvanization coats steel with zinc to prevent rust. While zinc is a necessary plant micronutrient, excessive leaching can occur if the soil is highly acidic. For plants that require acidic soil, or if a gardener is concerned about the galvanized coating, lining the interior with a food-grade plastic barrier is a simple precaution.